Showing papers in "Deep-sea Research Part Ii-topical Studies in Oceanography in 1998"

Biogeography, biodiversity and fluid dependence of deep-sea cold-seep communities at active and passive margins

Abstract: To date, several cold-seep areas which fuel chemosynthesis-based benthic communities have been explored, mainly by deployment of manned submersibles. They are located in the Atlantic and in the Eastern and Western Pacific oceans and in the Mediterranean Sea, in depths ranging between 400 and 6000 m in different geological contexts in passive and active margins. Our study is based on a review of the existent literature on 24 deep cold seeps. The geographic distribution of seeps, the variations of origin and composition of fluids, and rates of fluid flow are presented as they are important factors which explain the spatial heterogeneity and the biomass of biological communities. Methane-rich fluid of thermogenic and/or biogenic origin is the principal source of energy for high-productive communities; however, production of sulphide by sulphate reduction in the sediment also has a major role. The dominant seep species are large bivalves belonging to the families Vesicomyidae or Mytilidae. Other symbiont-containing species occur belonging to Solemyidae, Thyasiridae, Lucinidae bivalves, Pogonophora worms, Cladorhizidae and Hymedesmiidae sponges. Most of the symbiont-containing cold-seep species are new to science. Different symbiont-containing species rely on sulphide or methane oxidation, or both, via chemoautotrophic endosymbiotic bacteria. A total of 211 species, from which 64 are symbiont-containing species, have been inventoried. Patterns in biodiversity and biogeography are proposed. A large majority of the species are endemic to a seep area and the symbiont-containing species are mainly endemic to the cold-seep ecosystem. A comparison of species found in other deep chemosynthesis-based ecosystems, hydrothermal vents, whale carcass and shipwreck reduced habitats, reveals from the existing data, that only 13 species, of which five are symbiont-containing species occur, at both seeps and hydrothermal vents. The species richness of cold-seep communities decreases with depth. High diversity compared to that on hydrothermal vent sites is found at several seeps. This may be explained by the duration of fluid flow, the sediment substrate which may favour long-term conditions with accumulation of sulphide and the evolution of cold seeps.

Distribution and optical properties of CDOM in the Arabian Sea during the 1995 Southwest Monsoon

Abstract: Spatial and temporal variability in concentration and optical properties of colored dissolved organic matter (CDOM) were examined in the Arabian Sea during the 1995 Southwest (Summer) Monsoon on NRL/Seasoar Cruise, 21 June–12 July 1995. Classical upwelling patterns were observed near the coast, with strong fronts characterized by cold water, high chlorophyll a, and high nutrients. CDOM concentration showed a negative correlation with temperature in the surface waters throughout the study region, reflecting upwelling as the major source of CDOM to surface waters. Three water mass types were identified and found to have distinctive excitation–emission matrix (EEM) fingerprints. Persian Gulf Water had two fluorescence maxima due to UV and visible humic substance fluorescence. Maximum CDOM concentrations were found in the core of this water mass. Upwelling water had five or six fluorescence peaks, due to new and old CDOM. Elevated CDOM concentrations at the depth of the chlorophyll a maximum indicated production of new CDOM associated with the phytoplankton bloom. Oligotrophic water had extremely low CDOM concentrations, and only UV-absorbing and fluorescing compounds were found. This is consistent with CDOM which has been subjected to extensive exposure to solar irradiation at the sea surface. A new fluorophore with an Ex/Em maximum at 280/340 nm is described from highly productive waters. Two new EEM fingerprints are presented, one for oligotrophic blue waters showing photoresistant DOM, and one for upwelling water that may be of biological origin.

Temporal and spatial patterns of biological community development at nascent deep-sea hydrothermal vents (9°50 N, East Pacific Rise)

Abstract: The April 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9°45′N and 9°52′N on the East Pacific Rise provided a unique opportunity to follow temporal changes in biological community structure from the “birth” of numerous deep-sea hydrothermal vents. In March l992, DSV Alvin was used to deploy an on-bottom observatory, the Biologic–Geologic Transect, to monitor faunal succession along a 1.37 km segment of the axial summit caldera between 9°49.61′N and 9°50.36′N (depth ∼2520 m). Photo- and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the Transect were performed in March 1992, December 1993, October 1994, and November 1995. Photographic and chemical time-series analyses revealed the following sequence of events in low-temperature venting areas. (1) Immediately following the 1991 eruption, hydrogen sulfide and iron concentrations in diffuse fluids were extremely high (>1 mmol kg-1) and microbially derived material blanketed active areas of venting in the form of thick microbial mats. (2) Mobile vent fauna (e.g. amphipods, copepods, octopods, and galatheid and brachyuran crabs) and non-vent fauna (e.g. nematocarcinid shrimp) proliferated in response to this increased biological production. (3) Within 1 yr of the eruption, areal coverage of microbial mat was reduced by ∼60% and individuals of the vestimentiferan tube worm Tevnia jerichonana settled gregariously in areas where diffuse flow was most intense. (4) Two years after the eruption, maximum levels of H2S decreased by almost half (from 1.90 to 0.97 mmol kg-1) and dense thickets of the vestimentiferan Riftia pachyptila dominated vent openings previously inhabited by Tevnia jerichonana. (5) Three years after the eruption, maximum hydrogen sulfide levels declined further to 0.88 mmol kg-1 and mussels (Bathymodiolus thermophilus) were observed on basaltic substrates. (6) Four years after the eruption, galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0.72 mmol kg-1. Also by this time mussels had colonized on to tubes of Riftia pachyptila. (7) Between 3 and 5 yr after the eruption, there was a 2- to 3-fold increase in the number of species in the faunal assemblages. In the absence of additional volcanic/tectonic disturbance, we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5–10 yr following the eruption. We also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase. We hypothesize that the above series of events (1–7) represents a general sequence of biological successional changes that will occur at newly formed low-temperature deep-sea hydrothermal vents along the northern East Pacific Rise and contiguous ridge axes. Megafaunal colonization at deep-sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life-history strategies of individual species, physical oceanographic processes, and the dynamic hydrothermal environment. Our observations indicate that the successful sequential colonization of dominant megafaunal vent species, from Tevnia jerichonana to Riftia pachyptila to Bathymodiolus thermophilus, also may be strongly influenced by temporal changes in geochemical conditions. Additional evidence demonstrating the close link between diffuse vent flux, fluid geochemistry, and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow. These correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast-spreading mid-ocean ridges should consider not only the interplay of species-specific life-history strategies, community productivity, and physical oceanographic processes, but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species.

