Showing papers on "Water column published in 1994"

Effects of land use on water quality and aquatic biota of three North Carolina Piedmont streams

Abstract: Three streams in the Piedmont ecoregion of North Carolina were studied to evaluate the effect of land use (forested, agricultural, urban) on water quality and aquatic biota. In comparison with the forested stream, there were few changes in water quality at the agricultural and urban streams. Suspended-sediment yield was greatest for the urban catchment and least at the forested catchment. Suspended-sediment concentrations during storm events followed this same pattern, but at low-moderate flows suspended-sediment concentrations were greatest at the agricultural site. Most nutrient concentrations were highest at the agricultural site, and the amount of ‘available’ dissolved nitrogen was elevated at both the urban and agricultural sites. High concentrations of metals (totals) in the water column were sometimes observed at all sites, but maximum average concentrations were recorded at the urban site (especially Cr, Cu, and Pb). Maximum sediment metal concentrations, however, were not found at the urban site, but were usually recorded at the forested site. Only minor differences were noted between fish communities of the forested and agricultural sites, although both abundance and average size of some species increased at the agricultural site. The fish community at the urban site was characterized by low species richness, low biomass, and the absence of intolerant species. Invertebrate taxa richness, a biotic index, and the number of unique invertebrate species (found at only one site) indicated moderate stress (Fair water quality) at the agricultural site and severe stress (Poor water quality) at the urban site. At the agricultural site, declines in taxa richness within intolerant groups were partially offset by increases within tolerant groups. The agricultural stream had the highest abundance values, indicating enrichment. The urban site, however, was characterized by low species richness for most groups and very low abundance values. Analysis of seasonal patterns suggested detritus was the most important food source for invertebrates in the forested stream, while periphyton was of greater importance in the agricultural stream. Dominant macroinvertebrate groups shifted from Ephemeroptera at the forested site, to Chironomidae at the agricultural site, and Oligochaeta at the urban site. There was little between-site overlap in dominant species (8–7%), indicating that land use strongly influenced the invertebrate community. Chemical and physical parameters measured at the three sites did not seem sufficient to account for all of the observed differences in the invertebrate communities, suggesting some unmeasured toxicity. Biological measurements, especially macroinvertebrates community structure, consistently indicated strong between-site differences in water and habitat quality.

Benthic phosphorus regeneration, net primary production, and ocean anoxia: A model of the coupled marine biogeochemical cycles of carbon and phosphorus

Abstract: We examine the relationships between ocean ventilation, primary production, water column anoxia, and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon (C) and phosphorus (P). The elemental cycles are coupled via the Redfield C/P ratio of marine phytoplankton and the C/P ratio of organic matter preserved in marine sediments. The model assumes that on geologic timescales, net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone. The model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water-sediment interface. Evidence from marine and lacustrine settings, modern and ancient, demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia-dysoxia expands. Steady state simulations show that a reduction in the rate of thermohaline circulation, or a decrease of the oxygen content of downwelling water masses, intensifies water column anoxia-dysoxia and at the same time increases surface water productivity. The first effect reflects the declining supply of oxygen to the deeper parts of the ocean. The second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides. Sedimentary burial of organic carbon and authigenic calcium phosphate mineral (francolite), on the other hand, is promoted by reduced ocean ventilation. According to the model, global-scale anoxia-dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system. Consequently, higher rates of primary production and organic carbon burial can be achieved, even when the continental supply of reactive phosphorus to the oceans remains unchanged.

