Showing papers on "Water column published in 1990"

Hydrothermal scavenging of rare-earth elements in the ocean

Abstract: SUSPENDED participate samples collected from the TAG hydrothermal vent field, at 26° N on the Mid-Atlantic Ridge, contain ˜10% of the total rare-earth element content (by volume) of ambient sea water. Shale-normalized distribution patterns show both positive europium anomalies and negative cerium anomalies, indicating that the rare-earth elements in these hydrothermal precipitates come from both hydrothermal vent fluid and seawater sources. Rare-earth/Fe concentration ratios in the suspended p articulate material increase at increasing distances from their hydro-thermal source, indicating that rare-earth elements must be continuously extracted from sea water as hydrothermal precipitates are dispersed through the water column. Therefore, although high-temperature vent fluids escaping from the sea floor are typically enriched 10–10,000 times in rare-earth elements relative to sea water1–4, hydrothermal systems must nevertheless act as a net sink in the global ocean budget of the rare-earth elements. But as the maximum rare-earth/Fe ratios observed for suspended particles are ˜10 times lower than previously reported values for hydrothermal sediments5–8, it seems that most of the uptake of rare-earth elements from sea water must occur only after hydrothermal precipitates have settled to the underlying sediments.

Environmental factors affecting the formation of methylmercury in low pH lakes

Abstract: Recent studies have demonstrated elevated levels of mercury in fish from remote, low alkalinity and low pH lakes. The mechanisms of this enhanced bioaccumulation are poorly understood, but the amount of methylmercury produced in a lake can play a major role. Decreased pH stimulates methylmercury production at the sediment-water interface and possibly in the aerobic water column. Decreased pH also decreases loss of volatile mercury from lake water and increases mercury binding to particulates in water – factors that may increase methylation at low pH by enhancing the bioavailability of mercury for methylation. In anoxic subsurface sediments, decreased pH decreases the rate of mercury methylation, suggesting that methylmercury formation in the water column and at the sediment-water interface may be most important in acidified lakes. Sulfate-reducing bacteria are important mercury methylators in acidified lakes. Whether enhanced sulfate reduction stimulates methylmercury production in low pH lakes is presently unclear, although most of the available data do not support this hypothesis.

Temperature and salinity limits for growth and survival of some planktonic foraminifers in laboratory cultures

Abstract: The biological response to extreme temperatures and salinities is investigated in the laboratory for seven species of planktonic foraminifera: Globigerinoides sacculi/er (Brady), Globigerinoides ruber (d'Orbigny), Globigerinoides conglobatus (Brady), Globigerine/la siphonifera (d'Orbigny), Orbulina universa d'Orbigny, Neogloboquadrina dutertrei (d'Orbigny) and Globorotalia menardii (d'Orbigny). When one of the vital processes, food acceptance, growth or reproduction is inhibited by a culture variable, the absolute survival limit is reached. The measured in vitro temperature ranges compare well with the global temperature distribution patterns of these species, suggesting that this parameter plays a major role in their biogeographical distribution. The salinity ranges that are tolerated in laboratory cultures exceed the range encountered in modern oceans. Thus salinity does not limit the distribution of the species investigated herein. In general, larger mean final shell sizes are attained and the total shell length increase is larger at optimum temperatures and salinities than at extreme culture conditions, but the differences were not always statistically significant. Marginal temperature and salinity conditions do not induce contained growth in expatriated specimens. Under extreme culture conditions, the relative frequency of the different shell morphologies is altered relative to normal conditions. "Abnormal" phenotypes are more frequent under normal conditions and the "normal" morphology is found more often under extreme conditions. As opposed to previous reports, the frequency of kummerform chambers generally decreases toward extreme temperature and salinity culture conditions, indicating that kummerform phenotypes are not indicative of environmental stress. The incidence of sac-like chambers in G. sacculi/er and the formation of spherical chambers in adult 0. universa decrease toward extreme temperature and salinity culture conditions, demonstrating that maturation is suppressed in stress situations. SEM investigations show that changes in shell porosity are correlated with treatment variables in culture. The highest porosities are attained at higher temperatures and lower salinities. Generally, an increase in total porosity is achieved by an increase of the pore area accompanied by a reduction of the pore density. The in vitro experiments explain the changes that occurred in the Pleistocene foraminiferal assemblages from the Red Sea around 18 thousand years ago and earlier. During glacial periods, salinity approximated or even exceeded the upper thresholds that were tolerated under laboratory conditions. Under these circumstances, species disappeared from the water column. The order of disappearance as recorded in the sediments may be explained with the upper salinity limits found in this study. Also, the recurrent shifts of dominance between G. sacculi/er and G. ruber are well documented for this fossil assemblage. The present experiments support the conclusion that salinity is the driving mechanism behind this phenomenon. Observations in modern oceans suggest that the fertility of the water mass is probably also an important factor behind the shifts of dominance between G. sacculi/er and G. ruber.