Macrofaunal communities within and adjacent to a detritus-rich submarine canyon system

Abstract: Macrofaunal abundance, biomass, diversity and species assemblages within Scripps and La Jolla Submarine Canyons are compared with those on the nearby continental shelf and slope. Our primary objective was to examine the effect of detrital aggregates on infaunal communities within canyons. Two submarines, a remotely operated vehicle (ROV), and a Soutar box-corer were used to collect samples. Within the canyons, organic enrichment by macrophyte detritus was evident from canyon heads down to 550 m, and evidence of strong currents (coarse sediment) was found down to 700 m. Infaunal density and biomass were higher in the canyons than outside at all depths where comparative data were available (100–500 m). Infaunal assemblages in canyons were distinct from those at reference stations. Both the canyon and non-canyons samples showed community differentiation with depth. Species diversity was generally high, but decreased with depth outside of canyons and increased with depth within the canyons. Low diversity at shallow depths within the canyon is attributed to a combination of organic enrichment and physical disturbance. Submarine canyons are commonly found to contain distinct species assemblages or higher faunal densities and/or biomass than nearby non-canyon regions at similar depths. Canyons are regular features along most ocean margins and appear to be important as sites of enhanced secondary production, provide diverse habitats, and act as conduits of coastal detritus to the deep-sea.

Relationships between oxygen, organic matter and the diversity of bathyal macrofauna

Abstract: The relationships of environmental factors with measures of macrobenthic community diversity were examined for the total fauna, and for polychaetes only, from 40 bathyal stations in the North Atlantic, eastern Pacific and Indian Oceans (154–3400 m). Stepwise multiple regression revealed that depth, latitude, sediment organic-carbon content and bottom-water oxygen concentration are significant factors that together explained 52–87% of the variation in macrobenthic species richness (E[s100]), the Shannon–Wiener index (H′), dominance (D), and evenness (J′). Percent sand and percent clay were not significant factors. After removal of depth and latitudinal effects, oxygen and organic-carbon concentrations combined accounted for 47, 67, 52 and 32% of residual variation in macrobenthic E(s100), H′, D, and J′, respectively. Organic carbon exhibited a stronger relationship than oxygen to measures of community evenness, and appeared to have more explanatory power for polychaetes than total macrobenthos. When only stations with oxygen Examination of rarefaction curves for Indo-Pacific stations reveals that total macrobenthos, polychaetes, crustaceans and molluscs all exhibit reduced species richness within oxygen minimum zones (OMZs). However, representation under conditions of hypoxia varies among taxa, with polychaetes being most tolerant. Molluscs and crustaceans often (but not always) exhibit few individuals and species in OMZs, and sometimes disappear altogether, contributing to reduced macrobenthic diversity and elevated dominance in these settings. The linear negative relationship observed between bathyal species richness and sediment organic-carbon content (used here as a proxy for food availability) may represent the right side (more productive half) of the hump-shaped, diversity–productivity curve reported in other systems. These analyses suggest there are potentially strong influences of organic matter and oxygen on the diversity and composition of bathyal macrobenthos, especially in the Indo-Pacific Ocean.

Seasonal variation of hydrographic and nutrient fields during the US JGOFS Arabian Sea Process Study

Abstract: Between September 1994 and December 1995, the US JGOFS Arabian Sea Process Experiment collected extensive, high quality hydrographic data (temperature, salinity, dissolved oxygen and nutrients) during all seasons in the northern Arabian Sea. An analysis of this unique data suite suggests the presence of many features that are described in the canonical literature, but these new data provided the following insights. 1. Although the seasonal evolution of mixed-layer depths was in general agreement with previous descriptions, the deepest mixed-layer depths in our data occurred during the late NE Monsoon instead of the SW Monsoon. 2. The region exhibits considerable mesoscale variability resulting in extremely variable temperature-salinity (TS) distributions in the upper 1000 db. This mesoscale variability is readily observed in satellite imaging, in the high resolution data taken by a companion ONR funded project, and in underway ADCP data. 3. The densest water reaching the sea surface during coastal upwelling appeared to have maximum offshore depths of ∼150 m and σ θ ’s close to the core value (∼25) for the saline Arabian Sea Water (ASW), but salinities in these upwelling waters were relatively low. The densest water found at the sea surface during late NE Monsoon conditions has σ θ ’s>24.8 and relatively high salinities, suggesting that they are a source for the ASW salinity maximum. 4. Persian Gulf Water (PGW) with a core σ θ of 26.6 forms a widespread salinity maximum. Despite the considerable extent of this feature, Persian Gulf outflow water, with a salinity (4) of ∼39 at its source, can only be a minor contributor. Within the standard US JGOFS sampling grid, maximum salinities on this surface are ∼36.8 at stations near the Gulf, falling to values as low as ∼35.3 at the stations farthest removed from its influence. Even at our standard stations closest to the Gulf (N-1 and N-2), the high-salinity, low-nutrient Persian Gulf water has only a modest direct effect on nutrient concentrations. This PGW salinity maximum is associated with the suboxic portions of the Arabian Sea’s oxygen minimum zone. 5. The salinity maximum associated with Red Sea Water (RSW, core σ θ =27.2) in the JGOFS study region is clearly evident at the southermost sampling site at 10′N (S-15). Elsewhere, this signal is weak or absent and salinity on the 27.2 σ θ surface tends to increase towards the Persian Gulf, suggesting that the disappearance of this salinity maximum is due, at least in part, to the influence of the Persian Gulf outflow. 6. Inorganic nitrogen-to-phosphate ratios were lower (frequently much lower) than the standard Redfield ratio of 15/1–16/1 (by atoms) at all times and all depths suggesting that inorganic nitrogen was more important than phosphate as a limiting nutrient for phytoplankton growth, and that the effects of denitrification dominated the effects of nitrogen fixation. 7. The water upwelling off the Omani coast during the SW Monsoon has inorganic nitrogen to silicate ratios that were higher (∼2/1) than the ∼1/1 ratio often assumed as the ratio of uptake during diatom growth. 8. The temporal evolution of inorganic nitrogen-to-silicate ratios suggests major alteration by diatom uptake only during the late SW Monsoon cruise (TN050) in August–September 1995. 9. Widespread moderate surface layer nutrient concentrations occurred during the late NE Monsoon. 10. A zone of high offshore nutrient concentrations was encountered during the SW Monsoon, but instead of being associated with offshore upwelling it may represent offshore advection from the coastal upwelling zone, the influence of an eddy, or both. 11. Although our data do not contradict previous suggestions that the volume of subtoxic water may be reduced the SW Monsoon, they suggest a weaker re-oxygenation than indicated by some previous work. Similarly, they do not confirm results suggesting that secondary nitrite maxima may be common in waters with oxygen concentrations >5 μM.