Diurnal variation of denitrification and nitrification in sediments colonized by benthic microphytes

Abstract: Diurnal variation of denitrification in sediments with benthic microphytes was investigated by a 15N isotope-pairing technique in order to distinguish between coupled nitrification-denitrification and denitrification of nitrate supplied from the water column. Sediments were incubated in a continuous flowthrough system and exposed to diurnal light and dark cycles. Illumination of both limnetic and estuarine sediments doubled the rate of coupled nitrification-denitrification but reduced the rate of denitrification of nitrate supplied from the water column by -50%. Photosynthesis in the uppermost sediment layers during illumination led to deeper oxygen penetration and a resultant stimulation of nitrification. This stimulation may explain the increased rate of coupled nitrification-denitrification during illumination. The rate of denitrification of nitrate supplied from the water column was reduced during illumination due to a longer diffusion path between water and the anoxic denitrification zone. Denitrification, the bacterial reduction of nitrate to nitrogen gas via nitrite, is an important nitrogen sink in aquatic environments. There are two main sources of nitrate for sediment denitrification: nitrate diffusing into the sediment from the water column and nitrate produced by nitrification in the sediment (Jenkins and Kemp 1984; Seitzinger 1988). The processes of nitrification and denitrification are usually vertically separated within the sediment (Vanderborght and Billen 1975). Nitrification is strictly aerobic and therefore restricted to a thin oxic zone in the upper few millimeters of the sediment. Denitrification takes place in anoxic environments, and in many sediments, most activity is found just below the oxic-anoxic interface (Sweerts and de Beer 1989). The rates of nitrification and denitrification in the active microenvironments are controlled by the availability of substrates, which to a large extent are supplied by diffusion along

Diagnosing Climate Change and Ocean Ventilation using Hydrographic Data

Abstract: Changes in atmospheric forcing can affect the subsurface water column of the ocean by three different mechanisms. First, warmed mixed-layer water that is subducted into the ocean interior will cause subsurface warming; second, the subducted surface water can be freshened through changes in evaporation and precipitation; and third, the properties at a given depth may be changed by the vertical displacement of isotherms and isohalines without changes of water masses. These vertical displacements of the water column can be caused either by changes in the rates of renewal of water masses or by dynamical changes (such as changes in wind stress). A method for analysing the subsurface temporal changes in hydrographic data is described in terms of these three processes: “pure warming,” “pure freshening,” and “pure heave.” Linear relations are derived for the relative strength of each process in terms of the observed changes of potential temperature and salinity in two different coordinate frames: (i) con...

Production and transport of methane in oceanic particulate organic matter

Abstract: METHANE is an important component of the global carbon cycle1 and a potent greenhouse gas2,3. Surface ocean waters are typically supersaturated with dissolved methane relative to atmospheric equilibrium, presumably as a result of in situ microbial methane production4–8. Because methanogenic bacteria are strict anaerobes9and surface ocean waters are highly oxygenated, the observation of methane supersaturation has been termed the 'oceanic methane paradox'10. Although methanogenic bacteria have been isolated from oceanic particulate matter, faecal pellets and zooplankton11–14, no data are available on in situ rates of methane formation in these microenvironments. During a series of experiments in the North Pacific ocean, we have identified a previously unrecognized component of the oceanic methane cycle. We find that methane is associated with sinking particles, presumably as a dissolved constituent of the interstitial fluids of particulate biogenic materials, which exchanges with the water column as particles sink. This phenomenon provides a mechanism for the active transport in the water column of an otherwise passive, dissolved species. The particle-to-seawater methane flux that we measure is sufficient to replace all of the methane present in the upper water column in about 50 days and to produce the characteristic methane supersaturations in less than a month. We suggest that particulate production and transport may also be relevant to the redistribution and cycling of other bioreactive compounds in the marine environment.

Diel oxygen dynamics and anoxic events in an eutrophic estuary of Waquoit Bay, Massachusetts

