Showing papers in "Deep-sea Research Part Ii-topical Studies in Oceanography in 2005"
TL;DR: In this article, a third branch (through the Central Channel) onto the outer shelf is found, and the mean flow opposes the prevailing winds and is primarily forced by the sea-level slope between the Pacific and Arctic oceans.
Abstract: Mooring and shipboard data collected between 1992 and 1995 delineate the circulation over the north central Chukchi shelf. Previous studies indicated that Pacific waters crossed the Chukchi shelf through Herald Valley (in the west) and Barrow Canyon (in the east). We find a third branch (through the Central Channel) onto the outer shelf. The Central Channel transport varies seasonally in phase with Bering Strait transport, and is ∼0.2 Sv on average, although some of this might include water entrained from the outflow through Herald Valley. A portion of the Central Channel outflow moves eastward and converges with the Alaskan Coastal Current at the head of Barrow Canyon. The remainder appears to continue northeastward over the central outer shelf toward the shelfbreak, joined by outflow from Herald Valley. The mean flow opposes the prevailing winds and is primarily forced by the sea-level slope between the Pacific and Arctic oceans. Current variations are mainly wind forced, but baroclinic forcing, associated with upstream dense-water formation in coastal polynyas might occasionally be important. Winter water-mass modification depends crucially on the fall and winter winds, which control seasonal ice development. An extensive fall ice cover delays cooling, limits new ice formation, and results in little salinization. In such years, Bering shelf waters cross the Chukchi shelf with little modification. In contrast, extensive open water in fall leads to early and rapid cooling, and if accompanied by vigorous ice production within coastal polynyas, results in the production of high-salinity (>33) shelf waters. Such interannual variability likely affects slope processes and the transport of Pacific waters into the Arctic Ocean interior.
391 citations
TL;DR: In this paper, a year-long time-series of temperature, salinity and velocity from 12 locations throughout the Chukchi Sea from September 1990 to October 1991 document physical transformations and significant seasonal changes in the throughflow from the Pacific to the Arctic Ocean for one year.
Abstract: Year-long time-series of temperature, salinity and velocity from 12 locations throughout the Chukchi Sea from September 1990 to October 1991 document physical transformations and significant seasonal changes in the throughflow from the Pacific to the Arctic Ocean for one year. In most of the Chukchi, the flow field responds rapidly to the local wind, with high spatial coherence over the basin scale—effectively the ocean takes on the lengthscales of the wind forcing. Although weekly transport variability is very large (ca. - 2 to + 3 Sv ), the mean flow is northwards, opposed by the mean wind (which is southward), but presumably forced by a sea-level slope between the Pacific and the Arctic, which these data suggest may have significant variability on long (order a year) timescales. The high flow variability yields a significant range of residence times for waters in the Chukchi (i.e. one to six months for half the transit) with the larger values applicable in winter. Temperature and salinity (TS) records show a strong annual cycle of freezing, salinization, freshening and warming, with sizable interannual variability. The largest seasonal variability is seen in the east, where warm, fresh waters escape from the buoyant, coastally trapped Alaskan Coastal Current into the interior Chukchi. In the west, the seasonally present Siberian Coastal Current provides a source of cold, fresh waters and a flow field less linked to the local wind. Cold, dense polynya waters are observed near Cape Lisburne and occasional upwelling events bring lower Arctic Ocean halocline waters to the head of Barrow Canyon. For about half the year, at least at depth, the entire Chukchi is condensed into a small region of TS-space at the freezing temperature, suggesting ventilation occurs to near-bottom, driven by cooling and brine rejection in autumn/winter and by storm-mixing all year. In 1990–1991, the ca. 0.8 Sv annual mean inflow through Bering Strait exits the Chukchi in four outflows—via Long Strait, Herald Valley, the Central Channel, and Barrow Canyon—each outflow being comparable (order 0.1–0.3 Sv) and showing significant changes in volume and water properties (and hence equilibrium depth in the Arctic Ocean) throughout the year. The clearest seasonal cycle in properties and flow is in Herald Valley, where the outflow is only weakly related to the local wind. In this one year, the outflows ventilate above and below (but not in) the Arctic halocline mode of 33.1 psu. A volumetric comparison with Bering Strait indicates significant cooling during transit through the Chukchi, but remarkably little change in salinity, at least in the denser waters. This suggests that, with the exception of (in this year small) polynya events, the salinity cycle in the Chukchi can be considered as being set by the input through Bering Strait and thus, since density is dominated by salinity at these temperatures, Bering Strait salinities are a reasonable predictor of ventilation of the Arctic Ocean.
371 citations
TL;DR: In the spring and summer of 2002, a cruise track encompassed the shelf and continental slope area of the Chukchi and Beaufort Seas progressing into deep water over the Canada Basin this article.
Abstract: In the spring and summer of 2002 primary production in the Chukchi Sea was measured, using 14 C uptake experiments. Our cruise track encompassed the shelf and continental slope area of the Chukchi and Beaufort Seas progressing into deep water over the Canada Basin. The study area experienced upwards of 90% ice cover during the spring, with ice retreating into the basin during the summer. Production in the spring was light-limited due to ice cover, with average euphotic zone production rates of o0.3 g C m � 2 d � 1 . Values of 8 g C m � 2 d � 1 were observed in association with surface bloom conditions during the initial ice breakup. Considerable nutrient reduction in the surface waters took place between the spring and summer cruise, and although not observed, this was attributed to a spring bloom. Decreased ice cover and increased clarity of surface waters in the summer allowed greater light penetration. The highest rates of production during the second cruise were found at 25–30 m, coincident with the top of the nutricline. Daily euphotic zone productivity in the summer averaged 0.78 g C m � 2 d � 1 on the shelf and 0.32 g C m � 2 d � 1 on the edge of the Canada basin. These data provide an estimated annual production of 90 g C m � 2 yr � 1 in the study area.
270 citations
TL;DR: The dynamics of the flow of dense water through Barrow Canyon was investigated using data from a hydrographic survey in summer 2002 as discussed by the authors, where the focus is on the winter-transformed Bering water, the highest volumetric mode of winter water in the Chukchi Sea.
Abstract: The dynamics of the flow of dense water through Barrow Canyon is investigated using data from a hydrographic survey in summer 2002. The focus is on the winter-transformed Bering water—the highest volumetric mode of winter water in the Chukchi Sea—which drains northward through the canyon in spring and summer. The transport of this water mass during the time of the survey was 0.2–0.3 Sv. As the layer flowed from the head of the canyon to the mouth, it sank, decelerated, and stretched. Strong cyclonic relative vorticity was generated on the seaward side of the jet, which compensated for the stretching. This adjustment was incomplete, however, in that it did not extend across the entire current, possibly because of internal mixing due to shear instabilities. The resulting vorticity structure of the flow at the canyon mouth was conducive for baroclinic instability and eddy formation. Multiple eddies of winter-transformed Bering water were observed along the Chukchi–Beaufort shelfbreak. Those to the west of Barrow Canyon were in the process of being spawned by the eastward-flowing shelfbreak current emanating from Herald Canyon, while the single eddy observed to the east originated from the Barrow Canyon outflow. It is argued that such an eddy formation is a major source of the ubiquitous cold-core anti-cyclones observed historically throughout the Canada Basin. Implications for the ventilation of the upper halocline of the Western Arctic are discussed.