Atmospheric forcing in the Arabian Sea during 1994–1995: observations and comparisons with climatology and models

Abstract: Accurate, year-long time series of winds, incoming shortwave and longwave radiation, air and sea temperatures, relative humidity, barometric pressure, and precipitation were collected from a surface mooring deployed off the coast of Oman along the climatological axis of the Findlater Jet from October 1994 to October 1995. Wind stress, heat flux, and freshwater flux were computed using bulk formulae. The Northeast Monsoon was characterized by steady but moderate winds, clear skies, relatively dry air, and two months, December and January, in which the ocean, on average, lost 45 W m-2 to the atmosphere. The Southwest Monsoon had strong winds, cloudy skies, and moist air. Because of reduced latent and longwave heat loss, it was accompanied by sustained oceanic heat gain, with the strongest monthly mean warming, 147 W m-2, in August. Large differences are found between the observations and older climatologies. Recent climatologies agree better with the observations. The means of the Southampton Oceanography Center climatology for 1980–1995 are close to the buoy monthly means. Monthly means from that climatology show that 1994–1995 was in general a typical year, with surface meteorology and air–sea fluxes within one standard deviation of the long term means. Concurrent data from the NCEP, ECMWF, and FNMOC show that the models provide realistic surface winds. FNMOC winds show that the timing and character of the onset of the Southwest Monsoon in 1995 differed from 1994 and 1996 when variability within one month is resolved. The models fail to replicate other observed surface meteorology and to produce realistic heat fluxes. Annual and monsoonal mean net heat fluxes from the models differed from those of the buoy by 50 to 80 W m-2. Because of these differences, some care is warranted in selecting and using air-sea flux fields in studies of the Arabian Sea.

Upper ocean export of particulate organic carbon in the Arabian Sea derived from thorium-234

Abstract: Thorium-234 is used in the Arabian Sea as a tracer of sinking particle fluxes. Samples were collected from January to August 1995 on four cruises during the Northeast Monsoon, the Spring Intermonsoon and the mid- and late-Southwest Monsoon periods. In this study, 234Th activity distributions are used to quantify the 234Th flux on sinking particles, and the measured ratio of particulate organic carbon (POC) to particulate 234Th is used to convert from 234Th to POC export at 100 m. The calculated POC fluxes range from 25 mmols C m-2 d-1, and strong seasonal and spatial gradients are observed. The single largest feature is a basinwide export maximum associated with the late-SW Monsoon cruise when POC export rates are 17–28% of the observed primary production rates along the southern sampling line. During all other cruises, this export ratio is

An integrated approch to the foraging ecology of marine birds and mammals

Abstract: Birds and mammals are important components of pelagic marine ecosystems, but our knowledge of their foraging ecology is limited. We distinguish six distinct types of data that can be used in various combinations to understand their foraging behavior and ecology. We describe methods that combine concurrent dive recorder deployment, oceanographic sampling, and hydroacoustic surveys to generate hypotheses about interactions between the physical environment and the distribution, abundance, and behavior of pelagic predators and their prey. Our approach is to (1) map the distribution of whales in relation to the distribution of their prey and the physical features of the study area (bottom topography, temperature, and salinity); and (2) measure the foraging behavior and diet of instrumented whales in the context of the fine-scale distribution and composition of their prey and the physical environment. We use this approach to demonstrate a relationship between blue whale distribution, sea surface temperature, and concentrations of their euphausiid prey at different spatial scales offshore of the Channel Islands, California. Blue whale horizontal spatial distribution was correlated with regions of high acoustic backscatter. Blue whale dive depths closely tracked the depth distribution of krill. Net sampling and whale diet revealed that whales fed exclusively upon dense schools of Euphausia pacifica (between 100 and 200 m) and Thysanoessa spinifera (from the surface to 100 m). Whales concentrated foraging efforts upon those dense euphausiid schools that form downstream from an upwelling center in close proximity to regions of steep topographic relief. We propose that (1) the distribution of Balaenoptera whales in the coastal California Current region is defined by their attraction to areas of predictably high prey density; (2) the preferred prey of these whales are several species of euphausiids ( E. pacifica, T. spinifera, and N. simplex ) that are abundant in the California Current region; (3) blue whales concentrate their foraging efforts on dense aggregations of euphausiids found at discrete depths in the water column; (4) these localized areas of high euphausiid densities are predictable and sustained by enhanced levels of primary productivity in regions which are located downstream from coastal upwelling centers (indicated by sea surface temperature); (5) topographic breaks in the continental shelf located downstream from these upwelling centers work in concert with euphausiid behavior to collect and maintain large concentrations of euphausiids swarms, and (6) despite seasonal and inter-annual variability, these processes are sufficiently consistent that the distribution of Balaenoptera whales can be predicted.

Blue whale habitat and prey in the California Channel Islands

Abstract: Whale Habitat and Prey Studies were conducted off southern California during August 1995 (WHAPS95) and July 1996 (WHAPS96) to (1) study the distribution and activities of blue whales and other large whales, (2) survey the distribution of prey organisms (krill), and (3) measure physical and biological habitat variables that influence the distribution of whales and prey. A total of 1307 cetacean sightings included 460 blue whale, 78 fin whale and 101 humpback whale sightings. Most blue whales were found in cold, well-mixed and productive water that had upwelled along the coast north of Point Conception and then advected south. They were aggregated in this water near San Miguel and Santa Rosa Islands, where they fed on dense, subsurface layers of euphausiids both on the shelf and extending off the shelf edge. Two species of euphausiids were consumed by blue whales, Thysanoessa spinifera and Euphausia pacifica , with evidence of preference for the former, a larger and more coastal species. These krill patches on the Channel Island feeding grounds are a resource exploited during summer–fall by the world’s largest stock of blue whales.

Spatial patterns in phytoplankton growth and microzooplankton grazing in the Arabian Sea during monsoon forcing

Abstract: Spatial patterns in the rates of phytoplankton growth and microzooplankton grazing were investigated in the Arabian Sea during the Southwest Monsoon (August–September) and early Northeast Monsoon (December) seasons in 1995 using the seawater dilution technique. Nutrient-enhanced growth rates ( μ n ) averaged 1.2 d -1 in the upper euphotic zone for both cruises and were similar between higher and lower nutrient stations, the former (>1.0 μM NO 3 ) being characteristic of the upwelling-influenced western coastal portion of the study region and the latter ( 3 ) being typical of the central basin. Growth rates without added nutrients ( μ 0 ) were also comparable between cruises but strongly related to ambient nutrient conditions, averaging 1.1 d -1 (91% of μ n ) at the higher nutrient stations and 0.5 d -1 (44% of μ n ) at the lower nutrient stations. The rate estimates for phytoplankton losses to microzooplankton grazing ( m ) averaged 0.6 d -1 for the upper euphotic zone and did not vary systematically between low- and high- nutrient stations. As a consequence, μ 0 and m were largely in balance for the more oligotrophic stations, while the eutrophic stations showed a growth differential over grazing of about 0.6 d -1 . These experimental results are consistent with observed differences in community structure, namely the dominance of picoplankton in oligotrophic offshore regions and the increased importance of the large diatom-mesozooplankton grazing pathway in the richer coastal areas. Overall, the spatial patterns, if not magnitudes, of the community responses to Southwest and Northeast Monsoon forcing were remarkably similar in this study, allowing for a relatively simple interpretation of the influence of enhanced nutrient supply on the rates and fates of phytoplankton production.

