Distribution and community structure of estuarine copepods

Distribution, abundance, and community structure were studied over a 30 month period in the planktonic copepod community of the estuaries near Beaufort, North Carolina. Many of the copepod species showed a demersal distribution during the day and entered the surface waters at night. Several species were largely confined to vegetated littoral areas during the day. The copepod community showed consistent trends of seasonal abundance and succession of dominant species which differed greatly from those found by previous workers, whose methods were inadequate to sample quantitatively the small, demersal copepods which dominated the community. Copepod abundances were higher than found in previous studies and were correlated with water temperature. Species composition changed from a winter community dominated byCentropages spp., to a spring community dominated byAcartia tonsa, to a summer community jointly dominated byParacalanus crassirostris andOithona spp. Copepods were much more important grazers in these estuaries than previous studies had concluded.     

The annual cycle of zooplankton in a Long Island estuary

Zooplankton and chlorophyll-a samples and associated hydrographic data were collected at approximately weekly intervals in the Peconic Bay estuary for most of the period between May 1978 and June 1979. Surface zooplankton samples were obtained by simultaneously-towed 73 μm- and 202 μm-mesh nets, and subsurface samples were collected with 505 μm-mesh nets. Zooplankton numbers and displacement volumes fluctuated widely throughout the year, with highest values in early spring and summer. Juvenile or adult copepods accounted for means of 90.0% and 85.0% of the animals recorded for the 202 μm- and 73 μm-net samples, respectively. The combination of Acartia tonsa and A. hudsonica adults+copepodids accounted for a mean of 81.4% of the zooplankton recorded for the 202 μm-net samples, and the combination of copepod nauplii, Acartia spp. adults+copepodids, Oithona colcarva and Parvocalanus crassirostris accounted for a mean of 82.7% of the animals recorded for the 73 μm-net samples. Copepod nauplii were the most abundant zooplankters collected in the 73 μm-net samples, and they were generally collected in higher numbers than the total number of animals in the 202 μm-net samples. During the colder months, late copepodids and adults of larger copepod species comprised greater proportions of the total zooplankton than during the warmer months when nauplii and copepodids of smaller copepod species were predominant. The ctenophore Mnemiopsis leidyi and the medusa Cyanea capillata also had periods of abundance during warmer months. Differences between numbers of larger zooplankters collected over different depth intervals or in successive replicate tows over the same depth intervals, reveal the likely effects of both vertical and horizontal patchiness. Comparisons of zooplankton numbers from the present investigation, which were obtained with relatively fine-mesh nets, with values from previous studies in adjacent waters which used coarser-mesh nets, suggest that many previous investigations have seriously underestimated the numbers of smaller zooplankters, particularly copepod nauplii.     

Seasonal Factors Influencing Copepod Abundance in the Maryland Coastal Bays

Maryland Coastal Bays differ in hydrography from river-dominated estuaries because of limited freshwater inflow from tributary creeks and more marine influence. Consequently, the copepod community structure may be different from that of the coastal ocean and river-dominated estuaries in the mid-Atlantic region. A 2-year study was conducted to describe copepod species composition and seasonal patterns in abundance and factors influencing the community structure. Seven copepod genera, Acartia, Centropages, Pseudodiaptomus, Parvocalanus, Eurytemora, Oithona, and Temora, in addition to harpacticoids were found. The copepod community was dominated by Acartia spp. (64%), followed by Centropages spp. (30%), unlike in river-dominated estuaries in the region where the copepod community is usually dominated by Acartia spp. followed by Eurytemora affinis. Acartia tonsa was the most abundant in summer and fall whereas Centropages spp., Temora sp., Oithona similis, E. affinis, and harpacticoids were most abundant in winter and early spring. Parvocalanus crassirostris and Pseudodiaptomus pelagicus were present in fall and winter but at relatively low densities. The highest mean density of copepods occurred in winter 2012 (36,437 m^?3) and the lowest in spring 2013 (347 m^?3). Low densities occurred through early summer (614 m^?3) coinciding with peak spawning by bay anchovy (Anchoa mitchilli). Bottom-up control via low phytoplankton biomass coupled with top-down control by ctenophores (Mnemiopsis sp.), mysids (Neomysis americana), and bay anchovy was probably responsible for the low copepod densities in spring and early summer. Temperature and salinity were also important factors that influenced the seasonal patterns of copepod species occurrence. The observed seasonal differences in the abundance of copepods have important implications for planktivorous fishes as they may experience lower growth rates and survival due to food limitation in spring/early summer when copepod densities are relatively low than in late summer/fall when copepod abundance is higher.     

