Showing papers on "Water column published in 2002"

Gulf of Mexico Hypoxia, A.K.A. “The Dead Zone”

Abstract: The second largest zone of coastal hypoxia (oxygen-depleted waters) in the world is found on the northern Gulf of Mexico continental shelf adjacent to the outflows of the Mississippi and Atchafalaya Rivers. The combination of high freshwater discharge, wind mixing, regional circulation, and summer warming controls the strength of stratification that goes through a well-defined seasonal cycle. The physical structure of the water column and high nutrient loads that enhance primary production lead to an annual formation of the hypoxic water mass that is dominant from spring through late summer. Paleoindicators in dated sediment cores indicate that hypoxic conditions likely began to appear around the turn of the last century and became more severe since the 1950s as the nitrate flux from the Mississippi River to the Gulf of Mexico tripled. Whereas increased nutrients enhance the production of some organisms, others are eliminated from water masses (they either emigrate from the area or die) where the...

Iron-sulfur-phosphorus cycling in the sediments of a shallow coastal bay: Implications for sediment nutrient release and benthic macroalgal blooms

Abstract: We conducted a study to determine the seasonal relationship between iron, sulfur, and phosphorus in the upper sediments and pore waters of a shallow intercoastal bay. From April 1999 to September 2000, sediment cores were collected from Rehoboth Bay, Delaware. Analyses of the sediments in the upper 4 cm revealed that redox conditions controlled Fe-S-P concentrations in the sediments, pore waters, and overlying water. Monthly sampling showed a marked decrease in the reactive solid phase P pool (ascorbate leachable fraction, ASC-P) and sharp increases in soluble P (measured as PO ) in pore waters and overlying waters, as the conditions became more reducing through32 4 out the summer months. These changes were paralleled by decreases in the amorphous Fe(III) (ascorbate leachable fraction, ASC-Fe) and total Fe(III)oxyhydroxide pools [dithionite extracted fraction, Fe(III) oxide] and increases in solid FeS/FeS2. The release of soluble P from sulfidic sediments to oxygenated overlying waters only occurred during periods of solid FeS/FeS2 production, which indicates that Fe(III) oxides act as a barrier to diffusive P flux. During these anoxic conditions, the regenerative P appears to induce secondary benthic algal blooms and promotes eutrophication in these inland bays through late summer. By the late fall and into early spring, sulfide production diminished and oxic conditions were reestablished as indicated by increases in solid amorphous and crystalline Fe(III) oxides and decreases in FeS/FeS2 concentrations. During this period, increasing ASC-Fe concentrations correlated with increases in ASC-P concentrations and decreases in pore-water PO . The seasonal correlations 32 4 between Fe-S-P indicate that Fe redox chemistry controls sediment P flux to the overlying water column.

Nitrogen Fixation and Nitrogen Isotope Abundances in Zooplankton of the Oligotrophic North Atlantic

Abstract: Deep-water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean. 15N:14N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near-surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15N content relative to average deep-water nitrate. Herein we show that the low dΔ15N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N2 fixation. These results provide direct, integrative evidence that N2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.