Effects of orientation on acoustic scattering from Antarctic krill at 120 kHz

Abstract: Backscattering measurements of 14 live individual Antarctic krill (Euphausia superba) were made at a frequency of 120 kHz in a chilled insulated tank at the Long Marine Laboratory in Santa Cruz, CA. Individual animals were suspended in front of the transducers, were only loosely constrained, had substantial freedom to move, and showed more or less random orientation. One thousand echoes were collected per animal. Orientation data were recorded on video. The acoustic data were analyzed and target strengths determined from each echo. A method was developed for estimating the three-dimensional orientation of the krill based on the video images and was applied to five of them, giving their target strengths as functions of orientation. Scattering models based on a simplified distorted-wave Born approximation (DWBA) method were developed for five animals and compared with the measurements. Both measured and modeled scattering patterns showed that 120 kHz acoustic scattering levels are highly dependent on animal orientation. Use of these scattering patterns with orientation data from shipboard studies of E. superba gave mean scattering levels approximately 12 dB lower than peak levels. These results underscore the need for better in situ behavioral data to properly interpret acoustic survey results. A generic E. superba DWBA scattering model is proposed that is scalable by animal length. With good orientation information, this model could significantly improve the precision and accuracy of krill acoustic surveys.

Response of microbial community structure to environmental forcing in the Arabian Sea

Abstract: The effect of environmental forcing on microbial community structure was investigated in the Arabian Sea during four seasonal cruises during 1995: late Northeast Monsoon (January); Spring Intermonsoon (March–April); late Southwest Monsoon (August–September); and early Northeast Monsoon (December). The distributions of picoplankton populations—heterotrophic bacteria (HBac), Prochlorococcus ( Pro ), Synechococcus spp. ( Syn ) and picoeucaryotic algae (Peuc)—were determined by flow cytometric analysis. Seasonal variations in abundance maxima, vertical profiles, integrated abundance (0–200 m), and estimated carbon biomass were contrasted along two transects from the coast of Oman to 1500 km offshore. HBac were numerically dominant in surface waters in all regions (1–3×10 6 cells ml -1 ), with higher maximum abundances in coastal waters than at offshore stations. Conversely, Pro were most abundant at the oligotrophic offshore stations, and 100-fold lower, or absent, at coastal stations, except during the Spring Intermonsoon when abundances were extremely high along the entire southern transect. Syn abundances were highly variable, with no consistent trend between coastal and offshore stations. This variability may be explained by the prevalence of mesoscale eddies, but also could be due to overlapping distributions of multiple Syn populations distinguished by pigment type. Syn with a low phycourobilin (PUB) to phycoerythrobilin (PEB) ratio pigment type were more abundant at coastal stations, whereas Syn with a high PUB : PEB ratio increased in abundance offshore. Average depth profiles for Pro, Syn , and HBac displayed uniform abundance in the surface mixed layer, with a rapid decrease below the surface mixed layer depth; however, during the Spring Intermonsoon most profiles had a peak at the base of the surface mixed layer. Distributions of Peuc typically displayed a subsurface maximum near the base of the surface mixed layer, except during the SW Monsoon when abundance peaked near the surface. This report is the first to describe the seasonal and spatial variation in microbial community structure in the Arabian Sea over a complete monsoon cycle. Overall, the eucaryotic component was more important at coastal stations, and the procaryotic components were predominant at offshore stations. Pro abundance was restricted to warm oligotrophic waters and was inversely related to surface nitrate concentrations; thus, an increase in the % Pro as a fraction of total procaryote abundance was also indicative of oligotrophic conditions. The effects of SW Monsoonal forcing on microbial community structure resulted in an increase of Peuc, but this response was limited to coastal stations. Pro and Syn remained dominant at offshore stations.

Biology and ecology of the “Pompeii worm” (Alvinella pompejana Desbruyères and Laubier), a normal dweller of an extreme deep-sea environment: A synthesis of current knowledge and recent developments

Abstract: Alvinella pompejana, the “Pompeii worm” lives on active hydrothermal edifices at deep-sea vents of the East Pacific Rise. The physical and chemical patterns of its microhabitat were determined from temperature probe measurements, temperature time series, and on-board and shore-based chemical analyses based on discrete sampling (pH, H2S, CO2, CH4, S2O2-3, Ca, Mg, Cu, Cd, Zn). The microhabitat is characterised by high temporal and microscale spatial variability, with temperature values in the range of 20°–45°C at the immediate periphery of tubes but reaching higher, still undetermined, values inside the tubes. The difference observed between in vitro temperature limits for the stability of biomolecules and metabolic rates, and suggested in situ conditions seems to indicate a significant protective role of biological interfaces (tubes and cuticle). Temporal instability possibly also plays an important role in the ability for these worms to colonise such an extreme habitat. The functional role of dominant epibiotic bacteria is discussed in the light of recent biochemical and molecular data: the tube-worm–bacteria system can be considered as a symbiotic entity where carbon is probably metabolised and recycled. Sulphide detoxification occurs by oxidation at the gill level and possibly at the intracellular haemoglobin level. Heavy metals, ingested or absorbed, are trapped in spherocrystals and bound to metallothionein-like proteins. Anatomical, physiological and molecular adaptations to hypoxia allow the worm to successfully colonise the chimneys. A. pompejana lives in an ephemeral environment and must reproduce and disperse accordingly. It is a gonochoric species that displays a pseucopulatory behaviour allowing transfer of sperm to female spermathecae, thus avoiding dispersion of the gametes. The size of the oocytes suggests a lecithotrophic or benthic development. The population size structure is polymodal, indicating discontinuous recruitment. Population genetics data indicate the occurrence of a microscale level of population differentiation that does not increase with increasing geographical distances, thus suggesting the occurrence of a metapopulation-like system and/or the possibility that enzyme loci evolve under stabilising selective driving forces inherent to the vents’ highly variable conditions.