Zooplankton In An Australian Tropical Estuary

The composition of the zooplankton community in a macrotidal (8 m tidal range), tropical estuarine system (Darwin Harbour, Australia; 12^o28′ S, 130^o50′ E) was studied over a 2 year period with the goal of describing biodiversity and determining the environmental factors that have the greatest impact on community structure. Most (82–84%) of the >73 μm plankton was composed of copepod nauplii and copepodites, and plankton samples taken with larger, coarser meshed (150 and 350 μm) nets did not contain significant numbers of larger (non-copepod) organisms. In all, 32 copepod species were recorded, with small euryhaline marine copepod species such as Parvocalanus crassirostris, Bestiolina similis and Oithona aruensis dominating the zooplankton. Plankton abundances ranged between 30,000 and 110,000 m⁻³, and there were significant year (2003 > 2004), season (wet > dry) and site differences (inner harbour sites > outer harbour sites), but negligible diurnal differences. Multivariate analyses identified three sample groups: (1) middle and outer harbour sites, (2) inner harbour and river sites and (3) the river site during the wet seasons. Middle and outer harbour stations were characterised by a diverse mixture of coastal copepods, whereas inner harbour and river sites were dominated by P. crassirostris and O. aruensis. During the wet season, there was a distinct copepod community within the Blackmore River, dominated by Acartia sinjiensis, Oithona nishidai and Pseudodiaptomus spp. Environmental variables (nutrients and chlorophyll a) were correlated with salinity, which had the strongest influence on community structure. There was a significant drop in species richness from harbour to river sites. Small copepods of the families Paracalanidae and Oithonidae dominate tide-dominated Australian tropical estuaries, whereas copepods belonging to the family Centropagidae (such as Gladioferens spp.) appear to be characteristic of wave-dominated estuaries in southern Australia.     

Standing stock and estimated production rates of phytoplankton and zooplankton in Narragansett Bay,Rhode Island

Seasonal changes in phytoplankton biomass and production, total zooplankton biomass, and biomass and potential production rates of the two dominant copepods, Acartia hudsonica (formerly called Acartia clausi) and Acartia tonsa are described for several stations in Narragansett Bay, R.I. Plankton in the bay behaved as a single population with simultaneous changes occurring at the upper bay (Station 5) and the lower bay (Station 1). Phytoplankton biomass was higher in the upper bay ( \(\bar x\) =16.95 mg chl a·m^?3) than in the lower bay ( \(\bar x\) =6.37 mg chl a·m^?3) and these 0269 0101 V differences in biomass were reflected in the phytoplankton production rates. The zooplankton, which was dominated by A. hudsonica in the spring and early summer and A. tonsa during summer and fall, showed no such consistent differences between the stations. Mean A. hudsonica biomass (St 1, \(\bar x\) ;=82.7 mg dry wt·m^?3; St 5, _ \(\bar x\) ;=95.2 mg dry wt·m^?3) exceeded that of A. tonsa (St 1, \(\bar x\) ;=56.7 mg dry wt·m^?3; St 5, \(\bar x\) ;=60.0 mg dry wt·m^?3). Potential production rates of the two Acartia 0269 0101 V spp. were strongly temperature dependent. Despite the higher biomass levels of A. hudsonica, low temperatures resulted in lower potential production rates ( \(\bar x\) ; St 1=7.25 mg C·m^?3 day^?1; \(\bar x\) ; St 5=10.77mg C·m^?3 day^?1) and biomass doubling times of up to 9.6 days. Potential production rates of A. tonsa at summer temperatures were high ( \(\bar x\) ; St 1=19.0 mg C·m^?3 day^?1; \(\bar x\) ; St 5=22.9 mg C·m^?3 day^?1) and biomass doubling times were generally less than one day.     