Convection and restratification in the Labrador Sea, 1990-2000

Abstract: Temperature, salinity and other property distributions observed across the central Labrador Sea in the early summers between 1990 and 2000 reveal a 4-year period of exceptionally intense convection followed by 5 years of restratification. The intense convection led, in the centre of the Sea, to mixed layers increasing in density and depth to a maximum of 2300 m thereby creating a fresh deep pool of Labrador Sea Water (LSW). In the second half of the decade, warmer winter weather limited the depth of convection to ∼1000 m. The shallower convection isolated the deep reservoir of homogeneous LSW between 1000 and 2000 m from renewal: this reservoir slowly diminished in volume as the layer became more stratified. In addition, the mean temperature and salinity of the 1000–2000 m layer increased by 0.4°C and 0.025 as warmer more saline water was mixed into the central region from the boundaries. In the upper layer between 150 and 1000 m the restratification processes led to an increase in temperature of 0.6°C but no significant change in salinity. The upper 150 m also showed no discernible trends in salinity but did participate in the warming trend. Interannual variability in local atmospheric forcing accounts for much of the observed change in heat content in the convectively overturned part of the water column during both the convection and restratification phases. It is proposed that constant horizontal fluxes transport heat and salt from the boundaries into the centre of the Sea. When the heat loss from the sea surface is greater than the horizontal flux the mixed layer becomes colder and denser and the depth of convection increases. When the heat loss is less than the horizontal flux and the convection remains shallow the temperature rises in both the 0–1000 m and the 1000–2000 m layers and salinity increases in the deeper layer. In both situations salinity in the upper 1000 m remains roughly constant as the horizontal salinity flux approximately offsets the annual input of fresh water of 60±10 cm into the surface layer.

Nutrient-enhanced productivity in the northern Gulf of Mexico: past, present and future

Abstract: Nutrient over-enrichment in many areas around the world is having pervasive ecological effects on coastal ecosystems. These effects include reduced dissolved oxygen in aquatic systems and subsequent impacts on living resources. The largest zone of oxygen-depleted coastal waters in the United States, and the entire western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (<2 mg 1−1 02) in 1985–1992 averaged 8000 to 9000 km2 but increased to up to 16000 to 20 700 km2 in 1993–2001. The Mississippi River system is the dominant source of fresh water and nutrients to the northern Gulf of Mexico. Mississippi River nutrient concentrations and loading to the adjacent continental shelf have changed in the last half of the 20th century. The average annual nitrate concentration doubled, and the mean silicate concentration was reduced by 50%. There is no doubt that the average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring. There is considerable evidence that nutrient-enhanced primary production in the northern Gulf of Mexico is causally related to the oxygen depletion in the lower water column. Evidence from long-term data sets and the sedimentary record demonstrate that historic increases in riverine dissolved inorganic nitrogen concentration and loads over the last 50 years are highly correlated with indicators of increased productivity in the overlying water column, i.e. eutrophication of the continental shelf waters, and subsequent worsening of oxygen stress in the bottom waters. Evidence associates increased coastal ocean productivity and worsening oxygen depletion with changes in landscape use and nutrient management that resulted in nutrient enrichment of receiving waters. A steady-state model, calibrated to different observed summer conditions, was used to assess the response of the system to reductions in nutrient inputs. A reduction in surface layer chlorophyll and an increase in lower layer dissolved oxygen resulted from a reduction of either nitrogen or phosphorus loading, with the response being greater for nitrogen reductions.

Phytoplankton, light, and nutrients in a gradient of mixing depths: field experiments

Abstract: We studied the effects of water column mixing depth and background turbidity on phytoplankton biomass, light climate, and nutrients in two field enclosure experiments designed to test predictions of a dynamical model. In 1997 and 1998, we created gradients of mixing depth by enclosing the 100-μm-filtered phytoplankton community of a phosphorus-deficient lake in cylindrical plastic bags of varying depth (1.5–15 m) which were continuously mixed. To mimic different levels of background turbidity, we surrounded the transparent enclosure walls with a layer of opaque white (1997) or black (1998) plastic. The experiments were run for 4 wk (1997) and 6 wk (1998). The results supported two key assumptions of the model: specific production and specific sedimentation losses both decreased with increasing mixing depth. At all mixing depths, fast-sinking diatoms dominated the communities. In accordance with model predictions, algal biomass concentration and standing stock (summed over the mixed layer) showed a unimodal relationship to mixing depth when background turbidity was high (1998). When background turbidity was lower (1997), only the ascending limbs of the corresponding relationships were found, which supports the prediction that the mixing depth at which biomass peaks (i.e., becomes predominantly limited by light) increases with decreasing background turbidity. Also in accordance with predictions, light intensity at the bottom of the mixed layer decreased with increasing mixing depth and with increasing background turbidity. Finally, the data supported only the ascending limbs of the predicted inverse unimodal relationships among mixing depth and dissolved inorganic and total water column phosphorus. The absence of descending limbs in these relationships at low mixing depths was probably due to deviations of the experimental systems from two model assumptions. First, the remineralization rate of sedimented phosphorus may have been too slow to equilibrate with sedimentation losses over the experimental periods. Second, biomass yield per unit nutrient (the ratio of seston carbon to phosphorus) was not constant, but decreased with increasing mixing depth. To our knowledge, these are the first field experiments in which the effects of mixing depth on phytoplankton and its resources have been investigated systematically along a large gradient.