A comparison of phytoplankton populations of the Arabian Sea during the Spring Intermonsoon and Southwest Monsoon of 1995 as described by HPLC-analyzed pigments

Abstract: We used diagnostic pigments to estimate the relative abundances of different algal groups in the Arabian Sea during the Spring Intermonsoon (cruise TN045) and late Southwest Monsoon (cruise TN050) of 1995. Northern and southern transect lines were occupied during each cruise. These transects started near the coast of the Arabian Peninsula at 22.38°N and 18.50°N, respectively, and extended >1200 km offshore. The pigment concentrations at the offshore stations (>1000 km from the coast) were low during both cruises, and the composition of the phytoplankton resembled that found in oligotrophic, open-ocean waters. The pigment compositions at the inshore stations (<1000 km from the coast) were markedly different during both cruises. During TN045, the inshore waters of the northern transect were dominated by a mixture of diatoms and haptophytes. Large particles (>18 μm) accounted for up to 23% of the total chlorophyll a-related pigments (TCHLA, chlorophyllide a+monovinyl chlorophyll a+divinyl chlorophyll a). Pigment biomass (=500 ng TCHLA L-1) was homogeneously distributed in the upper water column at these stations. By contrast, pigment biomass at the inshore stations of the southern transect was concentrated in a pronounced deep chlorophyll maximum. The phytoplankton were clearly partitioned in the water column, with photosynthetic prokaryotes and eukaryotes dominating in the upper mixed layer and below, respectively. The high levels of divinyl chlorophyll a observed (up to 300 ng l-1) represent the highest concentrations ever recorded for this prochlorophyte marker. The size-fractionated pigment analyses revealed that 85% of the TCHLA passed through 2 μm filters, indicating dominance by the picophytoplankton. During TN050, the inshore portion of the northern transect displayed a clear example of a diatom-to-Phaeocystis successional event. Diatoms dominated at coastal upwelling stations and were progressively replaced by Phaeocystis at offshore stations. At the inshore stations of the southern transect, diatoms dominated the phytoplankton community, and a large fraction of the TCHLA (21–48%) was retained by 18 μm filters. A filament with high diatom biomass was observed 700 km offshore, documenting that horizontal advection is an important mechanism for removing phytoplankton from coastal waters in this region. However, lower than expected TCHLA concentrations were measured during this period relative to those observed in earlier years in the CZCS imagery. Several hypotheses are discussed to explain this enigma, including sampling biases, losses by grazing and sinking, phytoplankton dilution via upwelling, and Si limitation of diatom growth.

Particulate organic carbon fluxes: compilation of results from the 1995 US JGOFS Arabian Sea Process Study: By the Arabian Sea Carbon Flux Group

Abstract: Organic carbon fluxes in the Arabian Sea were measured as a function of depth, season and distance from the coast of Oman. We present here a compilation of measurements of primary production, water-column export flux and sediment accumulation of organic carbon over a full annual monsoon cycle on a 1500-km transect from the coast of Oman toward the central Arabian Sea. This represents an integration of measurements spanning one day (primary production) to 1000 yr (sediments) and gives a broad overview of organic carbon removal and remineralization in the highly productive, seasonally varying region of the northern Indian Ocean. Organic carbon fluxes decreased from the surface to the sediments by a factor of 500–10,000, with the largest rates of change in the upper ocean and at the sediment–seawater interface. Organic carbon fluxes generally decreased with distance offshore, with the largest gradient between surface and seafloor being at the offshore station. Sediment accumulation rates of organic carbon differed by a factor of 40 between nearshore and offshore, while primary productivity varied only by a factor of 2. The decrease in carbon flux with depth that occurs between the deepest traps and the sediment becomes a greater proportion of the total loss with increasing distance from shore. Thus, the influence of processes at the sediment–water interface on the proportion of primary productivity preserved in the sediment increases offshore relative to upper water column processes. Carbon fluxes changed greatly with season, with highest fluxes during the Southwest Monsoon. Export fluxes varied more with season than primary productivity or mid-water fluxes.

Seasonal response of zooplankton to monsoonal reversals in the Arabian Sea

Abstract: The US JGOFS Arabian Sea Process Study was designed to provide a seasonally and spatially resolved carbon budget for a basin exhibiting some of the highest and lowest concentrations of plant biomass in the world’s ocean. During the US JGOFS Process Study in the Arabian Sea (September 1994–January 1996), the absolute maximum in biomass of epipelagic zooplankton in the entire study was observed during the Southwest Monsoon season inshore of the Findlater Jet in the area of upwelling. The greatest contrast between high and low biomass in the study area also was observed during the Southwest Monsoon, as was the strongest onshore–offshore gradient in biomass. Lowest biomass throughout the study was observed at the most offshore station (S15), outside the direct influence of the monsoon forcing. The greatest day/night contrasts in biomass were observed nearshore in all seasons, with nighttime biomass exceeding daytime in the Northeast Monsoon season, but daytime exceeding nighttime in the Southwest Monsoon season. The diel vertical migration patterns in general reversed between the monsoons at all stations in the southern part of the study area. Virtually, no diel vertical migration of zooplankton took place in any season at the station with strong, persistent subsurface suboxic conditions (N7), suggesting that these conditions suppress migration. Based on the distribution of biomass, we hypothesize that inshore of the Findlater Jet, zooplankton grazing on phytoplankton is the dominant pathway of carbon transformation during both monsoon seasons, whereas offshore the zooplankton feed primarily on microplankton or are carnivorous, conditions that result in reduction of the carbon flux mediated by the zooplankton. Predation by mesopelagic fish, primarily myctophids, may equal daily growth of zooplankton inshore of the Findlater Jet during all seasons. This suggests that the food web inshore of the Findlater Jet is well integrated, may have evolved during past periods of intensified upwelling, and has a distinctly annual cycle.

Seasonal variability of bio-optical and physical properties in the Arabian Sea: October 1994-October 1995

