Preliminary results of an experimental study of the effects of mysid predation on estuarine zooplankton community structure

In a shallow estuarine system near Beaufort, North Carolina, a period of high winter abundance of the mysidsMysidopsis bigelowi andNeomysis americana was associated with a change in zooplankton species composition, from dominance byAcartia tonsa to dominance byCentropages spp. andSaphirella sp. Both mysids feed onA. tonsa at higher rates than the other copepods. Experiments were carried out in 600–1 000 liter enclosures, in which the initial mysid density was manipulated and the effects on the enclosed copepod community were monitored. Mysid predation had a significant effect on copepod densities. The effects of mysid predation on species composition appeared to depend on the relationship between their prey preferences and the dominant copepod species present in the communities. Under conditions favoring dominance byA. tonsa, the preferred prey species, the results suggested that mysid predation may reduce dominance and increase diversity. But when the less preferredCentropages was dominant, mysid predation had no effect on species composition.     

Assessment of the potential impact of the mysid shrimp, Neomysis mercedis, on Daphnia

Neomysis mercedis, an important invertebrate predator in somelakes and estuaries in North America, occurred at high densities(>2 mysids m^-3) in Lake Washington in the early 1960s, aperiod when Daphnia were scarce in the lake. Because Neomysisfeed selectively on Daphnia, it was hypothesized that mysidpredation contributed significantly to the scarcity of Daphnia.To evaluate this hypothesis, mysid abundance was monitored inthe lake (July 1989–February 1992), and whole lake predationimpacts on Daphnia were estimated. Mysid abundance varied from319 million, 0.1 mysids m^-3 (February 1992) to 4276 million,1.7 mysids m^-3 ( June 1991), and mysid biomass ranged from 1153(October 1991) to 4700 kg dry weight (November 1990). A peakin Daphnia consumption was noted during autumn, when total mysidbiomass was high. In late autumn/winter mysid consumption month^-1accounted for 28–95% of Daphnia biomass and 13–38%of production. Mysid densities in this study were 18x and 5xlower than in 1962 and 1975, respectively. By extrapolation,at a density of 3.5 mysids m^-3 observed in the early 1960s,total Neomysis consumption demand month^-1 exceeded 100% of Daphniabiomass (late autumn–early spring), and 100% of Daphniaproduction (late autumn–winter). Estimates of the totalmysid consumption demand in this study are thus in accord withthe hypothesis that Neomysis could control Daphnia abundancein Lake Washington.     

Predation by longfin smelt ( Spirinchus thaleichthys ) on the mysid Neomysis mercedis in Lake Washington

1. Longfin smelt and mysid samples collected in Lake Washington from 1988 to 1992 were examined, and historic data on smelt and mysid abundance were reviewed to assess the impacts of smelt predation on Neomysis mercedis. The feeding habits of smelt were also examined for differences between odd and even years, and seasonal and ontogenetic variations in smelt diet were described. 2. A reciprocal relationship in the abundance of 1+ smelt and Neomysis suggested that smelt regulate mysid abundance in the lake. 3. The proportion of smelt diet contributed by mysids was higher for less abundant year classes of smelt. 4. These results indicate that intraspecific competition for scarce mysids occurs in the even-year classes of smelt.     

Importance of seagrass vegetation for habitat partitioning between closely related species,mobile macrofauna <Emphasis Type="Italic">Neomysis</Emphasis> (Misidacea)

Seagrass meadows provide both habitats and a range of food sources for macrofaunal communities. These functions facilitate the coexistence of less mobile invertebrates (in comparison with mysids, such as amphipods) that are associated with seagrass leaves, and may also enhance the coexistence of highly mobile invertebrates such as mysid. We investigated the function of seagrass in supporting the coexistence of two mysid species, Neomysis awatschensis and N. mirabilis. These taxa are dominant in seagrass ecosystems of temperate coastal areas. We compared patterns of habitat use between the two species at mesoscales (among seagrass patches) and microscales (among seagrass leaves) by performing field surveys and laboratory experiments. The field survey results showed positive correlations in the abundance of the two mysid species, indicating that both species select similar habitats at the mesoscale level. In the laboratory experiments, the pattern of microhabitat selection (fundamental habitat) was similar for both species, even at increased densities and with the presence of an immobile habitat-competitor (the gastropod Barleeia angustata) on the leaves. However, this pattern changed significantly when a food source (epiphytic microalgae) was present on the leaves. This result indicates that (i) inter- and intraspecific interference competition does not affect microhabitat selection in these two mysids and (ii) both Neomysis species use similar habitats at the feeding stage. Although these two closely related mysids species may have similar requirements for microhabitat and food, the evidence that they did not act as competitors is attributable to unrestricted microhabitat and food (e.g., epiphytic algae) in the presence of seagrass vegetation.     