Abstract: As a result of nutrient loading from septic systems, a thick canopy of macroalgae covers the bottom of Waquoit Bay, an embayment on Cape Cod, Massachusetts. Using automated conductivity-temperature-oxygen recorders and manual profiles, we measured diel water column O2 changes during summer in the Childs River, the estuary of the bay with the highest housing density. At dawn in midsummer, bottom waters in the Childs River are chronically hypoxic due to high rates of benthic respiration. On sunny days benthic photosynthesis drives bottom water O2 to 10–15 mg l−1 by afternoon. The extent of the daily O2 excursion is directly proportional to daily irradiance. Large diel O2 excursions in bottom water are due to limited mixing of surface and bottom water. Density stratification exceeded two sigma-t units 85% of the time during midsummer in the Childs River. Because of stratification, hypoxia and even anoxia occur in this estuary. The first of several anoxic events was observed in Waquoit Bay in 1988, and we have attempted to evaluate factors that trigger anoxia. High rates of benthic respiration result in anoxia when replenishment of O2 during the day is limited by insufficient light. Our analysis of meteorological records during two recent anoxic events shows that anoxia develops overnight in midsummer during periods of peak summertime temperatures after several days of cloudy, moderately calm weather. Similarly critical conditions existed most summers since 1975, yet anoxic events in the bay have not been reported historically. If climatic warming occurs, anoxic events in the bay may occur more frequently even if algal stocks remain unchanged. Eutrophication of Waquoit Bay is similar to many other embayments in populated coastal areas, and anoxic events may indicate a chronic growing problem in these important ecosystems. However, in shallow, stratified embayments, anoxia may be transient and easily missed without frequent monitoring.

Comparison of continuous records of near-bottom dissolved oxygen from the hypoxia zone along the Louisiana coast

Abstract: Oxygen depletion is a seasonally dominant feature of the lower water column on the highly-stratified, riverine-influenced continental shelf of Louisiana. The areal extent of hypoxia (bottom waters ≤2 mg l−1 dissolved oxygen) in mid-summer may encompass up to 9,500 km2, from the Mississippi River delta to the upper Texas coast, with the spatial configuration of the zone varying interannually. We placed two continuously recording oxygen meters (Endeco 1184) within 1 m of the seabed in 20-m water depth at two locations 77 km apart where we previously documented midsummer bottom water hypoxia. The oxygen meters recorded considerably different oxygen conditions for a 4-mo deployment from mid-June through mid-October. At the station off Terrebonne Bay (C6A), bottom waters were severely depleted in dissolved oxygen and often anoxic for most of the record from mid-June through mid-August, and there were no strong diurnal or diel patterns. At the station 77 km to the east and closer to the Mississippi River delta (WD32E), hypoxia occurred for only 50% of the record, and there was a strong diurnal pattern in the oxygen time-series data. There was no statistically significant coherence between the oxygen time-series at the two stations. Coherence of the oxygen records with wind records was weak. The dominant coherence identified was between the diurnal peaks in the WD32E oxygen record and the bottom pressure record from a gauge located at the mouth of Terrebonne Bay, suggesting that the dissolved oxygen signal at WD32E was due principally to advection by tidal currents. Although the oxygen time-series were considerably different, they were consistent with the physical and biological processes that affect hypoxia on the Louisiana shelf. Differences in the time-series were most intimately tied to the topographic cross-shelf gradients in the two locations, that is, station C6A off Terrebonne Bay was in the middle of a broad, gradually sloping shelf and station WD32E in the Mississippi River Delta Bight was in an area with a steeper cross-shelf depth gradient and likely situated near the edge of a hypoxic water mass that was tidally advected across the study site.

Paleolimnology of the McMurdo Dry Valleys, Antarctica.