224 citations
TL;DR: A review of the hydrographic data from the 2002 Western Arctic Shelf-Basin Interactions (SBI) Process Cruises permits the following conclusions as discussed by the authors : temperature and salinity relationships were similar to canonical descriptions, but at five stations in the outer shelf/slope region, warm/high-salinity Atlantic Layer Water appeared to have risen, displaced the lower halocline, and mixed with shelf/upper halocine water.
Abstract: A review of the hydrographic data from the 2002 Western Arctic Shelf–Basin Interactions (SBI) Process Cruises permits the following conclusions. (1) Temperature–salinity relationships were similar to canonical descriptions, but at five stations in the outer shelf/slope region, warm/high-salinity Atlantic Layer Water appeared to have risen, displaced the lower halocline, and mixed with shelf/upper halocline water. (2) Primary production in the SBI study region was strongly influenced by the advection of dissolved inorganic nitrogen (DIN) entering via Bering Strait. This import of DIN (ammonium+nitrate+nitrite) is modified by local processes, but without the Bering Strait inflow, biological productivity in the SBI region would be much lower. (3) In comparison to the inflowing Atlantic waters, DIN+urea/phosphate and DIN+urea/silicate ratios in the Pacific waters that dominated the upper ∼150 m of the water column were low. They were also low relative to Redfield uptake ratios for phytoplankton. (4) Microbial processes continue to destroy DIN in significant quantities as the Pacific waters transit the SBI region. (5) Nitrate and ammonium were the principal contributors to DIN. Nitrite concentrations were always
208 citations
TL;DR: The Gulf of Maine Coastal Current (GMCC) was investigated from 1998 to 2001 by means of extensive hydrographic surveys, current meter moorings, tracked drifters, and satellite-derived thermal imagery as discussed by the authors.
Abstract: The Gulf of Maine Coastal Current (GMCC), which extends from southern Nova Scotia to Cape Cod Massachusetts, was investigated from 1998 to 2001 by means of extensive hydrographic surveys, current meter moorings, tracked drifters, and satellite-derived thermal imagery. The study focused on two principal branches of the GMCC, the Eastern Maine Coastal Current (EMCC) that extends along the eastern coast of Maine to Penobscot Bay, and the Western Maine Coastal Current (WMCC) that extends westward from Penobscot Bay to Massachusetts Bay. Results confirm that GMCC is primarily a pressure gradient-driven system with both principal branches increasing their transport in the spring and summer due to fresh-water inflows, and flowing southwestward against the mean wind forcing during this period. In the spring and summer the subtidal surface currents in the EMCC range from 0.15 to 0.30 ms 1 while subtidal WMCC currents range from 0.05 to 0.15 ms 1 . The reduction of southwestward transport near Penobscot Bay is accomplished via an offshore veering of a variable portion of the EMCC, some of which recirculates cyclonically within the eastern Gulf of Maine. The degree of summer offshore veering, versus leakage into the WMCC, varied strongly over the three study years, from nearly complete disruption in 1998 to nearly continuous through-flow in 2000. Observations show strong seasonal and interannual variability in both the strength of the GMCC and the degree of connectivity of its principal branches. r 2005 Elsevier Ltd. All rights reserved.
202 citations
TL;DR: Agarwal et al. as discussed by the authors used a combination of laboratory measurements of germination and autofluorescence and observations of cyst autoreluorescence in the field to yield an Alexandrium fundyense cyst distribution map for the Gulf of Maine.
Abstract: The flux of cells from germinated cysts is critical in the population dynamics of many dinoflagellates. Here, data from a large-scale cyst survey are combined with surveys in other years to yield an Alexandrium fundyense cyst distribution map for the Gulf of Maine that is massive in geographic extent and cyst abundance. The benthic cyst population extends nearly 500 km alongshore. Embedded within it are several distinct accumulation zones or “seedbeds,” each 3000–5000 km 2 in area. Maximal cyst abundances range from 2–20×10 6 cysts m −2 . Cysts are equally or more abundant in deeper sediment layers; nearshore, cysts are fewer by a factor of 10 or more. This cyst distribution reflects sedimentary dynamics and the location of blooms in overlying surface waters. The flux of germinated cells from sediments was estimated using a combination of laboratory measurements of cyst germination and autofluorescence and observations of cyst autofluorescence in the field. These measurements constrained a germination function that, when applied to the cyst distribution map, provided an estimate of the germination inoculum for a physical/biological numerical model. In the laboratory studies, virtually all cysts incubated at different temperatures and light regimes became autofluorescent, but the rate of development was slower at lower temperatures, with no difference between light and dark incubations. Germination rates were highest at elevated temperatures, and were 2-fold greater in the light than in the dark. Laboratory and field fluorescence measurements suggest that>70% of the cysts in the top cm of sediment would germinate over a 60–90 day period in offshore waters. The combination of laboratory germination experiments and numerical modeling predicts nearly 100% germination of cysts in the top cm of sediment and resulting early season cell concentrations that are comparable in magnitude to observed cell distributions. It cannot account for late-season peaks in cell abundance that are heavily influenced by vegetative growth. Cyst germination flux from deep-water (>50 m) cyst seedbeds is 14X the flux in shallow waters. A conceptual model is proposed that is consistent with observed and modeled A. fundyense cyst and motile cell distributions and dynamics in the Gulf of Maine. Cysts germinate within the Bay of Fundy seedbed, causing localized, recurrent blooms that are self-seeding and “propagatory” in nature, supplying cells that populate the eastern segment of the Maine Coastal Current (MCC) and eventually deposit cysts offshore of Penobscot and Casco Bays. These cysts serve as a seed population for western Maine blooms that are transported to the west by the western segment of the MCC, where cells are removed either by mortality or advection from the region. Without the localized, “incubator” characteristic of the Bay of Fundy bloom zone, A. fundyense populations in the Gulf of Maine should diminish through time. Their persistence over many decades highlights the effectiveness of the mechanisms described here.
194 citations
TL;DR: In this article, the authors constructed the annual cycle of the baroclinic, geostrophic component of the mass and freshwater transports and freshwater content in the Gulf of Alaska's Alaska Coastal Current (ACC) from CTD data to assess the processes controlling the freshwater budget of the ACC.