Transfer of hydrocarbons from natural seeps to the water column and atmosphere

Abstract: Results from surface geochemical prospecting, seismic exploration and satellite remote sensing have documented oil and gas seeps in marine basins around the world. Seeps are a dynamic component of the carbon cycle and can be important indicators for economically significant hydrocarbon deposits. The northern Gulf of Mexico contains hundreds of active seeps that can be studied experimentally with the use of submarines and Remotely Operated Vehicles (ROV). Hydrocarbon flux through surface sediments profoundly alters benthic ecology and seafloor geology at seeps. In water depths of 500‐2000 m, rapid gas flux results in shallow, metastable deposits of gas hydrate, which reduce sediment porosity and affect seepage rates. This paper details the processes that occur during the final, brief transition — as oil and gas escape from the seafloor, rise through the water and dissolve, are consumed by microbial processes, or disperse into the atmosphere. The geology of the upper sediment column determines whether discharge is rapid and episodic, as occurs in mud volcanoes, or more gradual and steady, as occurs where the seep orifice is plugged with gas hydrate. In both cases, seep oil and gas appear to rise through the water in close proximity instead of separating. Chemical alteration of the oil is relatively minor during transit through the water column, but once at the sea surface its more volatile components rapidly evaporate. Gas bubbles rapidly dissolve as they rise, although observations suggest that oil coatings on the bubbles inhibit dissolution. At the sea surface, the floating oil forms slicks, detectable by remote sensing, whose origins are laterally within � 1000 m of the seafloor vent. This contradicts the much larger distance predicted if oil drops rise through a 500 m water column at an expected rate of � 0.01 m s � 1 while subjected to lateral currents of � 0.2 m s � 1 or greater. It indicates that oil rises with the gas bubbles at speeds of � 0.15 m s � 1 all the way to the surface.

Seasonal and interannual variability in sources of nitrogen supporting export in the oligotrophic subtropical North Pacific Ocean

Abstract: Over timescales of months to years, the export of organic nitrogen from the oceanic euphotic zone (principally as sinking particulate nitrogen, PN) is believed to balance the input of exogenous combined inorganic nitrogen (i.e., export production balances new production). In the oligotrophic North Pacific subtropical gyre, there are two significant sources of new nitrogen: the upward flux of nitrate from deep water and the biological fixation of dissolved N2 in near-surface waters. Because these sources have distinct stable isotopic signatures, we were able to use the total PN and d 15 N measurements of sinking particles and an isotopic mass balance model to deconvolute the relative and absolute contributions of the nitrate flux and nitrogen fixation to the gravitational export of PN. The sinking flux of PN and its isotopic composition both varied widely between 1989 and 2001. Seasonally, N 2 fixation correlated inversely with mixed-layer depth, reaching a maximum in June‐August, whereas nitrate-supported export correlated inversely with sea surface temperature, reaching a maximum in February‐March. These patterns were consistent with summertime increases in diazotroph biomass and water column N2 fixation rates, as indicated by phycoerythrin pigment concentrations and 15 N2 tracer studies. Annually, the relative contribution of N 2 fixation to N export varied from 36 to 69% (mean 5 48%) and showed a significant increasing trend over the period of observation. Although total PN export correlated with the Southern Oscillation Index, the nitrate- and nitrogen fixation‐based components appeared to respond to climate forcing in distinct ways, complicating our interpretation of the mechanisms of climate-induced changes in particle export.