Abstract: The McMurdo Dry Valleys presently contain more than 20 permanent lakes and ponds, which vary markedly in character. All, with the exception of a hypersaline pond, have a perennial ice-cover. The dry valley lakes, and lakes in other ice-free regions of continental Antarctica, are unique on this planet in that they consistently maintain a thick year-round ice cover (2.8-6.0 m) over liquid water. The persistent ice covers minimize wind-generated currents and reduce light penetration, as well as restricting sediment deposition into a lake and the exchange of atmospheric gases between the water column and the atmosphere. From a paleolimnological perspective, the dry valley lakes offer an important record of catchment and environmental changes. These lakes are also modern-day equivalents of periglacial lakes that were common during glacial periods at temperate latitudes. The present lakes are mostly remnants of larger glacial lakes that occupied the valleys in the past, perhaps up to 4.6 Ma ago. Two of the valleys contain evidence of being filled with large glacial lakes within the last 10000 years. Repeated drying and filling events since then have left a characteristic impression on the salt profiles of some lakes creating a unique paleo-indicator within the water column. These events are also marked in the sediments by the concentration and dilution of certain chemical constituents, particularly salts, and are also corroborated by carbonate speciation and oxygen isotope analysis. Stratigraphic analysis of dry valley lake sediments is made difficult by the occurrence of an 'old carbon' reservoir creating spurious radiocarbon dates, and by the high degree of spatial variability in lake sedimentation. From a biological perspective, the lakes are relatively simple, containing various taxa of planktonic and benthic microorganisms, but no higher forms of life, which is an advantage to paleolimnologists because there is no bioturbation in the sediments. Useful biological paleo-indicators found in the sediments include cyanobacterial filament sheaths, diatom frustules and other eukaryotic algal cells, protozoan cysts, photosynthetic pigments, and minerals (e.g. carbonates) associated with microbial activity. Future work will benefit from fully characterizing the connection between the ice covers, environmental conditions, and paleo-indicators, thereby allowing refinement of inferences made concerning the paleoenvironment. New dating techniques need to be tested in this environment to overcome the problems associated with radiocarbon dating. The establishment of a detailed and focused paleolimnological campaign is proposed.

Effects of low dissolved oxygen on predation on estuarine fish larvae

Abstract: Low dissolved oxygen concentrations, caused by density stratification of the water column and excess nutrient inputs, occur in many aquatic habitats. Laboratory experiments we conducted indicated that low dissolved oxygen has the potential to strongly alter the absolute and relative importance of a suite of estuarine predators of fish larvae. At dissolved oxygen concentrations 22 mg l ' , predation on naked goby Gobiosolna bosc larvae by an important invertebrate predator of plankton in Chesapeake Bay [the sea nettle scyphoinedusa Chrysaora quinqueclrrha) increased. In contrast, at the same oxygen concentrations, predation by 2 vertebrate predators, juvenile striped bass Morone saxatllis and adult naked goby, decreased. Changes in consumption of larvae most likely resulted from impaired ability of larvae to escape the scyphornedusa, and decreased attack rates by adult and juvenile fishes. Fish predators increased gill ventilation rates even at oxygen levels higher than those leading to decreased predation. However, we could detect no comparable change in behavior of the sea nettle even at 1 mg 1-', the lowest oxygen concentration tested The observed changes in trophic interactions occurred at dissolved oxygen concentrations that are not lethal during short exposures, and that commonly occur in the Chesapeake Bay and other eutrophic estuaries during summer. Thus, low oxygen has the potential to cause significant changes in the importance of alternate trophic pathways in estuarine systems.

Environmental variability as a factor controlling spatial patterns in distribution and species diversity of zooplankton in the St. Lawrence Estuary

Abstract: Field studies on a variety of organisms have suggested that environmental variability plays a major role in determining spatial patterns in distribution and species diversity of estuanne organisms due to the effect of abiotic fluctuations on the physiology of animals. However, there is no study examining the effect of environmental variability on zooplankton distribution and diversity in estuaries. As vertical m~gration is obligatory behavior for the retention of planktonic animals at intermediate positions in the St. Lawrence Estuary (Canada), vertical stratification of the water column is considered the major source of environmental variability for zooplankton in this system. To evaluate the importance of this source of variability as a factor controlling the distribution and diversity of zooplankton in the estuary, we examined the relative contribution of each of the environmental factors of salinity, temperature, turbidity and vertical stratification in explaining spatial patterns of summer zooplankton distribution and diversity. Multivariate analyses revealed the presence of a longitudinal succession of seasonally stable species assemblages (tidal freshwater, true-estuarine and euryhalinemarine assemblages) whose spatial distribution was mainly a function of salinity and vertical stratification. Turbidity and temperature played a minor role in explaining spatial distribution. For all sampling periods, the limit between the true-estuarine and euryhaline-marine assemblages, the lowest number of population centers and the lowest zooplankton abundance all corresponded to the most vertically stratified waters. In contrast, population centers of all species were concentrated in the most abiotically stable parts of the estuary. We conclude that environmental variability is a major factor determining zooplankton distribution and diversity in the estuary. The spatio-temporal stability of the species assemblages and the seasonal variabihty in the abundance of some species also suggest that trophic interactions may play an important role in the regulation of zooplankton populations in the estuary.