Abstract: We constructed the annual cycle of the baroclinic, geostrophic component of the mass and freshwater transports and freshwater content (FWC) in the Gulf of Alaska's Alaska Coastal Current (ACC) from CTD data to assess the processes controlling the freshwater budget of the ACC. In most months the coastal freshwater discharge is balanced by the along-shelf freshwater transport (FWT). Freshwater is trapped to the ACC because offshore eddy freshwater fluxes appear to buffer onshore Ekman transport of low-salinity surface waters due to the cyclonic winds. The annual average FWT is 880 km 3 yr −1 , which compares favorably with the annually averaged runoff of 760 km 3 yr −1 . The barotropic component of the ACC might transport an additional 380 km 3 yr −1 of freshwater, with the imbalance due to underestimates in runoff, the neglect of freshwater influx from the British Columbian shelf, or the reference salinity (33.8). The mean FWC is 540 km 3 , so that the freshwater flushing time over the 1500 km portion of the ACC considered is ⩽1 yr. Our conclusions are tentative because of uncertainties in discharge estimates, structures of the cross-shelf flow and along-shelf winds, neglect of flow-topography interactions, and the barotropic transport. If our results hold, they imply that the ACC is an important freshwater source for the Bering Sea shelf and Arctic Ocean. Mass transports and FWT in late winter 1998 (El Nino) were nearly twice as large as in winter 1999 (La Nina), and the springtime onset of stratification was earlier in 1998 than in 1999. These differences were due to the winter 1997–98 being subject to anomalously large runoff and strong downwelling over the coastal Northeast Pacific Ocean. The comparison suggests that climate changes leading to warmer, rainier winters will advance the onset of springtime stratification on the inner shelf. November–May anomalies of ACC mass transport, FWT, and FWC are significantly correlated with runoff anomalies and the latter are significantly correlated with anomalies of Ketchikan–Seward atmospheric sea level pressure gradient. These results lead to a 20th century runoff record that permits retrospective examination of the ACC. It appears that the wettest (driest) decade was the 1920s (1900s) during which time ACC transports decreased (increased). While the Pacific Decadal Oscillation is correlated with this “runoff” time series, it explains
182 citations
TL;DR: An on-board microcosm experiment was performed during the CYCLOPS May 2002 cruise to track the biogeochemical response of Eastern Mediterranean surface seawater to a gradient addition of fresh and pre-leached Saharan dust, mimicking the potential fertilization effect as mentioned in this paper.
Abstract: An on-board microcosm experiment was performed during the CYCLOPS May 2002 cruise to track the biogeochemical response of Eastern Mediterranean surface seawater to a gradient addition of fresh and pre-leached Saharan dust, mimicking the potential fertilization effect as opposed to the impact of adding particles alone. Response parameters examined were P-turnover time, bacterial production and abundance, chlorophyll a, other phytopigments, abundance of different pico and nanophytoplankton groups, primary production rates, abundance of heterotrophic nanoflagellates and ciliates. The addition of fresh Saharan dust (range: 0.2– 4.9 mg l - 1 ) and the subsequent nutrient release triggered an increase in phytopigments and primary production, while no response was detected for pre-leached dust particles. Most responses were linearly related to the amount of fresh dust added. Synechococcus and prymnesiophytes increased in abundance along with cellular pigment content while Prochlorococcus disappeared, heterotrophic bacteria increased production rates, and ciliates showed a small increase in cell density. A less clear response was recorded by in situ measurements following a Saharan dust storm during a cruise in the Levantine Basin in May 2001. The calculated amount of nutrients and dust particles delivered by such an event to a 15-m thick mixed surface layer is low ( ∼ 0.3 nmol P l - 1 , ∼ 9 nmol N l - 1 and 0.06 mg dust l - 1 ) , falling close to the lowest dust addition in our microcosm experiment. Even so, an enhancement of phosphate turnover time, a sharp decline of Prochlorococcus abundance, and slight increases in chlorophyll a and bacterial activity were observed in response to the dust storm. Considering the linear effect of fresh dust concentrations on the bacterial activity, primary production and pigment concentration (total and per cell), and the likely stimulation of grazing, it is not surprising that changes due to moderate strength dust storms are mostly close to detection limit of either field or remote sensing measurements.
171 citations
TL;DR: In this paper, the authors investigated the mechanisms underlying these aspects using coupled physical-biological simulations that represent the population dynamics of Alexandrium fundyense within the seasonal mean flow.
Abstract: Observations of Alexandrium fundyense in the Gulf of Maine indicate several salient characteristics of the vegetative cell distributions: patterns of abundance are gulf-wide in geographic scope; their main features occur in association with the Maine Coastal Current; and the center of mass of the distribution shifts upstream from west to east during the growing season from April to August. The mechanisms underlying these aspects are investigated using coupled physical–biological simulations that represent the population dynamics of A. fundyense within the seasonal mean flow. A model that includes germination, growth, mortality, and nutrient limitation is qualitatively consistent with the observations. Germination from resting cysts appears to be a key aspect of the population dynamics that confines the cell distribution near the coastal margin, as simulations based on a uniform initial inoculum of vegetative cells across the Gulf of Maine produces blooms that are broader in geographic extent than is observed. In general, cells germinated from the major cyst beds (in the Bay of Fundy and near Penobscot and Casco Bays) are advected in the alongshore direction from east to west in the coastal current. Growth of the vegetative cells is limited primarily by temperature from April through June throughout the gulf, whereas nutrient limitation occurs in July and August in the western gulf. Thus the seasonal shift in the center of mass of cells from west to east can be explained by changing growth conditions: growth is more rapid in the western gulf early in the season due to warmer temperatures, whereas growth is more rapid in the eastern gulf later in the season due to severe nutrient limitation in the western gulf during that time period. A simple model of encystment based on nutrient limitation predicts deposition of new cysts in the vicinity of the observed cyst bed offshore of Casco and Penobscot Bays, suggesting a pathway of re-seeding the bed from cells advected downstream in the coastal current. A retentive gyre at the mouth of the Bay of Fundy tends to favor re-seeding that cyst bed from local populations.
169 citations
TL;DR: Using geographic information systems (GIS) software and geostatistical techniques, this article utilized three decades of water-column chlorophyll a data to examine the relative importance of autochthonous versus allochthonsous sources of reduced carbon to benthic communities that occur from the northern Bering to the eastern Beaufort Sea shelf.
Abstract: Using geographic information systems (GIS) software and geostatistical techniques, we utilized three decades of water-column chlorophyll a data to examine the relative importance of autochthonous versus allochthonous sources of reduced carbon to benthic communities that occur from the northern Bering to the eastern Beaufort Sea shelf. Spatial trend analyses revealed areas of high benthic biomass (>300 g m−2) and chlorophyll (>150 mg m−2) on both the southern and northern Chukchi shelf; both areas are known as depositional centers for reduced organic matter that originates on the Bering Sea shelf and is advected northward in Anadyr and Bering shelf water masses. We found a significant correlation between biomass and chlorophyll a in the Chukchi Sea, reflective of the strong benthic–pelagic coupling in a system that is utilized heavily by benthic-feeding marine mammals. In contrast, there was no significant correlation between biomass and chlorophyll in the Beaufort Sea, which by comparison, is considerably less productive (biomass and chlorophyll,
TL;DR: In this article, the structure of the Caribbean Current and its variability were determined from high-resolution 1 2 -degree maps of the mean velocity and eddy kinetic energy, which were used to identify discrete cyclones and anticyclones.