Egg production of the copepodAcartia tonsa in Florida Bay during summer. 1. The roles of food environment and diet

The diet and egg production rate ofAcartia tonsa were measured during the thermally stable period between June and October 1995 at four locations in inner and outer Florida Bay. We sought to characterize the role ofA. tonsa in the bay’s pelagic food web, which has been changing since 1987, when the dominant submerged vegetation began shifting from benthic seagrasses to planktonic algae. At Rankin Lake, a shallow basin on the north side of the inner bay, where extensive seagrass mortality and persistent cyanobacteria blooms have occurred, microplankton biomass was relatively high and dominated by heterotrophic protists and dinoflagellates. Nanoplankton at Rankin, Lake, while numerically abundant, usually contributed only a small portion of the biomass. The ingestion rate ofA. tonsa in Florida Bay varied independently of food concentration (i.e., total microplankton biomass), but rates were higher (mean±SD =3.88 ± 0.73 μg C copepod^?1 d^?1) on the north side of the bay than on the south side (0.78 ±0.11 μg C copepod^?1 d^?1). Microzooplankton and dinoflagellates were important dietary constituents, especially in the vicinity of Rankin Lake. Egg production in this region (mean ± SD = 14.2 ± 7.7 eggs female^?1 d^?1) was considerably high than the baywide mean (5.8±0.81 eggs female^?1d^?1), and principal components analysis revealed associations between egg production and both dietary microzooplankton and dinoflagellate biomass. However, although grazing rates were relatively high in the inner bay,A. tonsa removed only 1–6% of the primary production from the water column during the summer and its egg production rates were low relative to typical rates for the species.     

Effects of zooplankton grazing on phytoplankton size-structure and biomass in the lower Hudson River estuary

The impact of mesozooplankton (>210 μm, mostly adult copepods and late-stage copepodites) and micrometazoa (64–210 μm, mostly copepod nauplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various¹⁴C-labeled algal species as tracers of grazing on natural phytoplankton. During spring and summer, zooplankton grazing pressure, defined as %=mg C ingested m^?2 h^?1/mg C produced m^?2 h^?1 (depth-integrated rates)×100, on total phytoplankton ranged between 0.04% and 1.9% for mesozooplankton and 0.1% and 6.6% for micrometazoa. The greatest grazing impact was measured in fall when 20.2% and 44.6%, respectively, of the total depth-integrated primary production from surface water phytoplankton was grazed. Mesozooplankton exhibited some size-selective grazing on phytoplankton, preferentially grazing the diatomThalassiosira pseudonana over the larger diatomDitylum brightwelli, but this was not found for micrometazoa. Neither zooplankton group grazed on the dinoflagellateAmphidinium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estuary. Differences in the growth coefficients of various phytoplankton size-fractions—not grazing selectivity—may be the predominant factor explaining community size-structure.     

Trends in copepod communities in the Navesink and Shrewsbury rivers,New Jersey: 1962–1992

The calanoid copepod community was surveyed semi-monthly, from May to July 1992, at three stations in the Navesink-Shrewsbury rivers system, the southernmost branch of the Hudson-Raritan estuary (New York-New Jersey). The dominant species collected during the survey wasAcartia hudsonica, followed byA. tonsa. A comparison of this survey with three earlier surveys suggests that the calanoid copepod community and relative abundance of dominant species have not changed substantially since the 1960s. Findings from a 1972 study, which noted the absence ofA. hudsonica andPseudodiaptomus coronatus as dominant species, were probably reflecting a temporary situation. The variations may have been related to a change in water quality, caused by an upgrade in sewage treatment, completed just prior to the 1972 survey, and/or resulted from the residual effects of Tropical Storm Agnes on this estuary.     