Dimethyl sulfide in the Baltic Sea: Annual variability in relation to biological activity

Abstract: The in situ variation of dimethyl sulfide (CH3SCH3, DMS) at a fixed station in a coastal area of the Baltic Sea has been studied for a period of time (January 1987 to June 1988) covering the annual biological cycle. DMS in the surface waters of the brackish Baltic Sea showed a clear seasonal variation, ranging from 2 to 200 ng S L−1. Lowest concentrations were in winter, peak values followed the spring bloom, and a pronounced maximum was found during the summer (July–August). Concentrations above low winter levels occurred only when the trophogenic layer was depleted of inorganic nitrogen. From our data it is clear that the seasonal variation in DMS concentration is related to biological activity. However, we did not find any correlations between DMS concentration and gross parameters such as chlorophyll a, total phytoplankton biomass, or primary production on an annual basis. Further, we were not able to relate high DMS concentrations to any particular phytoplankton species or species assemblages. It appears that DMS production is primarily associated with phytoplankton growth under nitrogen-limited conditions and not with certain species. We found a significant correlation of ambient DMS concentration with copepod and total Zooplankton biomass, suggesting Zooplankton grazing pressure as the major factor responsible for the liberation of β-dimethylsulfoniopropionate (DMSP) from phytoplankton cells and thus for the DMS production. The turnover time of DMS in the water column was calculated to be of the order of 2 days, and the most effective sink process seems to be of microbiological and/or chemical origin. Previous to this study, no in situ data set has been available to test the relative importance of the various factors responsible for the DMS production in seawaters. We have demonstrated that variations in DMS concentration must be looked upon as the result of complex physiological as well as ecological interactions.

Carbon metabolism in a humic lake: Pool sires and cycling through zooplankton

Abstract: To characterize the major carbon pathways in a humic lake, we determined carbon pool sizes and main pathways by long-term tracer studies in enclosures. Dissolved organic carbon (DOC) was by far the largest pool and constituted 80–85% of total carbon. In the water column particulate organic carbon was partitioned between detritus, zooplankton, bacteria, and phytoplankton at ratios of 22 : 4 : 3 : 1. Phytoplankton and bacterioplankton production averaged 24 and 32 µg C liter−1 d−1, while crustacean zooplankton production was very low (<5 µg C liter−1 d−1) during the experiment. Total pelagic community respiration was high, giving a net CO2 flux to the atmosphere of 44 µg C liter−1 d−1, while losses by sedimentation were negligible. Most of the particulate carbon available for zooplankton was highly recycled detritus of low nutritional value. The loop of ingestion and defecation of detrital particles was a major carbon pathway, giving detrital particles a turnover rate of 0.39 d−1. Detritus was found to support 46–82% of body carbon in the surveyed species, with Acanthodiaptomus as the upper extreme. Bacterial carbon accounted for 11–42% of body carbon and phytoplankton for 6–19% in the surveyed species.

Airborne mercury deposition and watershed characteristics in relation to mercury concentrations in water, sediments, plankton, and fish of eighty northern Minnesota lakes

Abstract: Mercury concentrations in precipitation, lake water and sediment, zooplankton, and fish were measured and analyzed together with extensive watershed and lake chemistry data for 80 lake watersheds in the study region of northeastern Minnesota including the Superior National Forest, Voyageurs National Park, and Boundary Waters Canoe Area Wilderness. Atmospheric deposition of mercury, transport, water column lifetimes, and sedimentation in lakes are determined. The factors relating mercury concentrations within the lake watershed components are analyzed and discussed