Life history and production of Neomysis mercedis in two British Columbia coastal lakes

The life history and production characteristics of Neomysis mercedis from two British Columbia lakes were examined for their potential influence on zooplankton and limnetic fish communities. During the day, mysids in shallow Muriel Lake (45 m) were on or near the bottom; in Kennedy Lake (> 100 m), mysids remained deeper than 50 m. In both lakes, mysids spent summer nights at 0–15 m depths despite > 20 °C temperatures. Mysid density was not strongly correlated with lake depth. Mysids generally displayed spring to early summer minima and late summer to fall maxima in numbers and biomass. Single peaks in gravid females and juvenile mysid abundance, and the absence of pronounced seasonal size changes of gravid females suggest that N. mercedis produced a single generation each year. Fecundities of study lake mysids are the lowest on record, and although size-dependent, exhibited unusually high variability. Annual productivity of mysids averaged 485 mg m^–2 y^–1 (range 205–690). Calculations indicate mysids consume several times more zooplankton per annum than limnetic fish do. N. mercedis is likely an important competitor of juvenile sockeye salmon (Oncorhynchus nerka) since: (i) sockeye exhibit food limited growth and survival patterns in coastal lakes, (ii) mysids and sockeye consume similar zooplankton prey and (iii) mysids do not contribute greatly to sockeye diet (i.e. < 26% of summer and fall diets by numbers or weight).     

Predation by Neomysis mercedis: effects of temperature, Daphnia magna size and prey density on ingestion rate and size selectivity

1. Neomysis mercedis predation rates on Daphnia magna were determined under laboratory conditions. There were generally no consistent differences between the number of Daphnia ingested at 10 and 14°C. 2. At each temperature, the number of prey consumed increased with mysid size and decreased with Daphnia size. 3. For small prey the relationship between ingestion rate and prey density represented a Type II functional response. However, for larger prey there was no significant relationship between density of prey and consumption by mysids. 4. The pattern of size-selective predation by Neomysis was studied to test the optimal foraging hypothesis. For prey populations with mixed size classes, the smallest size of prey was consumed most frequently but intermediate size prey provided the greatest biomass. These observations are contrary to our predictions based on calculations of profitability of different sizes of prey.     

Neomysis integer in a shallow hypertrophic brackish lake: distribution and predation by three-spined stickleback (Gasterosteus aculeatus)

The mysid shrimp Neomysis integer is a common invertebrate predator in brackish waters of Western Europe and is thought to play a central role in the food web owing to its predation on zooplankton. Neomysis distribution and abundance were investigated for 3 years in brackish, shallow and hypertrophic Lake Ferring (surface area: 3.2 km², mean depth: 1.4 m, salinity: 3–6, total P: 0.29–0.78 mg P l^-1, Secchi depth: 0.14–0.22 m). Mean summer abundance of Neomysis varied from 53 to 882 ind. m^-2. Neomysis density within the lake was relatively uniform and not related to sediment type, but increased with increasing depth. The high abundance of Neomysis is considered to reflect a fish stock almost completely dominated by small-sized fish species (mainly three-spined stickleback, Gasterosteus aculeatus). Three-spined stickleback density was high and catch per unit effort ranged between 30 and 80 per gill net. Stomach analyses showed that the sticklebacks preyed on Neomysis, but preferred specimens smaller than 3–4 mm, and only occasionally consumed those larger than 5–7 mm. In summer, between 33 and 67% of the Neomysis ingested by sticklebacks were smaller than 3 mm, while in the lake as a whole, only 5–14% were smaller than 3 mm. The periods when Neomysis is vulnerable to stickleback predation are restricted to a few weeks in late May and late July, when the new generations emerge. Sticklebacks therefore have a limited capacity for controlling large Neomysis, including gravid females.     