Succession of copepod species in a Middle Atlantic estuary

A study of the seasonal succession of dominant copepod species was conducted during the period May, 1972 to June, 1973 in the Navesink River estuary, a tributary of the New York Bight. The replacement of the copepod Acartia tonsa by Acartia clausi, a phenomenon well-documented in the middle Atlantic estuaries for the late winter and early spring seasons, was not observed during this study, indicating that this succession may not take place in the Navesink. Instead, the more brackish-water calanoids, Pseudodiaptomus coronatus and Eurytemora affinis replaced A. tonsa, increasing in numbers markedly as the A. tonsa population declined. Although A. clausi is known to occur in temperatures and salinities comparable to those of the Navesink, this study supports the results of Yamazi (1966) that the occurrence of A. clausi in the Navesink is a rarity.     

Variations in stable carbon isotope ratio of the copepod Acartia tonsa during the onset of the Texas brown tide

The Laguna Madre of South Texas is a shallow coastal lagoon whose dominant primary producers shifted from seagrasses to phytoplankton with the onset of the Texas brown tide, which persisted from 1990 through 1997. Acartia tonsa is the dominant component of the mesozooplankton and forms an important link in both the phytoplankton and detritus-based pelagic food webs. Stable carbon isotope ratios of A. tonsa, as well as the two major primary producers: phytoplankton (as particulate organic carbon) and seagrasses, were measured from March 1989 to October 1991. Zooplankton samples were collected at four locations in the Laguna Madre: two in shallow water (c. 1 m) over seagrass beds and two in slightly deeper water (c. 2–3 m) over a muddy bottom in a secondary bay without seagrasses. We found seasonal trends in the isotopic composition of A. tonsa collected within both habitats as well as distinct differences between the average {ie995-1} values of individuals collected in the two regions. Isotopic ratios of animals collected during the summer months were generally 4–8‰ enriched in ¹³C compared with those collected in the winter, at all stations. A. tonsa collected over seagrass beds were 2–5‰ more enriched in ¹³C than those collected over muddy bottoms. These observations suggest carbon derived from seagrasses can be an important source of nutrition for these copepods in summer, especially for copepods living over seagrass beds. The effects of the persistent brown tide decreased the contribution of seagrasses as a carbon source for A. tonsa during the summer of 1991. The pathway by which seagrass carbon enters the diet of A. tonsa is unclear, but the two pathways considered most likely are through copepods feeding on microzooplankton that have fed on bacteria nourished on seagrass carbon, or by copepods feeding directly on particles of seagrass detritus.     

Feeding rates of the sea nettle,Chrysaora quinquecirrha,under laboratory conditions

The scyphomedusa, Chrysaora quinquecirrha, preys on a variety of estuarine organisms. A series of experiments were conducted to measure the feeding rate of adult medusae on zooplankton prey. Artemia was used as a substitute for the endemic and abundant copepod, Acartia tonsa. The feeding rates ranged from three Artemia per hour per ml of medusae volume to 107 per hour per ml in 51 trials. The consumption was also linearly related to the initial prey concentration. Certain results suggested that a toxic factor might be exuded by the medusae, affecting the observed feeding rates. The results verify the possibility that Acartia may be a major prey organism in estuarine environments.     