Significant Flux Of Atmospheric Nitrous-Oxide From The Northwest Indian-Ocean

Abstract: INTEREST in nitrous oxide (N2O) has increased considerably in the light of its deleterious effect on the ozone layer1, and contribution to the greenhouse effect2. There are many sources of atmospheric N2O, both anthropogenic (for example, combustion) and natural, but the global budget is still inadequately defined. Despite the fact that most of the world's oceans are close to equilibrium with the atmosphere3, water bodies depleted in oxygen have been identified as areas of N2O production4,5, and so the oceans represent a potential source of atmospheric N2O. Here we report elevated N2O concentrations in the northwest Indian Ocean, an area that exhibits upwelling and high oxygen consumption in the water column. We found that N2O was supersaturated in both oxygen-saturated surface waters (up to 246% N2O saturation) and oxygen-depleted sub-surface waters (1,264% N2O saturation). The calculated flux to the atmosphere indicated that upwelling in the northwest Indian Ocean (15–25° N) represents one of the most significant marine sources of N2O, contributing between 5 and 18% of the total marine flux from a surface area of only 0.43% of the world ocean. These data suggest that the oceanic flux of N2O to the atmosphere shows strong spatial heterogeneity which should be considered in global budgets and ocean–atmosphere models.

Effects of Fish and Plankton and Lake Temperature and Mixing Depth

Abstract: A comparative study of small temperate lakes (<20 square kilometers) indicates that the mixing depth or epilimnion is directly related to light penetration measured as Secchi depth Clearer lakes have deeper mixing depths This relation is the result of greater penetration of incident solar radiation in lakes and enclosures with high water clarity Data show that light penetration is largely a function of size distribution and biomass of algae as indicated by a relation between the index of plankton size distribution (slope) and Secchi depth Larger or steeper slopes (indicative of communities dominated by small plankton) are associated with shallower Secchi depth In lakes with high abundances of planktivorous fish, water clarity or light penetration is reduced because large zooplankton, which feed on small algae, are reduced by fish predation The net effect is a shallower mixing depth, lower metalimnetic temperature and lower heat content in the water column Consequently, the biomass and size distribution of plankton can change the thermal structure and heat content of small lakes by modifying light penetration

Diffusion of rare-earth elements in fish teeth from deep-sea sediments

Abstract: THE distribution of rare-earth elements (REEs) in biogenic phosphate phases in sediments (such as the remains of fish teeth) has been proposed as an indicator of the depositional environment in the water column, recording for example palaeo-redox conditions1 and isotope geochemical2–5 and hydrothermal6,7 processes. Enrichment of these phases in REEs takes place only during early diagenesis8,9 for which reason the REE signal has often been assumed to represent the unfractionated record of REE distribution in the water column overlying the sediment surface1,4,5,10. Here we report on studies of REE zoning in fish teeth from Pacific sediments, from which we extract a REE diffusion coefficient of ∼70 mm2 Myr–1. This value suggests that diffusion of REEs from sea water into the fish teeth before burial is too slow to produce the enrichments observed. Instead, the REE distributions must derive from the surrounding sediment pore fluids. The use of these distributions as palaeoceanographic indicators is therefore called into question.

Massive fish kills within the nyanza gulf of lake victoria, kenya

Abstract: The morphometry and hydrology of the Nyanza Gulf of Lake Victoria can be greatly affected by violent storms. This can result in a condition in which nutrient-rich bottom mud is mixed with the sediment-laden runoff water from neighbouring marshes and rivers. This situation can lead to massive fish kills. It is such an event in 1984 which will be described that caused the sudden death of over 400 000 fish weighing over 2400 tonnes. The unusually low precipitation in the catchment area during that year resulted in a drop in the lake level from 12.4 to 11.8 m. This low water level, combined with a severe storm created the conditions which caused this large fish kill. The species affected were Lates niloticus and Oreochromis niloticus. Mortality was attributed to several factors which included high levels of suspended material in the water column (detritus and algae) which clogged the gills of the fish, low dissolved oxygen, low pH, and high concentrations of algae.

Nutrient exchanges between the water column and a subtidal benthic microalgal community

Abstract: Exchanges of nutrients between a shallow (ca. 1 m) nodiment and the water column were measured in the York River, Virginia, during 1983, using transparent and opaque plexiglass hemispheres. Exchanges of nitrate and phosphate were not significantly different between dome treatments, but ammonium release was significantly reduced within the transparent domes. Within the dark domes, ranges of hourly rates of ammonium and phosphate exchange were 21 to 364 μmol N m2 and 3 to 76 μmol P m2 while those in the transparent domes were −162 to 244 and 6 to 80, respectively. Negative values denote uptake by the sediment. Nitrate 1 nitrite exchanges averaged only 15% and 17% of the total dissolved luorganle nitrogen exchange in the dark and transparent domes, respectively.