Seasonal Variability and Biogeochemistry of Phosphorus in the Scheldt Estuary, South-west Netherlands

Abstract: The geochemistry of phosphate in the highly polluted Scheldt Estuary has been studied during eight cruises in 1987-88. Property-salinity plots show significant seasonal variability in the environmental conditions which trigger phosphate behaviour, such as dissolved oxygen, pH and phytoplankton activity. Consequently, the behaviour of orthophosphate differs from season to season. During autumn and winter, when phytoplankton activity is low or negligible, the orthophosphate profiles show the occurrence of a buffering mechanism, probably caused by desorption of particle-bound phosphorus in response to the increase in pH with increasing salinity. During spring and summer, the geochemistry of orthophosphate is influenced by the presence of anoxic headwaters. High phosphate concentrations are observed in the anoxic river water entering the estuary, probably reflecting release from the sediments. However, phosphate is rapidly removed from solution in the low-salinity zone due to co-precipitation with iron oxyhydroxides, formed by reoxidation in the water column. Seaward from the low-salinity high-turbidity zone, phosphate is removed from solution during phytoplankton blooms, especially during spring. Blooms also affect the suspended matter composition, in that a shift towards biogenic elements (POC, N, P) at the expense of lithogenic elements (Fe, Al) occurs. Moreover, the speciation of particulate phosphorus shifts from iron- and aluminium-bound forms to POC-bound phosphate in zones of high primary productivity. These findings have implications for the future management of the Scheldt Estuary, as nutrient reduction policies are being implemented.

Transport of sludge-derived organic pollutants to deep-sea sediments at deep water dump site 106

Abstract: Linear alkylbenzenes (LABs), coprostanol and epi-co-prostanol, were detected in sediment trap and bottom sediment samples at the Deep Water Dump Site 106 located 185 km off the coast of New Jersey, in water depths from 2400 to 2900 m. These findings clearly indicate that organic pollutants derived from dumped sludge are transported through the water column and have accumulated on the deep-sea floor. No significant difference in LABs isomeric composition was observed among sludge and samples, indicating little environmental biodegradation of these compounds. LABs and coprostanol have penetrated down to a depth of 6 cm in sediment, indicating the mixing of these compounds by biological and physical processes

Suspension-feeding bivalves and the fate of primary production: An estuarine model applied to Chesapeake Bay

Abstract: A probabilistic mathematical model of bivalve suspension-feeding in estuaries is based on bivalve abundance, filtering capacities, and water mixing parameters. We applied the model to five regions of the upper Chesapeake Bay, ranging from shallow tidal fresh habitats to deep mesohaline habitats, for the years 1985 to 1987. Model results indicated that existing suspension-feeding bivalves could consume more than 50% of annual primary production in shallow freshwater and oligohaline reaches of the upper Chesapeake Bay and Potomac River. In deep mesohaline portions of the Chesapeake Bay and Potomac River, suspension-feeding bivalves could consume only 10% of primary production. Independent estimates of benthic carbon demand based on benthic production supported the model predictions. Hydrodynamics of large estuaries restrict the potential of benthic suspension-feeders to crop phytoplankton production because the width and depth of these estuaries limit transport of pelagic waters to the littoral flanks of the estuaries where benthic suspension-feeders can be abundant. Benthic suspension-feeders are dominant consumers in shallow segments of the Chesapeake Bay system, but are suppressed in deeper segments. The suppression is below that set by hydrodynamic limits, and may be due to periodic hypoxia or other factors. Our results suggest that the proposed use of suspension-feeding bivalves to improve water quality of large estuaries will be limited by the depth and width of the estuary, unless the bivalves are suspended in the water column by artificial means.