Remote-sensing systems and seabirds: their use, abuse and potential for measuring marine environmental variables

Abstract: We examined how seabirds might be used to study marine environmental variables, which necessitates knowing location and the value of the variable to be studied. Five systems can potentially be used for determination of location: VHF (Very High Frequency) telemetry, PTT (Platform Terminal Transmitters) telemetry, GLS (Global Location Service) geolocation methods, dead reckoning and GPS (Global Positioning System), each with its own advantages with respect to accuracy, potential number of fixes and size. Temperature and light were used to illustrate potential difficulties in recording environmental variables. Systems currently used on seabirds for measurement of temperature respond slowly to environmental changes; thus, they may not measure sea surface temperature adequately when contact periods with water bodies are too short. Light can be easily measured for light extinction studies, but sensor orientation plays a large role in determining recorded values. Both problems can be corrected. The foraging behaviour of seabirds was also examined in order to identify those features which would be useful for determination of marine environmental variables at a variety of spatial and temporal scales. Area coverage by birds is highly dependent on breeding phase and tends to be concentrated in areas where prey acquisition is particularly enhanced. The identification of these sites may be of particular interest to marine biologists. 'Plungers' and 'divers' are potentially most useful for assessment of variables deeper within the water column, with some divers spending up to 90% of their time sub-surface. Few seabirds exploit the water column deeper than 20 m, although some divers regularly exceed 50 m (primarily penguins and auks), while 2 species dive in excess of 300 m. The wide-ranging behaviour of seabirds coupled, in many instances, with their substantial body size makes them potentially excellent carriers of sophisticated environmental measuring technology; however, the ethical question of how much the well-being of birds can, and should, be compromised by such an approach needs to be carefully considered.

Bacteria of the γ-Subclass Proteobacteria Associated with Zooplankton in Chesapeake Bay

Abstract: The seasonal abundance of γ-subclass Proteobacteria, Vibrio-Photobacterium, Vibrio cholerae-Vibrio mimicus, Vibrio cincinnatiensis, and Vibrio vulnificus in the Choptank River of Chesapeake Bay associated with zooplankton was monitored from April to December 1996. Large (>202-μm) and small (64- to 202-μm) size classes of zooplankton were collected, and the bacteria associated with each of the zooplankton size classes were enumerated by fluorescent oligonucleotide direct count. Large populations of bacteria were found to be associated with both the large and small size classes of zooplankton. Also, the species of bacteria associated with the zooplankton showed seasonal abundance, with the largest numbers occurring in the early spring and again in the summer, when zooplankton total numbers were correspondingly large. Approximately 0.01 to 40.0% of the total water column bacteria were associated with zooplankton, with the percentage of the total water column bacteria population associated with zooplankton varying by season. A taxonomically diverse group of bacteria was associated with zooplankton, and a larger proportion was found in and on zooplankton during the cooler months of the year, with selected taxa comprising a larger percent of the Bacteria in the summer. V. cholerae-V. mimicus and V. vulnificus comprised the bulk of the large and small zooplankton-associated Vibrio-Photobacterium species. In contrast, V. cincinnatiensis accounted for less than 0.1 to 3%. It is concluded that water column and zooplankton bacterial populations vary independently with respect to species composition since no correlation was observed between taxa occurring with highest frequency in the water column and those in association with zooplankton.

Biogenic silica dissolution in the oceans: Reconciling experimental and field‐based dissolution rates