Fluxes and transformations of aquatic pigments in Lake Mendota, Wisconsin

Abstract: Concentrations and fluxes of chlorophyll and carotenoid pigments were measured in suspended and settling particulate matter and in surface sediments in Lake Mendota. Flux comparisons were used to calculate the extent of alteration or degradation within the water column and at the sediment surface. Losses within the water column for specific time intervals ranged from almost negligible for diatoxanthin to 96% for peridinin. The extent of loss was influenced by pigment type and transport process. Surface sediment diagenesis in Lake Mendota resulted in differential degradation rates among chlorophyll and carotenoid constituents. Deposition of pheophorbide a-a grazing indicator-was most important during late spring, as zooplankton populations increased and chlorophyll levels in the water column dropped below 1 nmol liter-‘. The ability to trace the magnitude of this annual event through the sedimentary record was obscured by selective degradation of pheophorbide relative to chlorophyll and pheophytin. Similarly, carotenoid composition was dramatically altered both within the water column and at the sediment surface. Fucoxanthin and peridinin, major watercolumn carotenoids, were degraded extensively before incorporation in the sediments.

Incidence of mussel culture on biogeochemical fluxes at the sediment-water interface

Abstract: Upward nutrient fluxes at the sediment-water interface were studied in a mussel farming zone (Carteau, Gulf of Fos, France) in order to estimate the impact of organic matter input from biodeposition. Nitrate, nitrite, ammonia, silicate, phosphate and oxygen were measured. Fluxes were estimated by means of polyacrylate benthic chambers placed at sites located under (UM) and outside (OM) the rope hanging structures. Transformation of biodeposited organic matter increases phosphate, silicate and ammonia fluxes. No variation in nitrite fluxes could be detected and only minor differences were observed in nitrate and the oxygen production/consumption equilibrium at the two stations. Phosphate and silicate fluxes, which were always higher at the UM than at the OM site, decreased from spring to winter. Ammonia fluxes were very high under mussel cultures in May and September and lower in November. The fact that ammonia flux was always higher at the UM than at the OM sites might be explained by degradation of mussel biodeposit, as well as by benthic macrafauna excretion. Discrepancies between fluxes of the nutrients studied at the UM and OM sites increased as organic particulate matter in the water column decreased. Variations of oxygen flux followed a different pattern, since they were correlated with presence and abundance of photosynthetic microphytes on the bottom and in the water. Bottom respiration exceeded production of oxygen only in May 1988 at the UM station. As it now stands, biodeposit input into the sediment under mussel ropes does not affect the ecosystem, although the flow of nutrients towards the water column is higher than in other areas.

The Importance of Emergent Vegetation in Reducing Sediment Resuspension in Wetlands

Abstract: Wind-induced resuspension of bottom sediment was measured with sediment traps in Sand Lake National Wildlife Refuge, South Dakota. Resuspension was significantly greater in open water areas than in areas protected from wind by emergent vegetation. Resuspended sediment was apparently uniformly distributed through the water column.

High productivity of northern Bering Sea benthic amphipods

Abstract: POLAR regions, particularly Arctic seas covered by ice for a significant portion of the year, have been considered to be relatively unproductive1–4. In the western Arctic, including the northern Bering Sea, high water column productivity5,6, and well developed benthic communities7–9 have recently been reported. Ampeliscid amphipod crustaceans are the benthic community dominants in vast areas of the northern Bering Sea and are the major prey of the migratory California grey whale, Eschrichtius robustus10,11. Our study indicates that productivity of the benthic amphipods is remarkably high, given the latitude. The dominant species, Ampelisca macrocephala, is the most productive benthic marine amphipod ever reported and the amphipod community is more productive then entire benthic communities investigated elsewhere. biomassPolar marine ecosystems clearly have the potential to be highly productive

Seasonal abundances of planktonic ciliates and microflagellates in mesohaline Chesapeake Bay waters