Abstract: A mooring instrumented with optical and physical sensors within the upper 300 m was deployed for two consecutive 6-month periods (October 15, 1994 through October 20, 1995; sampling intervals of a few minutes) in the central Arabian Sea (15°30′N, 61°30′E). Both the Northeast Monsoon (November 1, 1994–February 15, 1995) and Southwest Monsoon (June 1– September 15, 1995) were observed. During the NE Monsoon, wind speeds averaged 6 m s-1 and reached up to 15 m s-1 during the SW Monsoon. Intermonsoon periods (Spring February 16–May 31, 1995; and Fall September 16–October 15, 1995) were characterized by weak and variable winds. Shortwave radiation and photosynthetically available radiation (PAR) displayed half-yearly cycles, peaking during the Intermonsoon periods. Two mixed-layer depth definitions have been used to describe our results. The first is based on a temperature difference of 0.1°C of the surface temperature, MLD0.1°C, and the second is based on a difference of 1.0°C, MLD1.0°C. The maximum Winter mixed layer depth (MLD1.0°C∼110 m) was deeper than the Summer mixed layer (MLD1.0°C∼80 m), primarily because of surface cooling and convection. A half-yearly cycle in chlorophyll a was evident with greater values occurring during each Monsoon and into the Intermonsoon periods. High chlorophyll a values associated with cool mesoscale features were also apparent during each Monsoon. These mesoscale features and others have been identified using remotely sensed sea-surface height anomaly maps. Time-series of the 1% light level depth, h1%, tracked the depth-integrated chlorophyll a. In general, h1% was deeper than MLD1.0°C during the latter half of the Spring Intermonsoon (SIM) (with low chlorophyll a periods) and shallower than MLD1.0°C during the latter portions of the Monsoons (high chlorophyll a periods). During the SIM, the penetrative components of net solar radiation at the base of the mixed layers, En(MLD1.0°C) and En(MLD0.1°C), reached values of ∼40 and 75 W m-2, respectively, when the net surface heat flux was 120 W m-2. The highest mixed layer radiant heating rates occurred during the Intermonsoon periods with peak values greater than 0.2 and 0.1°C d-1 for MLD0.1°C and MLD1.0°C, respectively. Our results indicate that biological variability is significant for the upper ocean heat budget of the central Arabian Sea during the SIM. The present results, in conjunction with those presented in Marra et al. (1998) and Honjo et al. (1998), demonstrate strong coupling of upper ocean biological processes with deep ocean particulate organic carbon fluxes. These collective results suggest that the timing and amplitudes of phytoplankton blooms associated with both seasonal stratification and eddies are quite well-correlated with relatively rapid export flux of organic carbon to the deep ocean.

Particle fluxes in a coastal upwelling zone: sediment trap results from Santa Barbara Basin, California

Abstract: Particle fluxes were measured in the center of Santa Barbara Basin on a bi-weekly basis for a three-year period beginning in August 1993. Lithogenic material dominates the total flux throughout the entire year, although it is delivered to the basin primarily during the winter rainy period. It appears that both biological and physical processes control the eventual transport of detrital material to the deep part of the basin. Biogenic sedimentation in Santa Barbara Basin is dominated by opaline silica, with highest fluxes during the spring–summer upwelling period. Export ratios (ratios of organic carbon flux to primary production) are inversely related to primary production. This relationship may be due to increased advective transport and/or enhanced regeneration of organic carbon during the highly productive upwelling period. During the first half of the study period, Santa Barbara Basin was influenced by El Nino conditions and our data suggest that productivity in this region is reduced during such periods. Seasonal changes in the relative contributions of biogenic and lithogenic material to the total particle flux, combined with the lack of bioturbation on the sea floor, results in the accumulation of varved sediments in Santa Barbara Basin.

Mesozooplankton biomass in the upper 1000 m in the Arabian Sea: overall seasonal and geographic patterns, and relationship to oxygen gradients

Abstract: Mesozooplankton biomass distributions in the upper 1000 m of the Arabian Sea were studied as part of the US JGOFS Arabian Sea Process Study Samples were collected in vertically stratified MOCNESS tows during four seasons in 1995 at six stations spanning the Arabian Sea This paper describes results from the size-fractionated dry weight and wet weight biomass profiles and their relationship to the seasonal monsoon cycle and the pronounced oxygen minimum zone (OMZ) The total mesozooplankton biomass and most size fractions exhibited a significant onshore/offshore biomass gradient during most seasons and at most depths The gradient was strongest in August during the late Southwest Monsoon and weakest in March during the Spring Intermonsoon, when maxima of some size-fractions occurred offshore Seasonal changes in zooplankton biomass were small The late Southwest Monsoon was the time of higher biomasses for nearshore stations, while the Spring Intermonsoon was the time of higher biomasses for offshore stations Most of the zooplankton biomass present in the upper 1000 m of the water column occurred in the upper 200 m, but there was substantial diel vertical migration, especially in the large size class, down to 300–400 m, well within the suboxic water of the OMZ Subsurface biomass increased in the lower OMZ within the depth range of the 005–01 ml/l oxygen gradient (about 500–600 m) The enhanced zooplankton biomass, higher potential food levels, and possible short food chains suggested active biological modification of the sinking flux in this depth zone Biomass depth profiles showed close relationships to oxygen profiles The shapes of the zooplankton biomass and oxygen profiles varied with geographic location across the Arabian Sea but were remarkably consistent over the year at each station Geographic differences in the shapes of the profiles were dependent on the vertical extent of the suboxic zone and may correspond to broad regional differences in the processing of the export flux in midwater However, the consistency of the shapes over time implies long-term stability in the structure and function of the midwater ecosystem at each location

Sediment community response to a temporally varying food supply at an abyssal station in the NE pacific

Abstract: Long time-series measurements were made to examine the effect of episodic inputs of organic matter to the benthic boundary layer on the sediment community at a site in the eastern North Pacific (Sta. M). Chlorophyll a and pheopigments were used to assess sediment enrichment, and the response of the sediment community was examined with seasonal measurements of sediment community oxygen consumption (SCOC) and sediment ATP from February 1992 to July 1996. In addition, macrofaunal density and biomass were examined in 26 sediment grab samples taken over a 2-year period. In order to determine whether the presence or absence of detrital aggregates correlated with macrofaunal density and biomass, 54 tube core samples were collected with the submersible Alvin at times when detrital aggregates were present on (August/September of 1994) and absent (April 1995) from the sea floor. Large, seasonal inputs of particulate organic carbon (POC) generally resulted in sediment enrichment and significantly correlated with increases in SCOC. Summer maxima in SCOC occurred consistently over our study period, indicating that this trend is a regular seasonal occurrence at Sta. M. Protozoans, primarily agglutinating foraminiferans, and five of the six dominant metazoan taxa exhibited seasonal increases in density during winter months after detrital aggregates had disappeared from the sea floor and approximately eight months after peak SCOC and POC measurements, over a 2-year period. Alvin tube core samples showed that relative protozoan density and biomass increased significantly over a 4-week period following an input of phytodetritus, indicating that these organisms may respond to organic matter inputs on time scales as short as weeks.