Population density, prey selection, and predator avoidance of the burrowing anemone (Ceriantheopsis americanus) in Narragansett Bay, Rhode Island

The maximum population density of the burrowing anemone (Ceriantheopsis americanus) was estimated at 17–28 animals m^?2 in soft-bottom sediments of mid Narragansett Bay. The gut contents of the anemone indicated primary prey of harpacticoid and calanoid copepods. The consumption of calanoid copepods was higher in October than April, which may be due to decreased density of hapacticoids in the fall. The anemones apparently avoid fish predation in late summer by withdrawing into the sediments. After the seasonal fall migration of fish out of the bay, anemones reappear.     

Zooplankton grazing in a Potomac River cyanobacteria bloom

During summer, bloom-forming cyanobacteria, including Anacystis, Aphanizomenon, and Microcystis aeruginosa, dominate tidal-fresh waters of the upper Potomac River estuary with densities exceeding 10⁸ cells l^?1. In an attempt to determine the importance of these high cyanobacteria densities to planktonic herbivory in the system, short-term grazing experiments were conducted in July and August 1987. Using size-fractionated river phytoplankton assemblages, zooplankton grazing rates were determined for dominant or subdominant planktonic microzooplankton and mesozooplankton feeding on ¹⁴C-labeled river assemblages, ¹⁴C-labeled river assemblages enriched with unlabeled cyanobacteria, and unlabeled river assemblages enriched with ¹⁴C-labeled cyanobacteria. Grazing rates were estimated for the rotifers Polyarthra remata, Hexarthra mira, Asplanchna brightwelli, Brachionus angularis, Epiphanes sp., Trichocerca similis, and the cyclopoid copepod Cyclops vernalis. Neither rotifers nor the copepod grazed heavily on Microcystis. Rotifer grazing rates on labeled cyanobacteria ranged from 4 to 1,650 nl· [individual · h]^?1 while copepod rates ranged from undetectable to 135 μl · [copepod · h]^?1. Grazing rates on labeled river phytoplankton assemblages were 4–100 times higher than noted for zooplankton feeding on cyanobacteria. The addition of the colonial alga to labeled river phytoplankton assemblages resulted in mixed zooplankton responses, that is, lower and higher grazing rates than observed on river assemblages with no added cyanobacteria. Total zooplankton demand for cyanobacteria and river phytoplankton assemblages was estimated for the study period July–August 1987. Rotifer plus C. vernalis herbivory would have removed 1–5% and 49%, respectively, of the standing stock of the two autotroph pools each day. Literature-derived clearance rates for Bosmina indicate, however, that herbivory by this cladoceran could increase demand to 24% and 60%, respectively, in bloom and nonbloom assemblages. These data suggest that the majority of cyanobacterial production remains ungrazed and may be transported to the lower estuary for salinity-induced aggregation and sedimentation.     

Salinity stress in harpacticoid copepods

Three species of intertidal harpacticoid copepods,Tigriopus japonicus, Tachidius brevicornis andTisbe sp., were tested for their response to salinities ranging from 0‰ to 210‰. At 90‰Tigriopus became dormant, but could be reanimated if placed in 30‰ seawater within 18 hours.Tachidius became dormant at 60‰ but could also be revived if placed in 30‰ seawater.Tisbe died shortly after an exposure to seawater of 45‰. Death was brought about inTigriopus andTachidius by salinities of 150‰. Naupliar, copepodite, and adult stages ofTigriopus withstood the salinities equally well, while egg sacs could tolerate five times the length of exposure of these stages. The respiratory rate ofTigriopus was 3.76 (±1.32) μl 0₂ hr^?1 for active adults and 0.03 (±0.01) nl 0₂ hr^?1 mg dry wt^?1 for adults in the dormant state. The duration of dormancy, with successful revival, appears to be limited to the time before the loss of the vital water content of the tissues due to the high osmotic pressure. The ability to enter dormancy in times of stress may have high survival value to some intertidal copepods.     