The changing ecosystem of a shallow, brackish lake, Hickling Broad, Norfolk, U.K. I. Trophic relationships with special reference to the role of Neomysis integer

• 1 Hickling Broad, a shallow, brackish lake in England, changed from dominance by submerged aquatic plants to dominance by phytoplankton in the 1970s. These changes were ascribed to the effects of guanotrophicatioh by black-headed gulls, and increased salinity which together resulted in fish kills caused by a toxic alga, Prymnesium parvum. A mysid, Neomysis integer, was believed to be important in switching the system to plankton dominance through its presumed selective feeding on Cladocera and increasing population size as fish predation decreased.
• 2 Studies on laboratory predation rates of the mysid on Cladocera and on the population dynamics, predation rates and diet of the mysid and the populations of the major zooplankter, Eurytemora affinis in the Broad in the 1980s, have shown that the former explanations were incomplete. The mysid could have markedly reduced the cladoceran community, for it has potentially high predation rates, but Cladocera were probably lost through salinization. Neomysis feeds efficiently on Eurytentora affinis, but the latter reproduces rapidly and its populations are unlikely to be controlled by mysid predation. Nauplii and small copepodites are selectively taken. Alternative and probably major food sources for the mysid are periphyton and benthic algae and detritus.
• 3 Roach and bream readily take the mysid but the present fish stock in the Broad is very low and can now exert minimal pressure on the mysid population. An invertebrate predator on the mysid, Palaemometes varians, is also unlikely to have major effects. The former model of the operation of the Broad's ecosystem needs reconsideration in view of the findings.

     

Distribution and salinity tolerance of marine mysids from a subtropical estuary,Brazil

We studied the spatial and temporal distribution pattern of three marine mysids (Chlamydopleon dissimile, Metamysidopsis elongata atlantica and Mysidopsis coelhoi) from the Cananeia-Iguape Coastal System, southeast Brazil, in relation to environmental variables and zooplankton abundance. We also analysed mysid reproductive traits, and tested their tolerance to salinity gradients in laboratory experiments. Salinity was identified as a major factor influencing mysid spatial and temporal distribution, selectively limiting their distribution within the estuary. Chlamydopleon dissimile and M. coelhoi were common, but occurred at low abundances throughout the year, whereas the numerically dominant M. elongata atlantica had abundance peaks in spring, when thermohaline conditions were more favourable. Mysid brood size varied interspecifically; the occurrence of juveniles throughout the year indicated that mysids breed continuously in the region. The high biomass of M. elongata atlantica observed in spring confirms the importance of mysids in coastal regions' food webs.     

Diet of the Chilean sandperch, Pinguipes chilensis (Perciformes, Pinguipedidae) in southern Chile

A study of the Pinguipes chilensis Valenciennes 1833 diet was carried out on 414 specimens captured on the Talcahuano, Chile coastline (36°41′S; 73°06′E) between October 1998 and March 2000. P. chilensis, endemic to the southeast Pacific (from Tumbes, Peru to Magallanes, Chile), is confirmed as a general predator over a wide trophic spectrum including 61 taxa. Crustaceans (mostly mysids, but also a wide variety of decapods) made up the principal dietary component, followed by fish and annelids. Prey belonging to Mollusca, Ophiuroidea, Echinoidea, Rhynchocoela, Sipunculida, Echiura and Urochordata were also present. There were no significant differences between male and female diets, although they shared only 32 prey. A significant dietary difference was found, however, between analysed size groups: specimens under 30 cm total length (TL) preyed mainly on the amphipod, Ampelisca araucana; those over 30 cm TL preyed on the mysid, Neomysis sp.; and those over 40 cm TL on fish, annelids, molluscs and echinoderms. Seasonal differences, due in large part to the dynamics of the prey species, were also found to be significant. Crustaceans were the predominant prey throughout the study with the exception of autumn, when fish were the most common prey.     

Predatory disturbance and prey species diversity: the case of gray whale (<Emphasis Type="Italic">Eschrichtius robustus</Emphasis>) foraging on a multi-species mysid (family <Emphasis Type="Italic">Mysidae</Emphasis>) community