Abstract: Recent satellite-tracked surface drifter trajectories were analyzed to describe the mean currents and eddies in the Caribbean Sea. The structure of the Caribbean Current and its variability were determined from high-resolution 1 2 -degree maps of the mean velocity and eddy kinetic energy. Looping drifter trajectories were used to identify discrete cyclones and anticyclones, and their characteristics were described and related to the structure of the mean flow. The translation rate of eddies in different areas was found to be similar to the mean velocity of the local background flow fields, suggesting that the eddies were largely advected by the background flow. Ten energetic anticyclones translated westward at 13 cm/s in the Venezuela and Colombia Basins. These anticyclones tended to lie in two bands, centered near 15°N and 17°N, coinciding with two jets of the Caribbean Current. The northern weaker jet contains water primarily from the North Atlantic; the southern stronger jet contains water from the tropical and South Atlantic. The anticyclones are thought to have formed in the eastern Caribbean from the anticyclonic vorticity derived from North Brazil Current rings. The ring vorticity enters the eastern Caribbean through island passages and is probably amplified by the anticyclonic shear on the northern side of the jets. Southwest of Cuba a cyclone–anticyclone pair was observed to translate slowly (∼2 cm/s) westward into the Yucatan Current. The cyclone was tracked for 10.5 months with four drifters, making it the longest tracked of the Caribbean eddies.
TL;DR: A two-year study of the iron distributions in anticyclonic mesoscale Haida eddies of the eastern North Pacific Ocean has shown that such eddies are a major source of iron to the high nitrate-low chlorophyll (HNLC) waters of the central Gulf of Alaska basin this article.
Abstract: A two-year study of the iron distributions in anticyclonic mesoscale Haida eddies of the eastern North Pacific Ocean has shown that such eddies are a major source of iron to the high nitrate–low chlorophyll (HNLC) waters of the central Gulf of Alaska basin. These eddies, which are typically about 100 km in diameter, form off the west coast of Canada in winter and then track roughly westward into the open ocean. Therefore, they carry large quantities of iron-rich coastal waters into iron-limited waters of the oligotrophic gyre. When the eddies had first formed, their dissolved iron concentrations were nearly two orders of magnitude higher than is typically observed at Station Papa (50°N, 145°W; 3 n m versus less than 0.05 n m ), and the total iron in the eddies surface waters (10–40 m) was more than 13 n m higher than open ocean waters of the Alaska gyre (14 n m versus 0.5 n m ). While the overall iron content of the eddies decreased rapidly during the first year after they formed, the eddy we were able to track the longest still contained 1.5–2 times more iron than the surrounding waters 16 months after its formation. Therefore, although iron concentrations in the surface mixed layers of the eddies drop to levels observed outside very rapidly (within 4 months), upward transport along isopycnals and upwelling due to eddy decay, as well as vertical advective and diffusive transports typical of North Pacific waters, probably provide steady fluxes of iron into the euphotic zone from the still iron-rich eddy core waters throughout the lifetime of the eddy.
TL;DR: In this article, the authors focus on factors responsible for fluxes of carbon from the upper layers to the deep sea and observe higher fluxes are observed during southwest and northeast monsoon season in both the regions.
Abstract: The Arabian Sea and the Bay of Bengal are tropical basins experiencing monsoonal wind forcing that reverses semi-annually. This brings changes in physics, chemistry and biology of the upper water column on a seasonal scale and ultimately regulates the sinking fluxes of the region. An attempt is made here to focus on factors responsible for fluxes of carbon from the upper layers to the deep sea. Higher fluxes are observed during southwest and northeast monsoon season in both the regions. In contrast to the Arabian Sea, an immense quantity of freshwater runoff together with warmer SST (∼30 °C) makes the northern bay strongly stratified. The runoff also brings in billions of tonnes of fluvial matter as well. Stratification constrains subsurface nutrient input into the surface waters thereby reducing the primary production in the Bay of Bengal. The total living carbon content in the Bay of Bengal is much lower than in the Arabian Sea. Higher downward fluxes associated with deep mixed layer and high production in the Arabian Sea during summer and winter pinpoint importance of strong winds causing mixing and upwelling during summer and evaporative cooling and convection during winter. Inability of the low-speed winds to break the stratification in the Bay of Bengal keeps the region low productive throughout the year. Therefore, river water associated with the terrigenous material due to ballast effect appears to swipe off surface producers to the deep, thereby increasing the downward fluxes of total particulates, which are sometimes even higher than that of the more productive Arabian Sea.
TL;DR: In this paper, a line-transect survey of cetaceans was conducted across the shelf and slope, out to 150 km offshore from Newport, Oregon (44.6°N) to Crescent City, California (41.9°N), in conjunction with multidisciplinary mesoscale and fine scale surveys of ocean and ecosystem structure.
Abstract: Associations between cetacean distributions, oceanographic features, and bioacoustic backscatter were examined during two process cruises in the northern California Current System (CCS) during late spring and summer 2000. Line-transect surveys of cetaceans were conducted across the shelf and slope, out to 150 km offshore from Newport, Oregon (44.6°N) to Crescent City, California (41.9°N), in conjunction with multidisciplinary mesoscale and fine-scale surveys of ocean and ecosystem structure. Occurrence patterns (presence/absence) of cetaceans were compared with hydrographic and ecological variables (e.g., sea surface salinity, sea surface temperature, thermocline depth, halocline depth, chlorophyll maximum, distance to the center of the equatorward jet, distance to the shoreward edge of the upwelling front, and acoustic backscatter at 38, 120, 200 and 420 kHz) derived from a towed, undulating array and a bioacoustic system. Using a multiple logistic regression model, 60.2% and 94.4% of the variation in occurrence patterns of humpback whales Megaptera novaeangliae during late spring and summer, respectively, were explained. Sea surface temperature, depth, and distance to the alongshore upwelling front were the most important environmental variables during June, when humpbacks occurred over the slope (200–2000 m). During August, when humpbacks concentrated over a submarine bank (Heceta Bank) and off Cape Blanco, sea surface salinity was the most important variable, followed by latitude and depth. Humpbacks did not occur in the lowest salinity water of the Columbia River plume. For harbor porpoise Phocoena phocoena, the model explained 79.2% and 70.1% of the variation in their occurrence patterns during June and August, respectively. During spring, latitude, sea surface salinity, and thermocline gradient were the most important predictors. During summer, latitude and distance to the inshore edge of the upwelling front were the most important variables. Typically a coastal species, harbor porpoises extended their distribution farther offshore at Heceta Bank and at Cape Blanco, where they were associated with the higher chlorophyll concentrations in these regions. Pacific white-sided dolphin Lagenorhynchus obliquidens was the most numerous small cetacean in early June, but was rare during August. The model explained 44.5% of the variation in their occurrence pattern, which was best described by distance to the upwelling front and acoustic backscatter at 38 kHz. The model of the occurrence pattern of Dall's porpoise Phocoenoides dalli was more successful when mesoscale variability in the CCS was higher during summer. Thus, the responses of cetaceans to biophysical features and upwelling processes in the northern CCS were both seasonally and spatially specific. Heceta Bank and associated flow-topography interactions were very important to a cascade of trophic dynamics that ultimately influenced the distribution of foraging cetaceans. The higher productivity associated with upwelling near Cape Blanco also had a strong influence on the distribution of cetaceans.