Parasites and cyclopoid predators of age-0 fish in the Roanoke River,North Carolina

Altered river flow has been suggested as a cause for the low recruitment of striped bass,Morone saxatilis, in the Roanoke River (North Carolina) because of its effect on the proximity of zooplankton and larval striped bass. This results in unsuccessful feeding and subsequent starvation, which was considered to be a major mortality factor. Other mortality factors, such as parasitism and copepod predation on age-0 fish, may also be regulated to some extent by changes in river flow. The relationship of cestode plerocercoids, trematode metacercaria, mussel glochidia, and cyclopoid copepod predators with age-0 fish was evaluated in the lower Roanoke River and western Albemarle Sound from plankton net collections made in 1984 to 1986 and 1988. Plerocercoid prevalence was higher under low river flow conditions than under high flow conditions in darters (Percidae; 16.7% vs. 9.2%), minnows (Cyprinidae; 28.8% vs. 4.7%), andMorone (1.9% vs. 0%). Gut analysis of the age-0 fish revealed that copepods (source of the plerocercoids) were a major diet component ofMorone and darters but not of minnows or herring (Clupeidae). Decreases in river flow were associated with increases in copepod density (Pearson r=?0.62; p=0.0001) and plerocercoid prevalence inMorone (Pearson r=?0.29; p=0.03). The low correlation value forMorone may be quite strong considering the complexity of the variables associated with prevalence. Metacercaria were found only inMorone and minnows, and prevalence and mean intensity were less than that found for plerocercoids. Mussel glochidia prevalence was less than 0.5% for all affected taxa, an order of magnitude less that that found in other studies. The low value may indicate that the mussel population in the Roanoke River is declining. Prevalence of attacks by the predatory copepodMesocyclops edax on age-0 fish was similar to that in Chesapeake Bay, and striped bass was the primary prey. Spatial and temporal proximity of copepods and fish prey may be the key factors in regulating copepod attacks. The low prevalence of parasites and copepod predators seen in this tudy would suggest that mortality from these sources may not be a major factor in age-0 recruitment in this system. Confirmation of these conclusions would require a more controlled experimental approach.     

In situ feeding rates of the ctenophoreMnemiopsis mccradyi

Prey ingested byMnemiopsis mccradyi collected in Link Port (Indian River, east coast of central Florida) consisted mostly of copepod nauplii, barnacle nauplii, bivalve veligers, and adult copepods (Acartia sp.,Oithona sp.); abundances in that order. Compared to relative in situ densities, there was an increased consumption of barnacle nauplii, bivalve veligers, andAcartia sp., and a decreasesed consumption of copepod nauplii andOithona sp., implying that prey selection had occurred. In situ clearance rates (based on numbers of ingested prey, digestion rates, and in situ prey densities) forM. mccradyi (5 cm mean length) were 0.1 to 1.31 h^?1 individual^?1, depending on prey taxon. These rates are less than those measured previously in the laboratory; however, it is not possible to state if this difference is statistically significant.     

Zooplankton dynamics in an intertidal salt-marsh basin

We determined tidal, diel (day-night), and diurnal (day to day) patterns of occurrence for the summer zooplankton assemblage in an intertidal salt marsh basin at North Inlet Estuary, South Carolina. In one time series, 153 μm pump and 365 μm net collections were made every 1–2 h during four consecutive tidal cycles. Taxonomic composition remained unchanged throughout most of the 48-h period, but densities and proportionalities of individual taxa were highly variable. Recurring patterns of abundance were observed and taxon-specific relationships with the tidal and diel cycles were indicated. Zooplankton were not uniformly distributed within flooding-ebbing water masses and distributions could not be explained by simple passive advection with the tides. Diel differences in densities of copepods and bivalves resulted from behavioral responses to changing light conditions. Large pulses of crab and shrimp larvae originating from nocturnal hatching events within the intertidal basin exited but did not return during the next flood tide. Higher densities of postlarval decapods on flood tides indicated settlement and recruitment to the shallow basin. In a second time series, replicated collections of the 153 μm and 365 μm assemblages were made during the daytime ebb tide every 1–3 d from May through October 1991 to determine relationships between diurnal changes in depth, salinity, and temperature and zooplankton composition and abundance. Diurnal variations in densities and proportionalities were less than those observed during the 48-h study and patterns were not regular. For most taxa, relationships between depth and abundance were the same in both time series. During periods of reduced salinity, densities of copepods,Uca zoeae, and barnacle nauplii decreased and densities ofUca megalopae andPenaeus postlarvae increased. However, zoeae emerged and postlarvae recruited throughout the 5-mo period, indicating that considerable flexibility in responses and tolerances existed within the populations. The diversity of life-history strategies and behavioral adaptations found among the zooplankton assures continuous occupation of flooded intertidal habitats. We suspect that the evolution and maintenance of temporally staggered recurring patterns of occurrence results in reductions in the competition for resources.     