Why competitive exclusion does not limit the number of coexisting plankton species is a persistent question for community ecology. One explanation, the intermediate disturbance hypothesis (IDH), proposes that elevated species diversity is a product of moderate levels of disturbance that allow the subsequent invasion of less competitive species. Here, we assess the shifts in species diversity in a mysid (Mysidae Dana, 1850) zooplankton community, where at least 10 species have, over the last 15 years, have come to comprise the primary prey base of summer resident gray whales (Eschrictius robustus Lilljeborg, 1861) in Clayoquot Sound, British Columbia. We evaluate trends in the community structure of mysids (species dominance, diversity, and richness) across mysid habitat in the study area during the gray whale foraging season (May–September) for the period 1996 and 2008. Mysid species composition varies among years and diversity has increased as whales shifted their predatory focus from benthic amphipods (Ampeliscidae Costa, 1857) to mysids, near our starting point in 1996. Holmesimysis sculpta Tattersall, 1933 was the dominant species in early years; however, in 2007, the dominance shifted to Neomysis rayi Murdoch, 1885. The habitat restrictions and life history attributes of local populations of coastal mysids leave them vulnerable to the cumulative impacts of increased predation pressure by gray whales. This case study presents a unique examination implicating predation as an agent of disturbance capable of altering the species structure of a local prey community.     

Modelling catch per unit effort of the Black Volta near the Bui dam in Ghana

The chlorophyll a concentration and water level of the Black Volta near the Bui dam were studied in relation to fish production as measured by catch per unit effort (CPUE) between February 2011 and December 2012. The primary objective was to develop a simple linear regression model for predicting CPUE levels. The mean estimated CPUE for 2011 and 2012 was lower (6.23 kg canoe^?1 day^?1) in the postwet season than in the dry season (10.86 kg canoe^?1 day^?1) with a mean of 7.95 kg canoe^?1 day^?1. Hence, the dry season was the most important season for fish catches in the study area. Predictor variables that significantly explained CPUE levels were chlorophyll a (positive correlation) and water level (negative correlation) (P = 0.0002). The model was validated with independent data from the same Black Volta in 2011 and 2012. This model, CPUE = (0.062 × chlorophyll a) ? (0.456 × water level) + 3.363, explained 91% CPUE variability. Independent validation indicated that the model had the potential to predict CPUE (as a measure of fish production) in the Black Volta near the Bui dam. Hence, the model is also a valuable tool to predict future trends in the CPUE levels of the Black Volta.     

Development of a quantitative enzyme-linked immunosorbent assay for vitellin in the mysid Neomysis integer (Crustacea: Mysidacea)

Mysid crustaceans have been put forward by several regulatory bodies as suitable test organisms to screen and test the potential effects of environmental endocrine disruptors. Despite the well-established use of mysid reproductive endpoints such as fecundity, egg development time, and time to first brood release in standard toxicity testing, little information exists on the hormonal regulation of these processes. Control of vitellogenesis is being studied intensively because yolk is an excellent model for studying mechanisms of hormonal control, and vitellogenesis can be chemically disrupted. Yolk protein or vitellin is a major source of nourishment during embryonic development of ovigorous egg-laying invertebrates. The accumulation of vitellin during oocyte development is vital for the production of viable offspring. In this context, we developed a competitive enzyme-linked immunosorbent assay (ELISA) for vitellin of the estuarine mysid Neomysis integer. Mysid vitellin was isolated using gel filtration, and the purified vitellin was used to raise polyclonal antibodies. The ELISA was sensitive within a working range of 4 to 500 ng vitellin/mL. Serial dilutions of whole body homogenates from female N. integer and the vitellin standard showed parallel binding curves, validating the specificity of the ELISA. The intra- and interassay coefficients of variation were 8.2% and 13.8%, respectively. Mysid vitellin concentrations were determined from ovigorous females and eggs at different developmental stages. The availability of a quantitative mysid vitellin ELISA should stimulate further studies on the basic biology of this process in mysids. Furthermore, it could provide a means to better understand and predict chemically induced reproductive effects in mysids.     

Dietary shifts in a stressed fish assemblage: Consequences of a bivalve invasion in the San Francisco Estuary

We compared dietary patterns within a temperate estuarine fish assemblage (Suisun Marsh, CA, U.S.A.) during a period of high mysid shrimp abundance and after a major decline in mysid abundance caused by the invasion of the overbite clam Potamocorbula amurensis. Prior to the invasion, high dietary overlap, high stomach fullness, and low niche breadth occurred among the fishes in spring when mysid populations were high. Dietary overlaps decreased and niche breadth increased for all species but the endemic splittail Pogonichthys macrolepidotus in fall when mysid populations were low. Eight native species exhibited lower overall collective overlaps and fuller stomachs than five alien species, suggesting more efficient resource partitioning. After mysid abundance declined, only alien striped bass Morone saxatilis preyed upon mysids in greater than trace amounts. An alien mysid became an important prey for small striped bass, but striped bass also switched to piscivory at a smaller size than when mysids were abundant. Eight of 13 species exhibited significant declines in abundance during the study period, which were concordant with the original importance of mysids in their diets. Our results suggest that altered lower food web dynamics in the San Francisco Estuary caused by the invasion of the overbite clam changed fish diets and have contributed to declines in fish abundance.     