A preliminary mass balance model of primary productivity and dissolved oxygen in the Mississippi River Plume/Inner Gulf Shelf Region

Abstract: A deterministic, mass balance model for phytoplankton, nutrients, and dissolved oxygen was applied to the Mississippi River Plume/Inner Gulf Shelf (MRP/IGS) region The model was calibrated to a comprehensive set of field data collected during July 1990 at over 200 sampling stations in the northern Gulf of Mexico The spatial domain of the model is represented by a three-dimensional, 21-segment water-column grid extending from the Mississippi River Delta west to the Louisiana-Texas border, and from the shoreline seaward to the 30–60 m bathymetric contours Diagnostic analyses and numerical experiments were conducted with the calibrated model to better understand the environmental processes controlling primary productivity and dissolved oxygen dynamics in the MRP/IGS region Underwater light attenuation appears relatively more important than nutrient limitation in controlling rates of primary productivity Chemical-biological processes appear relatively more important than advective-dispersive transport processes in controlling bottom-water dissolved oxygen dynamics Oxidation of carbonaceous material in the water column, phytoplankton respiration, and sediment oxygen demand all appear to contribute significantly to total oxygen depletion rates in bottom waters The estimated contribution of sediment oxygen demand to total oxygen-depletion rates in bottom waters ranges from 22% to 30% Primary productivity appears to be an important source of dissolved oxygen to bottom waters in the region of the Atchafalaya River discharge and further west along the Louisiana Inner Shelf Dissolved oxygen concentrations appear very sensitive to changes in underwater light attenuation due to strong coupling between dissolved oxygen and primary productivity in bottom waters The Louisiana Inner Shelf in the area of the Atchafalaya River discharge and further west to the Texas border appears to be characterized by significantly different light attenuation-depth-primary productivity relationships than the area immediately west of the Mississippi Delta Nutrient remineralization in the water column appears to contribute significantly to maintaining chlorophyll concentrations on the Louisiana Inner Shelf

Size-fractionated iron concentrations in the water column of the western North Atlantic Ocean

Abstract: We used an improved adsorptive cathodic stripping square-wave voltametry method to determine vertical profiles of three operationally defined fractions of iron in the western North Atlantic Ocean. “Dissolved Fe” ( 0.4-pm fraction) increased from 0.1 nM in the surface water to 1.3 nM at depths below 1,000 m. Our results suggest the need to consider the temporal variation of iron and its size fractions in seawater if we are to understand the influence of iron on phytoplankton production in the ocean.

The role of the blue mussel Mytilus edulis in the cycling of nutrients in the Oosterschelde estuary (The Netherlands)

Abstract: The fluxes of particulate and dissolved material between bivalve beds and the water column in the Oosterschelde estuary have been measured in situ with a Benthic Ecosystem Tunnel. On mussel beds uptake of POC, PON and POP was observed. POC and PON fluxes showed a significant positive correlation, and the average C:N ratio of the fluxes was 9.4. There was a high release of phosphate, nitrate, ammonium and silicate from the mussel bed into the water column. The effluxes of dissolved inorganic nitrogen and phosphate showed a significant correlation, with an average N:P ratio of 16.5. A comparison of the in situ measurements with individual nutrient excretion rates showed that excretion by the mussels contributed 31–85% to the total phosphate flux from the mussel bed. Ammonium excretion by the mussels accounted for 17–94% of the ammonium flux from the mussel bed. The mussels did not excrete silicate or nitrate. Mineralization of biodeposition on the mussel bed was probably the main source of the regenerated nutrients.