Prochlorococcus and Synechococcus growth rates and contributions to production in the Arabian Sea during the 1995 Southwest and Northeast Monsoons

Abstract: We investigated the growth rates of Prochlorococcus and Synechococcus spp. and their relative contributions to carbon production at five stations in the Arabian Sea during the late Southwest and early Northeast Monsoon seasons in 1995. Estimates of Prochlorococcus growth rates were based on diel cell cycle analysis. Accurate determination of the duration of the cell cycle terminal event, e.g., t S+G2 was not possible for Synechococcus because of its highly variable cell cycle patterns (e.g., imperfect phasing, multiple DNA-replication peaks, and dark-arrested division). Consequently, growth and mortality rates of Synechococcus were estimated from diel variations in population abundance. The assumptions of this approach were validated by observations that Synechococcus cell division occurred only during the daytime as well as good agreement between growth rate estimates for Prochlorococcus compared to cell cycle analyses. Prochlorococcus growth rates were typically less than 1 doubling per day, although higher rates occurred in the surface waters at an offshore site (Stn. N7) during the SW Monsoon and at a coastal station (S2) during the NE Monsoon. For Synechococcus , maximum growth rates >2 d -1 were observed at mesotrophic (nitrate concentration 0.1–3 μm) onshore stations during both seasons. Synechococcus spp. grew much faster than Prochlorococcus in the upper water column at almost every station during both seasons, but the depth range of its maximum growth rate was shallower and its growth and abundance decreased sharply in deeper waters. In addition, growth rates of Synechococcus increased with nutrient availability whereas Prochlorococcus growth rates did not vary dramatically with nutrients. Although there was no significant difference in Synechococcus growth rates between the late SW and early NE Monsoon seasons, the estimated carbon production and relative contribution to primary production were greater during the early NE Monsoon due to the larger biomass of Synechococcus and lower total primary production. Maximum Prochlorococcus production was found only in the most oligotrophic regions, and Prochlorococcus was not a major contributor of primary production for the most part of the Arabian Sea during the SW and NE Monsoons. Overall, Prochlorococcus and Synechococcus were inversely related in terms of their relative contributions to primary production.

Diversity of deep-sea hydrothermal vent Archaea from Loihi Seamount, Hawaii

Abstract: Through an examination of SSU rDNA (genes coding for SSU rRNA), the molecular phylogeny of the domain Archaea (e.g. one of the three major lineages of life) was analyzed from a microbial mat at an active, deep-sea hydrothermal vent ecosystem located at Pele’s Vents on the summit of Loihi Seamount, Hawaii. These SSU rDNAs were amplified from extracted microbial mat genomic DNA by PCR, cloned into a plasmid vector, and sequenced. The derived archaeal sequences were then used to infer the evolutionary relationships between these microbial mat community members and their closest known relatives. Of the four clones initially chosen for sequence analysis, a cluster of three phylogenetically similar PVA (Pele’s Vents Archaea) clones all contained in the archaeal group I lineage of the marine Crenarchaeota were detected. A single PVA clone was contained in the archaeal group II lineage of the marine Euryarchaeota. All four of the PVA clones are novel and constitute the discovery of new archaeal taxa. From further rarefaction results of 75 archaeal SSU rDNA clones, we estimate the organismal diversity of this domain from the microbial mats located at Pele’s Vents to be significantly greater than that of the bacterial domain from this same ecosystem. Analyses of archaeal diversity at both the organismal (i.e. rarefaction) and phylogenetic level suggest that hydrothermal vents, such as Pele’s Vents, are intimately linked with marine archaeoplankton (a recently discovered component of marine picoplankton) detected from oceans around the world.

Spatial and temporal variations of total organic carbon in the Arabian Sea

Abstract: Concentrations of total organic carbon (TOC) were determined on samples collected during six cruises in the northern Arabian Sea during the 1995 US JGOFS Arabian Sea Process Study. Total organic carbon concentrations and integrated stocks in the upper ocean varied both spatially and seasonally. Highest mixed-layer TOC concentrations (80–100 μM C) were observed near the coast when upwelling was not active, while upwelling tended to reduce local concentrations. In the open ocean, highest mixed-layer TOC concentrations (80–95 μM C) developed in winter (period of the NE Monsoon) and remained through mid summer (early to mid-SW Monsoon). Lowest open ocean mixed-layer concentrations (65–75 μM C) occurred late in the summer (late SW Monsoon) and during the Fall Intermonsoon period. The changes in TOC concentrations resulted in seasonal variations in mean TOC stocks (upper 150 m) of 1.5–2 mole C m-2, with the lowest stocks found late in the summer during the SW Monsoon–Fall Intermonsoon transition. The seasonal accumulation of TOC north of 15°N was 31–41×1012 g C, mostly taking place over the period of the NE Monsoon, and equivalent to 6–8% of annual primary production estimated for that region in the mid-1970s. A net TOC production rate of 12 mmole C m-2 d-1 over the period of the NE Monsoon represented ≈80% of net community production. Net TOC production was nil during the SW Monsoon, so vertical export would have dominated the export terms over that period. Total organic carbon concentrations varied in vertical profiles with the vertical layering of the water masses, with the Persian Gulf Water TOC concentrations showing a clear signal. Deep water (>2000 m) TOC concentrations were uniform across the basin and over the period of the cruises, averaging 42.3±1.4 μM C.

Bathymetric patterns of body size: implications for deep-sea biodiversity

Abstract: The evolution of body size is a problem of fundamental interest, and one that has an important bearing on community structure and conservation of biodiversity. The most obvious and pervasive characteristic of the deep-sea benthos is the small size of most species. The numerous attempts to document and explain geographic patterns of body size in the deep-sea benthos have focused on variation among species or whole faunal components, and have led to conflicting and contradictory results. It is important to recognize that studying size as an adaptation to the deep-sea environment should include analyses within species using measures of size that are standardized to common growth stages. An analysis within eight species of deep-sea benthic gastropods presented here reveals a clear trend for size to increase with depth in both larval and adult shells. An ANCOVA with multiple comparison tests showed that, in general, size–depth relationships for both adult and larval shells are more pronounced in the bathyal region than in the abyss. This result reinforces the notion that steepness of the bathymetric selective gradient decreases with depth, and that the bathyal region is an evolutionary hotspot that promotes diversification. Bathymetric size clines in gastropods support neither the predictions of optimality models nor earlier arguments based on tradeoffs among scaling factors. As in other environments, body size is inversely related to both abundance and species density. We suggest that the decrease in nutrient input with depth may select for larger size because of its metabolic or competitive advantages, and that larger size plays a role in limiting diversity. Adaptation is an important evolutionary driving force of biological diversity, and geographic patterns of body size could help unify ecological and historical theories of deep-sea biodiversity.