Ecology of mysid shrimps in the Bornholm Basin (central Baltic Sea)

Mysid shrimps are an important trophic link in the food web of the Baltic Sea. In 2002 and 2003 we investigated species composition, distribution, life cycle and prey in the Bornholm Basin (central Baltic). Three species and one genus were identified: Mysis mixta, Neomysis integer, Mesopodopsis slabberi and Praunus sp. The dominant M. mixta and N. integer occurred in the whole study area but were more abundant in the peripheral, more shallow regions. Size frequency distribution indicated one generation for M. mixta and probably two for N. integer. Main prey of all size classes in both species was the cladoceran Bosmina coregoni maritima during summer and autumn. In spring and winter also copepods of the species Temora longicornis and Pseudocalanus acuspes were important prey items. Detritus and phytoplankton cells were of minor importance and many individuals had empty guts. Mysid shrimps in the deep basin might be advected from more shallow areas, as the abundance of all species was much lower than in coastal regions. The deep Bornholm Basin does not seem to be a favourable habitat, probably due to the inaccessibility of the oxygen deficient bottom layer. Under current hydrographic conditions mysids are probably not important predators of zooplankton and thus not competitors of planktivorous fish in the Bornholm Basin.     

The mysid-feeding guild of demersal fishes in the brackish zone of the Westerschelde estuary

The demersal fish fauna of the mesohaline zone of the Westerschelde estuary (south-west Netherlands) was sampled intensively in the period 1990–1992. Almost 500 beam trawl samples were taken in both subtidal (330 samples) and intertidal (144 samples) habitats. These yielded 44 fish species, mostly as juveniles. The area functioned as a nursery for several demersal fish species, and harboured large populations of hyperbenthic mysids. Three gobies, three flatfish, one clupeoid and one gadoid dominated the fish fauna, while three mysid species were important components of the holohyperbenthos. From c. 1500 stomach contents of 25 fish species, 44 prey species were identified, the most abundant of which were also common in the hyperbenthal. The demersal fish community consisted of a group that foraged subtidally on fast-moving epi-and hyperbenthic prey (for example gadoids, gobies and clupeoids) and a group that foraged on slow-moving or sessile endobenthic organisms, mainly in intertidal areas (for example most flatfish species). Mysidacea occurred in >50% stomachs analysed and were taken as prey by 19 of the 25 fish species. Mysids were most important in the diets of Pomatoschistus minutus, P. lozanoi, Trisopterus luscus and Merlangius merlangus, and were present in appreciable numbers in Pleuronectes flesus, Trigla lucerna, Clupea harengus and Pleuronectes platessa. These species fed mainly on the brackish water endemic Neomysis integer. Mesopodopsis slabberi (present in 35% of the gobiid stomachs) and Gastrosaccus spinifer(present in 25% of the gadoid stomachs) were of secondary importance. P. minutus and T. luscus showed a diet shift from calanoids (Eurytemora aYnis and Temora longicornis) to mysids at L_S of 30 and 50 mm, respectively. Only 1% of the standing stocks of the N. integer and M. slabberi populations was removed by the local demersal fish community, so top–down control of mysid populations in estuaries seems unlikely.     

The consequences of a drastic fish stock reduction in the large and shallow Lake Wolderwijd,The Netherlands. Can we understand what happened?