Organic geochemistry of sediments from chemosynthetic communities, Gulf of Mexico slope

Abstract: We used a research submersible to obtain 33 sediment samples from chemosynthetic communities at 541–650 m water depths in the Green Canyon (GC) area of the Gulf of Mexico slope. Sediment samples from beneath an isolated mat of H2S-oxidizing bacteria at GC 234 contain oil (mean = 5650 ppm) and C1–C5 hydrocarbons (mean = 12,979 ppm) that are altered by bacterial oxidation. Control cores away from the mat contain lower concentrations of oil (mean = 2966 ppm) and C1–C5 hydrocarbons (mean = 83.6 ppm). Bacterial oxidation of hydrocarbons depletes O2 in sediments and triggers bacterial sulfate reduction to produce the H2S required by the mats. Sediment samples from GC 185 (Bush Hill) contain high concentrations of oil (mean = 24,775 ppm) and C1–C5 hydrocarbons (mean = 11,037 ppm) that are altered by bacterial oxidation. Tube worm communities requiring H2S occur at GC 185 where the sea floor has been greatly modified since the Pleistocene by accumulation of oil, thermogenic gas hydrates, and authigenic carbonate rock. Venting to the water column is suppressed by this sea-floor modification, enhancing bacterial activity in sediments. Sediments from an area with vesicomyid clams (GC 272) contain lower concentrations of oil altered by bacterial oxidation (mean = 1716 ppm) but C1–C5 concentrations are high (mean = 28,766 ppm). In contrast to other sampling areas, a sediment associated with the methanotrophic Seep Mytilid I (GC 233) is characterized by low concentration of oil (82 ppm) but biogenic methane (C1) is present (8829 ppm).

Behavioral response of fish larvae to low dissolved oxygen concentrations in a stratified water column

Abstract: Density stratification and respiration lead to vertical gradients in dissolved oxygen in many aquatic habitats. The behavioral responses of fish larvae to low dissolved oxygen in a stratified water column were examined during 1990–1991 with the goal of understanding how vertical gradients in dissolved oxygen may directly affect the distribution and survival of fish larvae in Chesapeake Bay, USA. In addition, the effects of low oxygen on 24-h survival rates were tested so that results of behavior experiments could be interpreted in the context of risk to the larve. Naked goby [Gobiosoma bosc (Lacepede)] and bay anchovy [Anchoa mitchilli (Valenciennes)] larvae strongly avoided dissolved oxygen concentrations <1 mg 1-1, which were lethal within 24 h at 25 to 27°C. In addition, naked goby larvae, whose behavior was tested at a wider range of dissolved oxygen concentrations, also showed a reduced preference for an oxygen concentration of 2 mg 1-1, which leads to reduced survival during long-term exposures and to reduced feeding rates. There were no major differences in behavior or survival between the two species, or between the two age classes of naked gobies tested. Results suggest that behavioral responses to oxygen gradients will play a large role in producing marked vertical changes in abundance of feeding-stage larvae in Chesapeake Bay; mortality from direct exposure to low oxygen will likely be much less important in producing vertical patterns of larval abundance.

Denitrification processes in the Arabian Sea

Abstract: Recent information on some consequences of the acute mid-water oxygen deficiency in the Arabian Sea, especially on carbon-nitrogen cycling, is reviewed. An evaluation of published estimates of water column denitrification rate suggests an overall rate in the vicinity of 30Tg Ny-1, but the extent of benthic contribution remains unknown. A decoupling of denitrification from primary production, unique to the Arabian Sea, is revealed by nitrite, electron transport system (ETS) activity and bacterial production data. Results of both enzymatic and microbiological investigations strongly point to a major role of organic carbon other than that sinking from surface layers in supporting denitrification. Although denitrification is associated with an intermediate nepheloid layer, it seems unlikely that the excess carbon comes with particles re-suspended along the continental margins and transported quasi-horizontally into the ocean interior; instead, the particle maximum may directly reflect a higher bacterial abundance. It is proposed that denitrification may be predominantly fuelled by the dissolved organic matter.