TL;DR: In comparison to the open ocean, several additional processes including coastal upwelling, river discharge, tidal mixing, estuarine circulation and benthic remineralization enhance nutrient supply to the surface waters of the continental shelf.
Abstract: In comparison to the open ocean, several additional processes including coastal upwelling, river discharge, tidal mixing, estuarine circulation and benthic remineralization enhance nutrient supply to the surface waters of the continental shelf. In general, coastal waters become nitrate-limited during the phytoplankton growing season, whereas iron and dissolved silicate limit phytoplankton growth in the less productive oceanic waters of the Gulf of Alaska. If coastal processes supply ample amounts of macro and micro nutrients during the growing season, diatom communities dominated by species such as Skeletomema costatum, Chaetoceros spp. and Thalassiosira spp. will bloom. Growth rates of these bloom populations typically range from 0.5 to >1.5 doublings per day, which place a high demand on nutrients. Any transport of coastal waters away from the shelf will enhance productivity in oceanic waters. The general circulation of the eastern subarctic Pacific does not allow for offshore transport except in special circumstances. These include anticyclonic mesoscale eddy formation, which can export as much as 5000 km 3 of nutrient-rich waters from the shelf in a single eddy, and recirculation of waters away from southwestern Alaska due to the cyclonic circulation around the western edge of the Alaskan Gyre. Recirculation can carry nutrient-rich water from the coast to the vicinity of Ocean Station Papa (50°N, 145°W) within a few months. For both eddies and gyre recirculation, much of the water being carried into the open ocean lies below the euphotic zone. Iron enrichment occurs to a depth of at least 1000 m in the Gulf of Alaska as a result. Periods of enhanced eddy formation or recirculation may supply iron that enriches the open ocean for several years. Damming of the Columbia River and human uses of its waters have resulted in more winter and less summer discharge of fresh water and dissolved Si. Coastal currents in this area flow north in winter and south in summer. As a consequence, there has been a net temporal change in Si inputs to the ocean since the mid 1970s, with about 4×10 9 mol more Si flowing north in winter and 9×10 9 mol less Si flowing south in summer compared with the early 1900s. Since silicate limits diatom growth and diatoms are responsible for most carbon export in phytoplankton, implications for ocean productivity are significant.
TL;DR: In the Chukchi and Beaufort Seas, pigments and size-fractionated biomass of phytoplankton were analyzed in the spring and summer of 2002 as mentioned in this paper.
Abstract: Phytoplankton pigments and size-fractionated biomass in the Chukchi and Beaufort Seas showed spatial and temporal variation during the spring and summer of 2002. Cluster analysis of pigment ratios revealed different assemblages over the shelf, slope and basin regions. In spring, phytoplankton with particle sizes greater than 5 μm, identified as diatoms and/or haptophytes, dominated over the shelf. Smaller (
TL;DR: The results suggest that the role of temperature in controlling bacterial growth in the Western Arctic is similar to that in low-latitude ocean.
Abstract: Temperature is thought to have a disproportionate role in controlling bacterial growth in perennially cold waters like the Western Arctic Ocean. One impact of temperature is that bacteria in cold waters may require more dissolved organic material (DOM) in order to approach growth rates observed at higher temperatures (the Wiebe–Pomeroy hypothesis). To explore these issues, this study examined the effect of DOM additions and temperatures shifts on bacterial assemblages during short (2 h) and long (up to 10 days) incubations. We found that the temperature response for bacterial assemblages in the Western Arctic was similar to that observed in temperate waters; the Q 10 values for leucine and thymidine incorporation were 3.1±2.6 and 1.9±0.56, respectively, not significantly different from values observed in the equatorial Pacific Ocean. In contrast to what would be predicted from the Wiebe–Pomeroy hypothesis, the impact of DOM additions on leucine incorporation either was the same or greater at higher, not lower temperatures. Increasing the incubation temperature did stimulate leucine incorporation more quickly than did DOM additions, but DOM seems as important as temperature in controlling bacterial growth. Leucine incorporation rates per cell (an index of community growth rates) observed in these experiments varied greatly and approached rates observed in waters warmer by 25 °C. These results suggest that the role of temperature in controlling bacterial growth in the Western Arctic is similar to that in low-latitude ocean.
TL;DR: Three different molecular methods were used with traditional brightfield microscope techniques to enumerate the toxic dinoflagellate Alexandrium fundyense in samples collected in the Gulf of Maine in 1998, 2000, 2001, and 2003, to compare and contrast the specificity and utility of these probe types and assay approaches.
Abstract: Three different molecular methods were used with traditional brightfield microscope techniques to enumerate the toxic dinoflagellate Alexandrium fundyense in samples collected in the Gulf of Maine in 1998, 2000, 2001, and 2003. Two molecular probes were used in fluorescent whole-cell (WC) microscopic assays: a large-subunit ribosomal RNA (LSU rRNA) oligonucleotide probe (NA1) and a monoclonal antibody probe thought to be specific for Alexandrium spp. within the tamarense / catenella / fundyense complex. Cell abundance estimates also were obtained using the NA1 oligonucleotide probe in a semi-quantitative sandwich hybridization assay (SHA) that quantified target rRNA in cell lysates. Here we compare and contrast the specificity and utility of these probe types and assay approaches. WC counts of the 1998 field samples demonstrated that A. fundyense cell densities estimated using the antibody approach were higher than those using either the NA1 oligonucleotide or brightfield microscopy due to the co-occurrence of A. ostenfeldii with A. fundyense , and the inability of the antibody to discriminate between these two species. An approach using cell size and the presence or absence of food vacuoles allowed more accurate immunofluorescent cell counts of both species, but small cells of A. ostenfeldii that did not contain food vacuoles were still mistakenly counted as A. fundyense . For 2001, a dual-labeling procedure using two oligonucleotide probes was used to separately enumerate A. ostenfeldii and A. fundyense in the WC format. In addition, the SHA was used in 2001 and 2003 to enumerate A. fundyense . Some agreement was observed between the two oligonucleotide methods, but there were differences as well. Not including samples with cell numbers below empirically determined detection limits of 25 cells l –1 , good correlation was observed for surface samples and vertical profiles in May 2001 and June 2003 when the SHA estimates were, on average, equivalent, and 1.5× the WC counts, respectively. The worst correlations were for virtually all samples from the June 2001 cruise where the SHA both over- and under-estimated the WC counts. Some differences were expected, since the SHA and the WC assays measure different, but related parameters. The former quantifies intact cells and particulate material that might contain non-viable cells or fragments, whereas the latter measures only intact cells that survive sample processing and are visible in a sample matrix. A variety of factors can thus affect results, particularly with the WC method, including variable uptake of the oligonucleotide probe due to cell permeability changes, cell lysis during sampling, preservation and processing; variable rRNA content or accessibility due to nutritional or environmental factors; and the variable detection of intact cells or cell fragments in fecal pellets and detritus. The SHA offers dramatic increases in sample throughput, but introduces uncertainties, such as those due to sample matrix effects (non-specific labeling and cross-reactions), variable rRNA levels in intact cells or to the possible presence of target rRNA in cell fragments, fecal pellets, or detritus. Molecular probes are powerful tools for monitoring and research applications, but more work is needed to compare and refine these different cell enumeration methods on field samples, as well as to assess the general validity of brightfield or fluorescent WC approaches.