Abstract: [1] On average, 50-60% of diatomaceous opal produced in the euphotic zone redissolves within the upper 100 m of the water column. High specific silica dissolution rates in the surface ocean contrast with order-of-magnitude lower values in deep-sea sediments. The lack of a mechanistic understanding of these large variations in dissolution kinetics is a major source of uncertainty in models of nutrient silicon cycling in the oceans. Here we show that the observed variability in biogenic silica reactivity is consistent with results of experimental dissolution studies. Differences in aluminum content and specific surface area of the diatom skeletons, plus differences in temperature and degree of undersaturation, all contribute to lowering silica dissolution rates in deep-sea sediments relative to the euphotic zone. When variations in these material and environmental properties are accounted for, the predicted specific opal dissolution rate in average surface ocean water is over 2 orders of magnitude faster than the rate measured in recently deposited biosiliceous oozes of the Southern Ocean. The predicted specific dissolution rate, however, is approximately 5 times higher than the value obtained from global estimates of the depth-integrated dissolution rate and biogenic silica concentration in the upper water column. This discrepancy likely reflects the inhibitory effect of protective organic coatings on fresh diatom shells in surface waters. The experimental data further imply that aluminum incorporation in the silica matrix, during biomineralization or after death of the organisms, enhances the preservation efficiency of biogenic opal in marine sediments.

Spatial-temporal variability in surface layer deepening and lateral advection in an embayment of Lake Victoria, East Africa

Abstract: Vertical and horizontal exchanges in Pilkington Bay, a shallow (9 m) embayment of Lake Victoria, were determined from a surface energy budget, time series measurements of temperature, and quasi synoptic transects of conductivity, temperature, and depth conducted over a 2-d period. The surface energy budget is the first from a tropical lake over a diurnal timescale. Strong stratification developed during morning and early afternoon (.40 cycles h 21 ) but was eroded beginning in the afternoon by the combination of wind and heat loss. Surface heat losses contributed .70% of the energy for surface layer deepening 82% of the time from midafternoon until midmorning. Circulation times of the surface layer were ,2 min as it deepened to 1.5 m in the afternoon and were ,12 min at night even when mixing extended to the lake bottom. Spatial differences in the rates of heating and cooling and in the depth of wind mixing caused fronts to develop on spatial scales of kilometers within the bay. Convergence of these fronts led to downwelling of surface waters and upwelling of deep waters during the stratified period. Horizontal pressure gradients due to differential heating contributed to thermocline downwelling, lateral movement of deep, anoxic waters, and generation of high-frequency internal waves, all of which contribute to vertical and horizontal transports. Although wind and heat loss at one location generally determine the depth of the surface layer and thermocline, the depths of these key features may be strongly influenced by rates of heating and cooling elsewhere in a basin. The temporal and spatial scales of advection and vertical mixing determine the flux paths of particles and solutes in aquatic ecosystems and, ultimately, lacustrine productivity. Time series measurements of surface meteorology, from which surface energy budgets can be calculated, and water column temperatures can be used as an indirect approach to determine these pathways. The data also can be used to determine internal wave dynamics, the energetics of mixing, and the role of intrusions and density driven flows in advection. With this information, the rates of circulation within the upper mixed layer and vertical and horizontal transports can be assessed. The diurnal cycle of stratification and mixing is deter

In situ calcium carbonate dissolution in the Pacific Ocean

Abstract: Over the past several years researchers have been working to synthesize the WOCE/ JGOFS global CO2 survey data to better understand carbon cycling processes in the oceans. The Pacific Ocean data set has over 35,000 sample locations with at least two carbon parameters, oxygen, nutrients, CFC tracers, and hydrographic parameters. In this paper we estimate the in situ CaCO3 dissolution rates in the Pacific Ocean water column. Calcium carbonate dissolution rates ranging from 0.01 1.1 mmol kg1 yr1 are observed in intermediate and deepwater beginning near the aragonite saturation horizon. In the North Pacific Intermediate Water between 400 and 800 m, CaCO3 dissolution rates are more than 7 times faster than observed in middle and deep water depths (average = 0.051 mmol kg1 yr1). The total amount of CaCO3 that is dissolved within the Pacific is determined by integrating excess alkalinity throughout the water column. The total inventory of CaCO3 added by particle dissolution in the Pacific Ocean, north of 40S, is 157 Pg C. This amounts to an average dissolution rate of approximately 0.31 Pg C yr1. This estimate is approximately 74% of the export production of CaCO3 estimated for the Pacific Ocean. These estimates should be considered tomore » be upper limits for in situ carbonate dissolution in the Pacific Ocean, since a portion of the alkalinity increase results from inputs from sediments.« less