The Thermoregulatory Function of Diel Vertical Migration for a Juvenile Fish, Cottus extensus

Abstract: Juvenile sculpin (Cottus extensus) less than 30 mm long exhibit a diel vertical migration in the limnetic zone of Bear Lake (Utah-Idaho). Using mid-water and bottom trawls we found that these fish inhabit the bottom of the lake (5° C) during the day but migrate 30–40 m into the water column at night where they reside in the metalimnion or epilimnion at temperatures near 13–16°C. Larger fish do not migrate into the water column. Stomach analyses demonstrated that the young-of-the-year fish do not migrate into the water column to feed: from July to October their diet is 70–93% benthic ostracods and copepods, and pelagic prey are rarely consumed. Furthermore, gut fullness of the sculpin increases through the daylight period and decreases through the night, reaching minimum levels just before the dawn descent. Laboratory experiments demonstrated that the diel migration would increase digestion rate from 3%/h at profundal temperatures, to 22%/h in the warmer surface water, thus allowing the fish to empty their guts overnight and permit feeding the following day. Additionally, sculpin held in a temperature and feeding regime that mimicked that experienced by migrating fish grew 300% faster than those reared at 5° C. Given the overwhelming importance of fast growth for juvenile fishes, a post-feeding thermotaxs that increases digestion may be a common phenomenon increasing growth, and affecting the distribution and bioenergetic relationships of fish.

Phytoplankton dynamics in relation to the biogeochemical cycle of silicon in a coastal ecosystem of western Europe

Abstract: The Bay of Brest, France, a typical semi-enclosed coastal ecosystem (159 km2) of western Europe, was studied during spring 1992 with respect to the biogeochemical cycle of silicon. Three periods of nutrient and phytoplankton dynamics (chlorophyll a, biogenic silica, species composition), were distinguished during spring 1992, each corresponding, respectively, to a bloom of Thalassiosira sp. and Skeletonema costaturn during early spring (April], Rhizosolenia sp. during mid-spring (May) and Chaetoceros sociale during late spring (June). During each period the production of biogenic silica (the mean rate of spring biogenic silica production was 13 mm01 Si m-' dl ) , derived from I4C primary production measurements, size fractionation experiments and appropriate Si: C ratios, has been compared with the sum of the silicic acid inputs to the bay originating from rivers, from the adjacent Iroise Sea and from the sediments. From this comparison, it is concluded that (1) the early spring diatom bloom was mainly sustained by silicic acid from the watershed, (2) recycling of silicic acid within the water column played a major role during mid-spring to sustain the bloom of Rhizosolenia sp. and (3) silicic acid recycling at the sediment-water interface was the main contributor to the silica production during the late spring bloom. On a seasonal basis, the riverine inputs of Si (net source) balance the Si burial in sediments (net sink), and the contribution of the sediment to the silica production equals that of the watershed. The factors that govern the quantitative and qualitative variations of phytoplankton blooms during these periods are discussed. In these nitrate-rich coastal waters, support is given to the hypothesis of Si-limitation of the diatom growth, at least during the early spring period when inputs of silicic acid from the watershed represented the major contribution to the silica production.

Relationships between Suspended Particulate Matter and Sinking Flux along a Trophic Gradient and Implications for the Fate of Planktonic Primary Production

Abstract: We measured water column variables and the sinking flux of C, N, P and pigments in 15 lakes which varied in algal biomass to determine (1) the relationship between sinking flux and suspended particulate concentrations, (2) if sinking rates of particles changed as a function of trophic status, and (3) the importance of sinking as a fate for phytoplankton production along a trophic gradient. Sinking flux was well predicted by metalimnetic algal pigment concentrations (chlorophyll + phaeopigments) and epilimnetic C:N ratios (R2 = 83–97%). Sinking rates of algal pigments were not significantly higher in lakes with higher chlorophyll concentrations. Predictions based on observed C sinking fluxes, water column chlorophyll, and an empirical relationship between primary production and chlorophyll concur with published observations in suggesting a slight negative relationship between production and the ratio of sinking flux to production. Our results challenge the notion that plankton communities in oligotrophic l...