TL;DR: In this article, the distribution of diatom assemblages in the southern Bering Sea shows not only differences in species composition between the suspended particulate matter (SPM) and surface sediment samples, but also preferential dissolution of different species in the upper water column or surface sediments.
Abstract: Diatom assemblages in suspended particulate matter (SPM), surface sediments, and piston core samples in the southern Bering Sea and the western subarctic Pacific were examined. The distribution of diatom assemblages in the southern Bering Sea shows not only differences in species composition between the SPM and surface sediment samples, but also preferential dissolution of different species in the upper water column or surface sediments. Furthermore, we present the downcore profiles of selected diatom taxa for four piston cores to indicate changes in paleoceanographic conditions, e.g., seasonal productivity, sea-ice and surface-water circulation, of the glacial and deglaciation periods during the late Quaternary. In particular, we propose five models for the annual cycling of the water masses in the western subarctic Pacific and the southern Bering Sea during the late Quaternary. Significant regional differences in diatom accumulation rates and taxonomic composition between the southern Bering Sea and the western subarctic Pacific reflect spatial shifts in water masses caused by the variable extent of sea-ice cover and cold low-salinity surface waters in these regions.
TL;DR: In this paper, the authors used satellite telemetry and remotely sensed satellite data to examine relationships between oceanographic features and the movements of adult female fur seals in the Bering Sea and North Pacific Ocean during early, middle and late portions of their winter migration.
Abstract: Northern fur seals breeding on the Pribilof Islands are characterized by pelagic migrations that begin each fall and last approximately eight months. Previous studies have examined the early phases of the migration with respect to timing, location, and effects of ocean surface currents on movement. We used satellite telemetry and remotely sensed satellite data to examine relationships between oceanographic features and the movements of adult female fur seals in the Bering Sea and North Pacific Ocean during early, middle and late portions of their winter migration. Physical locations of 13 female fur seals were monitored during 2002–2003, and diving data were collected on a subset of the animals. Remotely sensed data were obtained to assess sea-surface temperatures, chlorophyll a concentrations, and seasurface height anomalies encountered by the fur seals. Data from historical pelagic collection of fur seals also were summarized to describe winter diet and the distribution of different age and sex classes of the general migration of fur seals to the eastern North Pacific. Seals departed from the Pribilof Islands in November and moved in a southeasterly direction over the continental shelf as they left the Bering Sea. Their travel routes did not follow coastal or bathymetric features as they crossed the North Pacific Ocean, but instead corresponded to complementary water movement of the Alaska Gyre and the North Pacific Current. Winter foraging areas varied geographically and were associated with eddies, the subarctic–subtropical transition region, and areas that undergo coastal mixing due to the California Current. The results indicate that fur seals may cue on a variety of oceanographic features that aid in reducing energetic expenditures and optimize foraging opportunities. Published by Elsevier Ltd.
TL;DR: In this paper, an extensive deployment of on-board nanomolar nutrient measurements in this low nutrient low chlorophyll system showed that free ammonia (50-80 nM) was present in the surface waters while nitrate was less than 10 nM, confirming the results obtained elsewhere in the CYCLOPS addition experiment results.
Abstract: The south east Levantine basin of the eastern Mediterranean is a uniquely P starved system with a nitrate:phosphate ratio in the deep water of 25–28:1, a PON:POP ratio of 27–32:1 and a DON:DOPUV ratio of � 100:1 (which probably represents a DON:DOPTOTAL of � 50:1). The C:N:P ratio of nutrients accumulated in the deep water from decomposed organic matter was 106:8.5–10.8:0.34–0.43 similar to the measured ratios for dissolved and particulate organic matter and much higher than the Redfield ratio. It is concluded that the P limitation of the eastern Mediterranean is due to the lack of P within the system and not in the preferential removal of P relative to N. Results from the first extensive deployment of on-board nanomolar nutrient measurements in this low nutrient low chlorophyll system showed that free ammonia (50–80 nM) was present in the surface waters while nitrate was less than 10 nM, confirming the results obtained elsewhere in the CYCLOPS addition experiment results that grazing/nutrient recycling is a dominant process in this system. The total DIN:DIP ratio in the nutrient depleted waters above the chlorophyll maximum was predominantly greater than 16:1, suggesting that the system has not switched to N limitation. A primary nitrite maximum was observed immediately below the chlorophyll maximum at the top of the nutricline, which was similar to those found previously in oligotrophic ocean locations. Where nanomolar technology was deployed through the nutricline, it was found that the phosphocline started at the same depth as the nutricline for nitrate and silicate, a conclusion that would not have been made if only conventional micromolar technology had been available. An intercomparison of nutrient procedures suggested that freezing samples is acceptable for samples with a concentration above 20 nM (DIP) and 400 nM (nitrate and nitrite), which represent most of the previously published data from intermediate and deep waters from the Levantine basin. However for concentrations lower than this, which in practice
TL;DR: In this article, the authors examined the cross-shelf distribution of major zooplankton species on the northern Gulf of Alaska (GOA) shelf during the production season for four years, between October 1997 and October 2001.
Abstract: The cross-shelf distribution of major zooplankton species was examined on the northern Gulf of Alaska (GOA) shelf during the production season for four years, between October 1997 and October 2001. The zooplankton community on the northern GOA shelf consisted of oceanic and neritic species of the North Pacific subarctic species complex. Cross-shelf distribution of the major zooplankton species was influenced by their depth preferences, vertical migration behavior, salinity-temperature preferences, and by cross-shelf water-mass distribution and movement. The neritic community, dominated by Pseudocalanus spp., Metridia pacifica and Calanus marshallae, had highest abundances on the inner shelf, in the Alaska Coastal Current, and in the adjacent fjords in late spring and early summer. The oceanic community, which contained primarily Neocalanus cristatus and Eucalanus bungii, was observed in the Alaskan Stream and adjacent waters near the shelf break. A mid-shelf transition zone contained a mixture of oceanic and neritic species. Prince William Sound (PWS) contained a unique species complex of large mesopelagic copepods, amphipods and shrimp. Neocalanus flemingeri and Oithona similis were abundant in all four regions during spring and early summer. The transition zone commonly crossed much of the shelf between the shelf break and the ACC, but satellite images and CTD data indicate that occasionally a narrow shelf-break front can form, in which case distinct zooplankton species groups are observed on either side of the front. Satellite data also revealed numerous large and small eddies, which probably contribute to cross-shelf mixing in the transition zone.