Diel vertical migration of zooplankton in contrasting habitats of the Chesapeake Bay

Two different habitats in the Chesapeake Bay were tested for the occurrence of diel vertical migration (DVM) by members of the zooplankton community. During July of 1997 diel samples were collected from Jones Creek, a small, protected, 3-m deep tidal gut, and a station in Hampton Roads, a 20-m deep, expansive section of the bay. Temperature, salinity, light penetration, water clarity, dissolved oxygen, total particulate nitrogen and carbon, chlorophyll concentration, and algal density were also determined for depth increments at these stations, ca. midday and midnight. Significant differences in zooplankton depth distribution occurred over the day at both stations. The calanoid copepod,Acartia tonsa, dominated the zooplankton community and the larger stages were strong migrators. Mysid shrimp, the largest sized crustaceans, displayed significant DVM at both stations. Although diel differences were also detected in chlorophylla concentrations, with 25% reduction at night in Jones Creek and 18% in Hampton Roads, counts of algal cells revealed no differences between night and day numbers. The nightly losses of chlorophyll were not driven by mesozooplankton grazers. Using grazing rates from the literature, we estimated that mesozooplankton harvested less than 5% of the standing crop of phytoplankton each day in both locations. Food quantity and quality (as determined by chlorophyll concentrations, cell counts, and C:N ratio) were uniform in the Jones Creek water column, but higher near the surface in Hampton Roads. While it was clearly advantageous for zooplankton to migrate to surface waters at night for feeding in Hampton Roads, it is paradoxical for DVM to persist in Jones Creek when ample food was found in deeper water.     

Selenium in San Francisco Bay zooplankton: Potential effects of hydrodynamics and food web interactions

The potential toxicity of elevated selenium (Se) concentrations in aquatic ecosystems has stimulated efforts to measure Se concentrations in benthos, nekton, and waterfowl in San Francisco Bay (SF Bay). In September 1998, we initiated a 14 mo field study to determine the concentration of Se in SF Bay zooplankton, which play a major role in the Bay food web, but which have not previously been studied with respect to Se. Monthly vertical plankton tows were collected at several stations throughout SF Bay, and zooplankton were separated into two operationally defined size classes for Se analyses: 73–2,000 μm, and ≥2,000 μm. Selenium values ranged 1.02–6.07 μg Se g^?1 dry weight. No spatial differences in zooplankton Se concentrations were found. However, there were inter- and intra-annual differences. Zooplankton Se concentrations were enriched in the North Bay in Fall 1999 when compared to other seasons and locations within and outside SF Bay. The abundance and biovolume of the zooplankton community varied spatially between stations, but not seasonally within each station. Smaller herbivorous-omnivorous zooplankton had higher Se concentrations than larger omnivorous-carnivorous zooplankton. Selenium concentrations in zooplankton were negatively correlated with the proportion of total copepod biovolume comprising the large carnivorous copepodTortanus dextrilobatus, but positively correlatid with the proportion of copepod biovolume comprising smaller copepods of the family Oithonidae, suggesting an important role of trophic level and size in regulating zooplankton Se concentrations.