Abstract: Ciliate, heterotrophic microflagellate (hflag) and autotrophic microflagellate (aflag) abundances are reported for mesohaline Chesapeake Bay waters based on samples gathered from April through October 1985–1987. Total water column averages for ciliate and microflagellate abundances were typical of eutrophic marine systems. Ciliate density ranged from 17·2 cells ml−1 in April to 1·8 cells ml−1 in September; hflag ranged from 3·7 × 103 cells ml−1 in June to 1·1 × 103 cells ml−1 in October. In spring the majority of ciliate and hflag standing stocks (70% and 64%, respectively) were located in bottom and transition waters; during summer months the majority (approximately 85% of both groups) were in surface and transition waters. During fall, ciliate stock was concentrated (72%) in surface waters and hflag were relatively evenly distributed in the three water column zones. Ciliate and microflagellate numbers were not directly related to chlorphyll α concentration except in the bottom layer, where simultaneous declines accompanied anoxia. Ciliate concentrations correlated with total numbers of microflagellates and hflag abundance, but not aflag density. We discuss the relative importance of predation and food availability in regulating ciliate and hflag concentrations in mesohaline Chesapeake Bay waters.

Life in the benthic boundary layer: connections to the mid-water and sea floor

Abstract: The abyssal benthopelagic zone is enriched in biomass and numbers of species, relative to the overlying water column. It is inhabited by (i) pelagic species whose ranges are truncated by the sea bed, (ii) normally benthic species who use the zone as a refuge, for dispersal or for locating their food, (iii) a specialized fauna, which includes many novel species with primitive characteristics. The increase in the standing-crop in the zone is related to the greater availability of organic material on the sea bed, and the extent of its resuspension and diffusion from off the bottom. Several species are known to be capable of responding to both predictable and erratic pulses of input, but otherwise their physiologies seem to differ little from other pelagic and benthic species. These communities contribute a small but significant percentage of the benthic recycling.

Microbial community structure and biomass estimates of a methanogenic Antarctic Lake ecosystem as determined by phospholipid analyses.

Abstract: Phospholipid analyses were performed on water column particulate and sediment samples from Ace Lake, a meromictic lake in the Vestfold Hills, Antarctica, to estimate the viable microbial biomass and community structure in the lake. In the water column, methanogenic bacterial phospholipids were present below 17 m in depth at concentrations which converted to a biomass of between 1 and 7×108 cells/liter. Methanogenic biomass in the sediment ranged from 17.7×109 cells/g dry weight of sediment at the surface to 0.1×109 cells/g dry weight at 2 m in depth. This relatively high methanogenic biomass implies that current microbial degradation of organic carbon in Ace Lake sediments may occur at extremely slow rates. Total microbial biomass increased from 4.4×108 cells/ liter at 2 m in depth to 19.4×108 cells/liter at 23 m, near the bottom of the water column. Total nonarchaebacterial biomass decreased from 4.2 ×109 cells/g dry weight in the surface sediment (1/4 the biomass of methanogens) to 0.06×108 cells/g dry weight at 2 m in depth in the sediment. Phospholipid fatty acid profiles showed that microeukaryotes were the major microbial group present in the oxylimnion of the lake, while bacteria dominated the lower, anoxic zone. Sulfate-reducing bacteria (SRB) comprised 25% of the microbial population at 23 m in depth in the water column particulates and were present in the surface sediment but to a lesser extent. Biomass estimates and community structure of the Ace Lake eco-system are discussed in relation to previously measured metabolic rates for this and other antarctic and temperate ecosystems. This is the first instance, to our knowledge, in which the viable biomass of methanogenic and SRB have been estimated for an antarctic microbial community.

Seasonal variability in the Mercury speciation of Onondaga Lake (New York)

Abstract: Dissolved and particulate Hg speciation was determined on four occasions in the Spring to Fall interval of 1989, at three depths of the water column of Onondaga Lake, New York; an urban system in which the sediments and fish flesh are contaminated with Hg. Species determined included total Hg (Hgt), reactive (‘ionic’) Hg (Hgi), monomethylmercury (CH3HgX), elemental Hg (Hg°) and dimethylmercury (CH3)2Hg). Onondaga Lake was found to contain very high levels of Hgt (2 to 25 ng L−1 Hg), Hgj (0.5 to 10 ng L−1 Hg), and CH3HgX (0.3 to 7 ng L−1 Hg), which generally increased with depth in the lake. These concentrations represent a significant level of contamination, based upon comparisons with other polluted and pristine sites. Elemental Hg levels were typically about 0.05 ng L−1 and (CH3)2Hg was near the limits of detection (−0.001 ng) L−1 in most samples. The greatest CH3HgX concentrations in the hypolimnion, as well as the largest gradients of both CH3HgX and (Hgt), were observed upon the first onset of stratification, in early summer. These concentrations did not become more pronounced, however, as stratification and H2S levels in the hypolimnion increased throughout the summer. The very low concentrations of (CH3)2Hg in these MeHg and sulfide-rich waters calls into question the belief that CH3HgX and H2S will react to yield volatile dimethyl-mercury, which can then escape to the atmosphere by diffusion. Mercury speciation was highly dynamic, indicating active cycling within the lake, and an apparent sensitivity to changes in attendant Iimnological conditions that track the stratification cycle.