TL;DR: In this paper, measurements of the seasonal variation in the export flux of particulate organic carbon (POC) are reported for the upper waters of the Chukchi Sea, where POC fluxes were quantified by determination of 234Th/238U disequilibrium and POC/234Th ratios in large ( > 53 μ m ) aggregates collected using in situ pumps.
Abstract: As part of the 2002 Shelf-Basin Interactions (SBI) process study, measurements of the seasonal variation in the export flux of particulate organic carbon (POC) are reported for the upper waters of the Chukchi Sea. POC fluxes were quantified by determination of 234Th/238U disequilibrium and POC/234Th ratios in large ( > 53 μ m ) aggregates collected using in situ pumps. Samples were collected at 35 stations on two cruises, one in predominantly ice-coved conditions during the spring (May 6–June 15) and the other in predominantly open water during the summer (July 17–August 26). Enhanced particle export was observed in the shelf and slope waters, particularly within Barrow Canyon, and there was a marked increase in particle export at all stations during the summer (July–August) relative to the spring (May–June). 234Th-derived POC fluxes exhibit significant seasonal and spatial variability, averaging 2.9 ± 5.3 mmol C m - 2 d - 1 ( range = 0.031 – 22 mmol C m - 2 d - 1 ) in the spring and increasing ∼ 4 -fold in the summer to an average value of 10.5 ± 9.3 mmol C m - 2 d - 1 ( range = 0.79 – 39 mmol C m - 2 d - 1 ) . The fraction of primary production exported from the upper waters increases from ∼ 15 % in the spring to ∼ 32 % in the summer. By comparison, DOC accumulation associated with net community production represented ∼ 6 % of primary production ( ∼ 2 mmol C m - 2 d - 1 ) . The majority of shelf and slope stations indicate a close agreement between POC export and benthic C respiration in the spring, whereas there is an imbalance between POC export and benthic respiration in the summer. The implication is that up to ∼ 20 % of summer production ( ∼ 6 ± 7 mmol C m - 2 d - 1 ) may be seasonally exported off-shelf in this productive shelf/slope region of the Arctic Ocean.
TL;DR: Sediment-laden sea ice is widespread over the shallow, wide Siberian Arctic shelves, with off-shelf export from the Laptev and East Siberian Seas contributing substantially to the Arctic Ocean's sediment budget.
Abstract: Sediment-laden sea ice is widespread over the shallow, wide Siberian Arctic shelves, with off-shelf export from the Laptev and East Siberian Seas contributing substantially to the Arctic Ocean's sediment budget. By contrast, the North American shelves, owing to their narrow width and greater water depths, have not been deemed as important for basin-wide sediment transport by sea ice. Observations over the Chukchi and Beaufort shelves in 2001/02 revealed the widespread occurrence of sediment-laden ice over an area of more than 100,000 km2 between 68 and 74°N and 155 and 170°W. Ice stratigraphic studies indicate that sediment inclusions were associated with entrainment of frazil ice into deformed, multiple layers of rafted nilas, indicative of a flaw-lead environment adjacent to the landfast ice of the Chukchi and Beaufort Seas. This is corroborated by buoy trajectories and satellite imagery indicating entrainment in a coastal polynya in the eastern Chukchi Sea in February of 2002 as well as formation of sediment-laden ice along the Beaufort Sea coast as far eastward as the Mackenzie shelf. Moored upward-looking sonar on the Mackenzie shelf provides further insight into the ice growth and deformation regime governing sediment entrainment. Analysis of Radarsat Synthetic Aperture (SAR) imagery in conjunction with bathymetric data help constrain the water depth of sediment resuspension and subsequent ice entrainment (>20 m for the Chukchi Sea). Sediment loads averaged at 128 t km–2, with sediment occurring in layers of roughly 0.5 m thickness, mostly in the lower ice layers. The total amount of sediment transported by sea ice (mostly out of the narrow zone between the landfast ice edge and waters too deep for resuspension and entrainment) is at minimum 4×106 t in the sampling area and is estimated at 5–8×106 t over the entire Chukchi and Beaufort shelves in 2001/02, representing a significant term in the sediment budget of the western Arctic Ocean. Recent changes in the Chukchi and Beaufort Sea ice regimes (reduced summer minimum ice extent, ice thinning, reduction in multi-year ice extent, altered drift paths and mid-winter landfast ice break-out events) have likely resulted in an increase of sediment-laden ice in the area. Apart from contributing substantially to along- and across-shelf particulate flow, an increase in the amount of dirty ice significantly impacts (sub-)ice algal production and may enhance the dispersal of pollutants.
TL;DR: The hypothesis that the observed lack of phytoplankton biomass increase response to a mesoscale in situ P-enrichment experiment in the P-limited Eastern Mediterranean was a consequence of co-limitation by P and N availability is supported.
Abstract: An on-board microcosm experiment was set up to test the hypothesis that the observed lack of phytoplankton biomass increase response to a mesoscale in situ P-enrichment experiment in the P-limited Eastern Mediterranean (Krom et al., 2005a) was a consequence of co-limitation by P and N availability in this ultraoligotrophic environment. Six microcosms were filled with subsurface seawater (ambient DIN: 90–100 nM) taken from inside a P-enriched patch (IN), which in the absence of biological activity would have had ca. 22 nM of PO4 3 . Another six microcosms were filled with unfertilized (o 2n M PO4 3 ) subsurface seawater from outside the patch (OUT). The bottles were either supplemented with 1600 nM NH4 or not, incubated on-deck and subsampled daily, or at the first and last (fourth) day of the experiment, for a suite of biological parameters. The addition of N to OUT water did not induce cell abundance increases in either the phototrophic or heterotrophic sides of the food chain, in line with previous assessments that the Eastern Mediterranean is not purely Nlimited. The IN and OUT treatments, to which no NH4 was added, mimicked the behavior of the in situ experiment, with an order of magnitude higher bacterial production of IN vs. OUT water, but no noticeable phytoplankton response. The addition of N to IN water, previously exposed to P, led to substantial responses of the entire microbial community, including 4 to 80-fold increases in chlorophyll, other pigments, bacterial activity, and the abundance of ciliates—relative to IN water to which N was not added. The ca. 10-fold increase in chlorophyll within 4 days was mostly due to a major increase in both abundance ( � 4) and fluorescence per cell ( � 17) of Synechococcus, whereas Prochlorococcus disappeared. These changes were accompanied by removal from the water of 570 nM of the added NH4 , equivalent to 570/22 or N : P ratio of 26 : 1, similar to the ratio measured for POM in the area. Possibly, non-Redfield ratios were maintained, still leaving by day 4 some 1100 nM of N that could not be used due to the lack of P. These results support our hypothesis that the lack of response of phytoplankton to the mesoscale P-enrichment was due to their concurrent N-starvation, i.e. N and
TL;DR: In this article, principal components analysis (PCA) was applied to obtain a robust comparison of biomarker compositions in Arctic Ocean sediments, and geometric mean (GM) linear regression of the PCA variables to estimate the relative contributions of labile/marine and stable/terrigenous sources to each biomarker and provided a novel measure of the removal rate of each biomarkers relative to phytol.