Accumulation and fate of phytodetritus on the sea floor

Abstract: Phytoplankton blooms sometimes result in the mass sinking of phytodetritus through the water column to the sea floor. The accumulation of a phytodetrital fluff layer on the sea floor is an episodic or seasonal event in some marine environments. This review provides a comprehensive list of locations in the world where the accumulation of phytodetritus has been observed on the sea floor. The microscopic and chemical composition of phytodetritus sampled from the sea floor at shallow to abyssal depths is also summarised. In addition, this review provides an overview of the mechanisms leading to mass sinking events, rates of accumulation of fluff layers, the impact of phytodetritus on fluxes of dissolved and particulate matter at the sediment/water interface, and the fate of phytodetritus on the sea floor. More studies are needed to understand the importance of these ephemeral phenomena for the ecology of benthic organisms, benthic-pelagic coupling in the carbon cycle, and the geological record in marine sediments.

Love handles in aquatic ecosystems: The role of dissolved organic carbon drawdown, resuspended sediments, and terrigenous inputs in the carbon balance of Lake Michigan

Abstract: During the unstratified (winter) and stratified (summer) periods of 1999 and 2000, we examined carbon (C) dynamics in the upper water column of southern Lake Michigan. We found that (a) bacterial respiration (BR) and planktonic respiration (PR) were major sinks for C, (b) C flux through bacteria (CFTB) was diminished in winter because of reduced bacterial production (BP) and increased bacterial growth efficiency (BGE) at colder temperatures, and (c) PR exceeded primary production (PP) during the spring‐summer transition. Drawdown of dissolved organic C (DOC), resuspended organic matter from the lake floor, and riverine organic matter likely provided organic C to compensate for this temporal deficit. DOC in the water column decreased between winter and summer (29 ‐91 mg C m 2 d 1 ) and accounted for 20%‐53% of CFTB and 11%‐33% of PR. Sediment resuspension events supported elevated winter heterotrophy in the years that they occurred with greatest intensities (1998 and 2000) and may be important to interannual variability in C dynamics. Further, riverine discharge, containing elevated DOC (5) and dissolved P (10) relative to lake water, peaked in the winter‐spring season in southern Lake Michigan. Collectively, terrigenous inputs (river, stream, and groundwater discharges; storm water runoff; and atmospheric precipitation) may support approximately 10%‐20% of annual in-lake heterotrophy as well as autotrophy. Terrestrial subsidies likely play a key role in the C balance of even very large lakes, representing a critical linkage between terrestrial and aquatic ecosystems.

Examination of coupling between primary and secondary production in a river-dominated estuary: Apalachicola Bay, Florida, U.S.A.

Abstract: To examine the influence of river-borne organic material on estuarine communities, we conducted a dual stable isotope study in an estuary heavily influenced by alluvial runoff. Despite significant alluvial influence, secondary production in Apalachicola Bay depends more upon estuarine primary production than upon a detrital food web supported by floodplain primary production. Two simple mixing models, floodplain-marine and floodplain-estuarine, indicated that the upper limits for the contribution of terrestrial organic matter to estuarine consumer diets averaged 37, 25, and 27% and 20, 19, and 25% for East Bay, Cat Point, and Dry Bar, respectively. Systematic δ 13 C variation of consumer organisms was found for differing locations and attributed to increasing influence of terrestrial organic matter and 13 C-depleted dissolved inorganic carbon (DIC) closer to the river mouth. The δ 34 S data exhibited significant variation with river flow that was attributed to an admixture of terrestrial floodplain detritus with estuarine and marine organic matter. Both δ 13 C and δ 34 S isotope data demonstrated clear distinctions between benthic and water column feeding types. Our results suggest that the estuary is dependent on riverine inflows to provide floodplain detritus during the high-flow period and dissolved nutrients for estuarine primary productivity during the low-flow season. Any alteration of river hydrology may adversely affect estuarine secondary production, especially during the low-flow period when the estuary is dependent on input of dissolved nutrients to maintain a high level of primary productivity.