In 1990 an experiment started in the large and shallow lake Wolderwijd (2700 ha, mean depth 1.5 m) to improve the water quality. About 75% of the fish stock was removed (425 000 kg fish). The fish was mainly composed of bream and roach. In May 600000 young pikes (3–4 cm) were introduced.In May 1991 the water became very clear (Secchi depth 1.8 m) during a spring bloom of large Daphnia. Then the grazing by zooplankton was eight times higher than the primary production of algae and the total suspended matter concentration became very low. Compared to the situation before the fish reduction, the grazing had increased only slightly, while the primary production had decreased significantly in early spring. The fish stock reduction might have contributed to the reduction in primary production by a reduced internal nutrient load. The clear water period lasted six weeks. Daphnia disappeared in July due to food limitation, the algal biomass increased and the Secchi depth became 50 cm. Daphnia did not recover during summer, due to predation that was not caused by 0 + fish but by the mysid shrimp Neomysis integer. Neomysis could develop abundantly, because of the reduced biomass of the predator perch. The production of young fish had been low because of the cold spring weather. The cold weather was probably also responsible for the slow increase in density of macrophytes. After 1991, perch probably can control Neomysis. Due to lack of spawning places and shelter for 0 + pike, pike was probably not able to control the production of 0 + fish. In a lake of this scale, it will not be easy to get more than 50% coverage of macrophytes, which seems necessary to keep the algal biomass low by nutrient competition. Therefore, we expect also in the future a decrease in transparency in the summer. Locally, especially near Characeae, the water might stay clear.     

Diet and predators of Tenagomysis tasmaniae Fenton, Anisomysis mixta australis (Zimmer) and Paramesopodopsis rufa Fenton from south-eastern Tasmania (Crustacea:Mysidacea)

The diets of three mysid species Tenagomysis tasmaniae Fenton, Anisomysis mixta australis (Zimmer) and Paramesopodopsis rufa Fenton were investigated together with a number of potential fish predators by gut contents and stable isotope analysis. Gut content analysis of the three mysis species revealed that all have an omnivorous diet, although the stomachs of P. rufa contained a greater percentage of crustacean remains, whereas the stomach contents of T. tasmaniae were largely composed of fragments of macroalgae. The stomach contents of A. mixta australis were composed mainly of fine particulate matter. Diel feeding rhythms were examined from mysids collected during a 24-hour sampling session in October. P. rufa appeared to be primarily a day-time feeder while T. tasmaniae fed more intensively at night. The results for A. mixta australis were inconclusive. Gut contents analysis of fish collected at the study site revealed predation on mysids by Arripis trutta (juveniles), Aldrichetta forsteri, Vincentia conspersa and Pseudolabrus fucicola. The mysid species most frequently eaten was Tenagomysis australis Fenton. The mysid species most frequently eaten was Tenagomysis australis Fenton.     

Vulnerability of early life intervals of Coregonus hoyi to predation by a freshwater mysid,Mysis relicta

Synopsis The bloater, Coregonus hoyi, deposits its eggs on deep sediments (70–100 m in Lake Michigan), where its eggs and embryos can be exposed to epibenthic predators. We investigated the vulnerability of early life intervals of bloaters to predation by mysids, Mysis relicta, which are mostly epibenthic by day and planktonic at night. Bloaters were raised from spawn in the laboratory and presented to field-collected mysids in laboratory predation trials. Eggs were not ingested by the mysids. Embryonic bloaters were vulnerable to predation by mysids only during the interval between hatching and swim up, usually 1–24 h under laboratory conditions. The mysids required about a day of exposure to this novel prey before they were able to kill significant numbers of the bloater embryos by making successive attacks with their thoracic legs. In experiments with experienced (2 and 3 days with bloater embryos) mysids, a functional response between embryo density and mysid predation rates was apparent. Temperature and the presence of alternative prey (zooplankton) did not alter the ‘kill rate’ (about 2.5 embryos mysid^-1d^-1) of experienced mysids at high bloater densities (>4 bloaters/mysid). However, more embryos were partially, rather than completely, ingested at 4 versus 9° C and in the presence of zooplankton.     

Zooplankton Communities of a Dammed Estuary in the Bay of Fundy,Canada

The upper estuary of the macrotidal Petitcodiac River, New Brunswick, Canada, was converted to a freshwater impoundment by construction of a causeway in 1968. Thirty years later, zooplankton community structure in the impoundment was characteristic of a disturbed ecosystem. The depauperate zooplankton consisted of a mixture of freshwater and marine/brackish taxa, but was not typical of regional estuaries or lakes. The causeway dam was an obstacle to fish migration, and several anadromous species that would formerly have been represented in the ichthyoplankton were absent. The invertebrate plankton community was dominated by rotifers, likely as a result of frequent water level fluctuations, and highly suspended sediment levels in the water column. Estuarine/marine calanoids (Eurytemora affinis, Temora longicornis, Centropages typicus, Tortanus discaudatus), the mysid Neomysis americana, and Cancer crab zoeae were collected in fresh water up to 14 km upstream of the causeway.