Deep-sea benthic foraminiferal species diversity in the NE Atlantic and NW Arabian sea: a synthesis

Abstract: We present a synthesis of species diversity data (Fishers’ alpha index, Shannon–Wiener (log 2 ), ES (100), Rank 1 Dominance) for “live” (stained) foraminifera from five bathyal (1340 m depth) and abyssal (4450–4950 m depth) sites in the NE Atlantic and a 3400 m-deep site in the Arabian Sea. Three Atlantic sites (Porcupine Seabight, BIOTRANS, Porcupine Abyssal Plain) are subject to seasonal phytodetritus inputs that support low diversity populations (8–17 species). In other respects the foraminifera are highly diverse. The meiofaunal fractions (>45 or >63 μm; including fragmented and phytodetritus species) of abyssal Atlantic samples yielded >110 and >170 species in the 0–1 cm and 0–10 cm layers, respectively; the Arabian Sea sample (0–1 cm layer only) yielded 232 species. In both cases, values for diversity measures were very high. Diversity was rather lower in bathyal Porcupine Seabight samples (0–1 cm layer), which yielded 500 μm; Porcupine and Madeira Abyssal Plains) was also speciose (113–133 species), but diversity measures were lower and dominance higher than for the meiofauna. All assemblages contained numerous undescribed species, many belonging to poorly known monothalamous, soft-bodied taxa. Sample diversity was influenced by several factors. Combining phytodetrital and sediment populations reduced diversity and increased dominance slightly; the inclusion of deeper sediment layers and finer sieve fractions had the opposite effect. The inclusion of fragments had more impact on macrofaunal than on meiofaunal diversity, although in both cases the effect was inconsistent (either positive or negative). Porcupine and Madeira Abyssal Plain multicore samples (>63 μm fraction) contained substantially more foraminiferal species than nematode species; the numbers of foraminiferal species in boxcore samples (>500 μm fraction) were comparable to, or greater than, literature values for macrofaunal taxa such as polychaetes and isopods. Few of the more abundant species at our Atlantic abyssal plain and Arabian Sea sites are endemic. This is consistent with literature evidence that many common deep-sea foraminiferal species are cosmopolitan and implies that global foraminiferal diversity may be more modest than the high sample diversity might suggest. Calcareous foraminifera, which are well-known taxonomically and have a good fossil record, may provide a model for diversity patterns among the deep-sea benthic biota in general.

A programmable acoustic recording tag and first results from free-ranging northern elephant seals

Abstract: A hydrophone-equipped tag recorded exposure to noise, as well as physiological and behavioral sounds, on free-ranging northern elephant seals (Mirounga angustirostris). The compact acoustic probe (CAP) consisted of the hydrophone, a thermistor, and a pressure transducer in a 36 cm long, 10 cm diameter cylindrical hydrodynamic housing capable of withstanding 2000 m depth. The enclosed logging electronics included a programmable “TattleTale 7” data acquisition engine and a 340 Mb hard disk. A custom low-power operating system supported multi-channel interrupt-driven sampling at 5 kHz. The complete tag weighed 0.9 kg in water and displaced 2.1 l. During five deployments on juveniles translocated from and returning to Ano Nuevo, California, CAP tags measured dive pattern, ambient and vessel noise exposure, oceanographic ranging (RAFOS) and thermometry (ATOC) beacons, acoustic signatures of swim stroke, surface respiration, and cardiac function, and possible vocalizations.

Spatial and temporal dynamics in marine aggregate abundance, sinking rate and flux: Monterey Bay, central California

Abstract: Seasonal profiles of marine aggregate abundance and in situ sinking rate experiments were carried out using a uniquely instrumented ROV platform in the midwater column of Monterey Bay California between 1991 and 1994. Variations of an order of magnitude in the midwater 100–500 m abundance of aggregates within the 0.5 to >5 mm size range were observed on an inter- and intra-annual basis at the study site. Maximum midwater aggregate abundances of 15–40 aggregates l -1 were common during the 1991 spring/summer upwelling season, with values of less than 5 aggregates l -1 being more typical of the non-upwelling winter months and the 1992 El Nino period. Midwater aggregate peaks represented the temporal signal of sinking aggregate material produced in the overlying waters, with no correlation observed between the vertical distribution of aggregates and the density structure, or the relative suspended particle abundence measured as c p . Additionally, significant injection of aggregate material to the midwater profiles via lateral advection was not evidenced by the combined aggregate, hydrographic, and physical flow data sets obtained simultaneously at the study site. In situ aggregate sinking rates were measured using an ROV-mounted settling chamber. Mean rates ranged from 16.3 to 25.5 m day -1 with a trend of increasing sinking rate with aggregate diameter observed. An analysis of aggregate shape showed a decrease in spherical shape with increasing aggregate diameter and sinking rate. Seasonal aggregate POC fluxes at 450 m for 1991–1992 were calculated from the in situ aggregate property data sets to compare with coincident, 450 m trap-measured POC fluxes. The calculated aggregate POC fluxes ranged from 26.3 to 481.2 mgC m -2 d -1 and were at least 1.5–2 times greater than the trap POC fluxes, with the exception of the non-upwelling winter months where values are similar. Trapping efficiency and interannual variations in aggregate sinking rates are believed to explain the differences between the aggregate and trap-based POC fluxes.

Upper ocean currents in the northern Arabian Sea from shipboard ADCP measurements collected during the 1994–1996 U.S. JGOFS and ONR programs

Abstract: A large upper-ocean velocity data set was obtained using a shipboard acoustic Doppler current profiler (ADCP) on seventeen RV T.G. Thompson cruises during the JGOFS and ONR expedition to the northern Arabian Sea from September 1994 through January 1996. Seven of the cruises followed a large area survey track centered over the Arabian Basin, four cruises conducted SeaSoar surveys on either side of the Findlater jet axis, and six cruises were for the deployment and maintenance of moored instrumentation, together providing some 380 cruise-days and 96,000 track kilometers of coverage. The ADCP data extended over the upper 250–400 m of the water column depending upon the temporal/spatial distributions of acoustic scatterers. The velocity data revealed several items that differed significantly from the historical perspective. Maximum current magnitudes in this area varied from more than 1 m s -1 along the Arabian coast to 10–20 cm s -1 well offshore. Perhaps the most important result was the complete dominance of the velocity field by eddies that had offshore correlation length scales of roughly 100 km, a spectral peak at around 300 km, and kinetic energies that ranged from 70 to more than 90% of the total kinetic energy. The total kinetic energy was highest within about 300 km of the shore and decreased significantly in magnitude and vertical extent offshore. Within the coastal region, the temporal variability was such that currents of 50 cm s -1 or more could completely reverse within a two-week period, seemingly independent of any local atmospheric forcing. Mean and seasonal velocities also differed from historical results. There was a large anti-cyclonic feature located for most of the year south of Ras ash Shabatat (∼58°E) which intensified during the southwest monsoon. There was a strong jet-like current off Ras al Hadd (∼22°N), which also intensified during the southwest monsoon, although itflowed southwestward against the wind. In contrast to the historical ship drift data, which indicated that the surface currents followed the monsoonal winds, the ADCP data over the upper 200–400 m was highly variable, with overall seasonal means that were often directed against the wind. Except within 100–200 km of the Arabian coast, the mean currents were generally toward the southwest with a mean transport in the upper 150 m between the coast and 10°N of approximately 9×10 6 m 3 s -1 .