Seasonal ecosystem variability in remote mountain lakes: implications for detecting climatic signals in sediment records

Abstract: Weather variation and climate fluctuations are the main sources of ecosystem variability in remote mountain lakes. Here we describe the main patterns of seasonal variability in the ecosystems of nine lakes in Europe, and discuss the implications for recording climatic features in their sediments. Despite the diversity in latitude and size, the lakes showed a number of common features. They were ice-covered between 5–9 months, and all but one were dimictic. This particular lake was long and shallow, and wind action episodically mixed the water column throughout the ice-free period. All lakes showed characteristic oxygen depletion during the ice-covered-period, which was greater in the most productive lakes. Two types of lakes were distinguished according to the number of production peaks during the ice-free season. Lakes with longer summer stratification tended to have two productive periods: one at the onset of stratification, and the other during the autumn overturn. Lakes with shorter stratification had a single peak during the ice-free period. All lakes presented deep chlorophyll maxima during summer stratification, and subsurface chlorophyll maxima beneath the ice. Phosphorus limitation was common to all lakes, since nitrogen compounds were significantly more abundant than the requirements for the primary production observed. The major chemical components present in the lakes showed a short but extreme dilution during thawing. Certain lake features may favour the recording of particular climatic fluctuations, for instance: lakes with two distinct productive periods, climatic fluctuations in spring or autumn (e.g., through chrysophycean cysts); lakes with higher oxygen consumption, climatic factors affecting the duration of the ice-cover (e.g., through low-oxygen tolerant chironomids); lakes with higher water retention time; changes in atmospheric deposition (e.g., through carbon or pigment burial); lakes with longer stratification, air temperature changes during summer and autumn (e.g., through all epilimnetic species).

Vertical budgets for organic carbon and biogenic silica in the Pacific sector of the Southern Ocean, 1996-1998

Abstract: During the 1996–1998 Antarctic Environment and Southern Ocean Process Study (AESOPS), a component of US JGOFS, we obtained seasonal or longer-term data sets on the rates of production, vertical transport, remineralization and burial of particulate organic carbon (POC) and biogenic silica (BSiO2) in the Southern Ocean at 170°W between 55°S and 68°S. The AESOPS data records enable us to construct vertical C and Si budgets for the water column and upper sediments, with all estimates derived from direct measurement of the relevant fluxes. We constructed annual C and Si budgets for each of four ecologically distinct zonal bands within the system. For both POC and BSiO2 the greatest annual delivery to the sea floor (∼200 and 1400 mmol m−2 yr−1, respectively) and burial (∼6 and 160 mmol m−2 yr−1, respectively) were observed in the southern Antarctic Circumpolar Current (ACC) between 61.5°S and 65.5°S. That pattern is consistent with our observation that a diatom bloom propagated southward through the southern ACC during the spring and summer of 1997–1998, following the receding ice edge, and that this bloom was the main source of both POC and BSiO2 in the system on an annual basis. In the other zones the annual fluxes of POC and BSiO2 to the sea floor ranged from 19% to 67% of those in the zone traversed by the summer diatom bloom. The higher benthic fluxes of both POC and BSiO2 in the southern ACC imply that blooms similar to the one we observed in 1997–1998 occur commonly in the southern ACC, and that their high-productivity signature is transmitted to the sea floor. The data show preferential preservation of BSiO2 over POC throughout the water column and upper seabed. In the four zonal bands we consider, BSiO2 and POC are produced in mole ratios of 0.1–0.4, exported from the upper 100 m in ratios of 0.2–0.6, arrive at 1000 m in ratios of 1.5–4.5, reach the sea floor in ratios of 2.2–7.6, and are buried in ratios of 11.6–28. Despite the preferential preservation of BSiO2, accumulation of opal-rich sediments beneath the ACC does not result from unusually efficient preservation of siliceous material. The estimated BSiO2 preservation efficiency (burial-production) ranges from 1.2% to 5.5%, indistinguishable from the global average of 3%. Instead, opal-rich sediment accumulation in this region reflects very high annual rates of BSiO2 production in surface waters, along with very low accumulation rates of other sedimentary components (e.g., CaCO3 and detrital material). The observed high ratios of BSiO2 production to POC production in surface waters are consistent with the known tendency for the Si/C ratio of diatoms to increase when [Fe] is low. If greater Fe availability during the last glacial maximum permitted diatoms in the Southern Ocean to grow with lower, more normal Si/C ratios, export of diatom-produced POC could have occurred at 2–3 times its present rate. A corresponding increase in opal export or opal sediment accumulation is unlikely because silicic acid is almost totally depleted north of 65°S under present conditions. Thus, even large increases in POC production and export during glacial periods would not be reflected in the opal accumulation record.