Primary productivity and particulate organic matter flow in an estuarine mangrove-wetland in Hong Kong

Abstract: Net primary productivity and organic matter flow of a mangrove-dominated wetland was estimated by following production and detritus dynamics in a tidal pond in north west Hong Kong in 1986–1988 (9.1 ha). Total productivity was 12.47 t dry wt ha−1 yr−1, of which >90% was from emergent macrophytes (the mangroveKandelia candel and the reedPhragmites communis). High turbidity and high summer temperatures probably limited respective production by phytoplankton and benthic macroalgae (dominated byEnteromorpha crinata). Despite the high total productivity, little detritus was exported from the emergent macrophyte stands, due to the low inundation frequency. This created a net water column carbon deficit which was provided for by the high organic matter import (mean = 4.42 g ash free dry wt m−2 d−1) from the incoming water. This same sediment and particulate organic carbon input giving a high accretion rate of 1.7 cm yr−1 was probably also the force behind progressive dis-coupling of emergent macrophyte production from water column consumers. This resulted in a tendency to retain production in the emergent macrophyte stands while the water column community increasingly relied on allochthonous carbon. This shift from a net exporter to a net importer of carbon in landward wetlands is probably characteristic of the transition into nutrient-conservative terrestrial systems.

Nitrogenous nutrient transformations in the spring and fall in the Chesapeake Bay

Abstract: Measurements of the standing stock of phytoplankton, bacterioplankton and particulate carbon and nitrogen, and dissolved pools of inorganic nitrogen were made at a series of stations located in the main channel of the Chesapeake Bay during cruises in the spring and fall of 1984. On each cruise, we conducted two transects of the long axis (N-S) of the bay; comparison of these transects revealed that the horizontal and vertical distribution of O2, chlorophyll a, and dissolved inorganic nitrogen can change dramatically on a time scale of days. Experiments were carried out with 15N-labelled substrates (NO2−, NO3−, and NH4+) at a subset of the stations. These experiments were designed to measure the fluxes of nitrogen between the dissolved pools as well as the flux into particles. During the spring cruise, a surprisingly high rate of uptake of NO2− by particles was found. In the fall, uptake by particles followed previously reported patterns more closely. The flux of nitrogen between dissolved inorganic pools was often greater than the flux into particles. The rates of individual reactions varied spatially, and significant activity throughout the water column was often found. The occurrence of typically anaerobic reactions in well-oxygenated waters suggests that anaerobic microzones may be an important site for nitrogen transformations in the bay. Our results also indicate that the rates of a number of microbially-mediated processes were greatly enhanced by storm-induced mixing of the water column during our fall cruise. Such transient enhancements may have a large, and usually undocumented, effect on the nitrogen cycle.

Sea scallop larvae Placopecten magellanicus on Georges Bank: vertical distribution in relation to water column stratification and food

Abstract: Vertical distribution of sea scallop larvae was studied from pump samples in mixed, stratified, and frontal areas of Georges Bank. The extent of larval aggregation was positively related to the degree of water column stratification. In mixed areas larvae were distributed evenly over the 40 to 50 m water column, while in stratified waters larvae showed subsurface peaks in concentration above the pycnocline. Where the pycnocline was well developed, differences in the larval centre of mass (ZCM) were associated with differences in the position of the pycnocline. Food for sea scallop larvae was measured as the concentration of chlorophyll a < 15 Km and particles 2.5 to 16 pm. In stratified areas food was usually greatest within the upper 10 m. Sea scallop larvae were most concentrated below 10 m and d ~ d not aggregate in relation to food concentration. The capacity of sea scallop larvae for depth regulation on Georges Bank appears limited, since there were no die1 differences in the ZCM, and sizedependent differences in vertical distribution were weak and inconsistent.