Abstract: An understanding of the carbon cycle within arctic sediments requires discrimination between the terrigenous and marine components of organic carbon, insight into the removal mechanisms for labile carbon during burial and appreciation of shelf-to-basin processes. Using a large data set of multiple molecular organic markers (alkanes, alkanols, sterols, saturated and unsaturated fatty acids, dicarboxylic acids), we apply (1) principal components analysis (PCA) to obtain a robust comparison of biomarker compositions in Arctic Ocean sediments, (2) geometric mean (GM) linear regression of the PCA variables to estimate the relative contributions of labile/marine and stable/terrigenous sources to each biomarker and (3) the slope of the GM regression of each biomarker with TOC to provide a novel measure of the removal rate of each biomarker relative to phytol. The PCA- and TOC-based indices generally increase together: biomarkers with very high TOC-based removal rates such as the saturated and unsaturated n-alkanoic acids generally have a high labile/marine content from PCA, while the sterols have low removal rates, but exhibit a range of labile/marine content values and the n-alkanes and n-alkanols have low values for both. A dominant feature of all PCA models examined is a progressive decrease in the autochthonous/marine biomarkers with each increase in sediment core depth, which points to a universal diagenetic alteration of organic carbon with depth in the cores. The PCA model also displays a shelf to basin trend that is non-diagenetic and implies the ongoing (centuries or more) delivery of long-chain n-alkanes, n-alcohols and n-alkanoic acids in a matrix that is pre-formed and well-preserved within the sediments. Terrigenous biomarker distributions within the PCA model suggest that atmospheric transport of plant waxes in aerosols and the water borne transport of very fine plant macerals likely have significant roles in the export of these vascular plant biomarkers to the basins. Biomarker ratios and profiles of the PCA-based labile/marine content with core depth indicate that the PCA model is more strongly influenced by the biomarker lability than the marine content, while increases in the marine content are largely responsible for the shifts in composition for near-surface core sections.
TL;DR: Isolate-specific A. fundyense responses to the environment incorporated into modeling efforts may ultimately enhance predictive and monitoring capabilities for PSP outbreaks.
Abstract: The objective of this study was to determine the effects of temperature, irradiance, and salinity on photosynthesis, growth rate, total toxicity, and toxin composition for two Alexandrium fundyense isolates in the laboratory. The A. fundyense cultures studied were isolated from coastal waters off Monhegan Island (MI) in the GoM and the Bay of Fundy (BoF). Laboratory experiments demonstrated that temperature exerted the greatest impact on chlorophyll-specific maximal photosynthetic rates with negligible effects by light or salinity. Temperature and irradiance exerted the greatest influence on species-specific growth rates, with the MI isolate exhibiting greater maximal growth rates. The BoF isolate was generally more toxic, with temperature inducing the greatest change in cellular toxicity, followed by irradiance and salinity. Variations in toxicity were due to changes in toxin concentration, the relative toxin composition, or both depending on the isolate and experimental condition. The lack of a clear relationship between photosynthesis or growth and toxicity suggests that toxicity is, at least in part, driven directly by environmental conditions. Isolate-specific A. fundyense responses to the environment incorporated into modeling efforts may ultimately enhance predictive and monitoring capabilities for PSP outbreaks.
TL;DR: A. fundyense bloom was exceptional in several ways: high toxin levels were measured farther south than ever before in New England; levels of toxicity in many locations were higher than previously observed at those stations; for the first time toxicity at some locations was above quarantine levels; cell concentrations far exceeded those observed in the coastal waters of southern New England in the past; and for the region the governors of Maine and Massachusetts officially declared the red tide to be a disaster, clearing the way for federal assistance as discussed by the authors.
Abstract: From May to July, 2005, an extensive bloom of Alexandrium fundyense occurred along the coast of southern New England. The outbreak eventually closed shellfish beds from central Maine to Massachusetts, including Nantucket Island and portions of Martha’s Vineyard, and resulted in the closure of 40,000 km 2 of offshore federal waters as well. The coastal Alexandrium bloom was exceptional in several ways: high toxin levels were measured farther south than ever before in New England; levels of toxicity in many locations were higher than previously observed at those stations; for the first time toxicity at some locations was above quarantine levels; cell concentrations far exceeded those observed in the coastal waters of southern New England in the past; and for the first time in the region the governors of Maine and Massachusetts officially declared the red tide to be a disaster, clearing the way for federal assistance. Initial observations suggest that several factors contributed to this bloom. Abundant rainfall and heavy snowmelt substantially increased the amount of fresh water entering the Gulf of Maine. Combined with other freshwater inputs, we hypothesize that this provided macro- and micro-nutrients, a stratified water column, and a transport mechanism that led to high cell abundances and broad, region-wide dispersal of the organism. Warm temperatures in western waters also would have favored A. fundyense growth. In addition, several storms with strong winds out of the northeast occurred at times when cells were abundant and in locations where the winds could advect them into Massachusetts and Cape Cod Bays and keep them there, leading to high cell concentrations and toxicity. Another contributing factor may have been the high abundance of newly deposited cysts in western Gulf of Maine sediments, as documented in a fall 2004 survey. Here, we evaluate this bloom and the patterns of toxicity in light of the conceptual models for A. fundyense dynamics developed during the Ecology and Oceanography of Harmful Algal Blooms (ECOHAB)–Gulf of Maine (GOM) program. Several features of the 2005 bloom conform to the mechanisms proposed in those models, including the alongshore transport of cells in major water masses and episodic intrusions of cells toward shore due to downwelling-favorable wind forcings. The
TL;DR: In this paper, the authors present evidence for cross-shelf exchange due to three different mechanisms: drift propagation, shelf-break circulation, and shelf-origin water from the formation region into the basin and by interacting with the shelf break circulation.
Abstract: The continental shelf of the Gulf of Alaska (GOA) is a complex system characterized by large freshwater runoff and strong winds. The GOA supports one of the world's richest ecosystems, including numerous species of fishes, marine mammals and sea birds. The mechanisms that provide nutrients to support this ecosystem are not well understood. The rivers and streams that provide freshwater to the shelf are low in nitrate, and the regional winds favor downwelling. High concentrations of nitrate are available in the deep basin of the GOA, but these must be introduced to the shelf in order to support the high productivity. We present evidence for cross-shelf exchange due to three different mechanisms. Episodes of downwelling relaxation result in a flux of saline, nutrient-rich water onto the shelf at depth. Eddies, formed in the northeastern GOA, propagate along the shelf-break influencing cross-shelf exchange by carrying shelf-origin water from the formation region into the basin and by interacting with the shelf-break circulation. Bathymetric steering in the many canyons that incise the GOA shelf results in flow into the canyons where strong tidal mixing results in cross-isobath movement of water properties.