Factors affecting microphytobenthic biomass, species composition and production in the Colne Estuary (UK)

Abstract: Biomass, species composition and primary production of the microphytobenthos (MPB) were investigated along environmental gradients in the Colne Estuary between August 1996 and March 1998 Mean monthly sediment chlorophyll a (chl a) concentrations varied between 6 and 241 mg m -2 There was no seasonal pattern in chl a distribution Low chl a concentrations were asso- ciated with the estuary mouth, which was characterised by exposed sandy sediments and low water column nutrient concentrations Position on the shore affected biomass, with greater chl a concentra- tions on the high shore Chl a concentrations followed the same pattern at both high and low shore positions There was a significant relationship between concentrations of sediment colloidal carbo- hydrate and chl a Epipelic diatoms dominated the MPB, with cluster analysis identifying 10 distinct species assemblages Navicula spp were an important component of all assemblages Peaks in chl a biomass occurred when a single species made up >65% of the cell numbers The distribution of the diatom assemblages was related to salinity, temperature and dissolved inorganic nitrogen (DIN) con- centrations Net production of oxygen occurred in sediments under illumination at all sites on almost all occasions Oxygen uptake by sediment in the dark was positively correlated with temperature at the head of the estuary, but not at any other site Mean annual chl a concentrations correlated posi- tively with total annual dark oxygen uptake Annual primary production was estimated by 4 models using chl a concentrations and a model based on gross O2 fluxes across the sediment-water interface Production was estimated to be in the range of 25 to 1199 gC m -2 yr -1 , depending on site and model There was greater production on the high shore than on the low shore, due to higher chl a concen- trations and longer emersion periods Total production of MPB in the estuary was estimated to be between 117 and 852 tons C yr -1 Assimilation by the MPB provided a temporary sink for DIN, equiv- alent to 128% of the annual 1996 load to the Colne Estuary N assimilation rates by the MPB were in the same order of magnitude as denitrification, indicating that assimilation by the MPB is an impor- tant process affecting the flux of DIN through this mesotidal estuary

Lipid biomarker fluxes in the Arabian Sea, with a comparison to the equatorial Pacific Ocean

Abstract: Fluxes and distributions of organic carbon (OC) and lipid biomarkers were measured in the western Arabian Sea as a function of season, depth, and distance from the coast of Oman during the US JGOFS Arabian Sea Process Study in 1994–1995. A strong seasonal pattern in OC and lipid flux is related to the annual monsoon cycle in the western Arabian Sea, with the highest fluxes measured during the Southwest Monsoon. Fluxes were greatly attenuated with depth in the water column and in surface sediments as remineralization effectively consumed particulate organic matter. A comparison of water-column fluxes with OC and lipid accumulation rates in sediments confirms that the water–sediment interface is a “hot-spot” for organic matter degradation. Biomarker compositions also varied seasonally and with distance offshore, reflecting seasonal and spatial succession of their biological sources and their subsequent export through the water column. Degradation of OC and biomarkers was extremely efficient, with only a tiny fraction (