Abundance Of Planktivores Research Articles

Changes in reef fish assemblages in a cross-shelf euphotic-mesophotic gradient in tropical SW Atlantic

Despite a vast number of studies about shallow reef fish assemblages, and an increasing amount of studies about mesophotic ones, there are very few cross-shelf assessments including both, shallow and mesophotic depths. Herein, we analyze changes in reef fish assemblages in a cross-shelf euphotic-mesophotic (19–72.6 m) gradient in tropical SW Atlantic. Data was obtained using a novel low-cost 360° stationary video system. Sixty-four species from thirty families were identified. Depth, distance offshore and proportion of consolidated substrate were the main predictors of fish assemblages’ structure. Four distinct fish assemblages were identified across the depth gradient (20–30, 30–40, 40–50 and 50–75 m), with marked shifts in composition, abundance, and trophic structure. Abundance of invertebrate feeders and large roving herbivores declined across the shelf and with increasing depth, while abundance of planktivores and piscivores increased, particularly below 50 m. These changes were likely caused by changes in food availability and habitat suitability. Our low-cost video system yielded high-resolution images that enabled us to accurately survey reef fish assemblages, being therefore a useful method for studies focusing on both, euphotic and mesophotic reef ecosystems. Our results contribute to a better understanding of how reef fish assemblages change with increasing depth in SW Atlantic reefs and corroborate the idea that the breakpoint between euphotic and mesophotic fish assemblages is context-dependent rather than previously defined as 30 m depth.

Bottom-up linkages between primary production, zooplankton, and fish in a shallow, hypereutrophic lake.

Nutrient supply is a key bottom-up control of phytoplankton primary production in lake ecosystems. Top-down control via grazing pressure by zooplankton also constrains primary production and primary production may simultaneously affect zooplankton. Few studies have addressed these bidirectional interactions. We used convergent cross-mapping (CCM), a numerical test of causal associations, to quantify the presence and direction of the causal relationships among environmental variables (light availability, surface water temperature, NO3 -N, and PO4 -P), phytoplankton community composition, primary production, and the abundances of five functional zooplankton groups (large cladocerans, small cladocerans, rotifers, calanoids, and cyclopoids) in Lake Kasumigaura, a shallow, hypereutrophic lake in Japan. CCM suggested that primary production was causally influenced by NO3 -N and phytoplankton community composition; there was no detectable evidence of a causal effect of zooplankton on primary production. Our results also suggest that rotifers and cyclopoids were forced by primary production, and cyclopoids were further influenced by rotifers. However, our CCM suggested that primary production was weakly influenced by rotifers (i.e., bidirectional interaction). These findings may suggest complex linkages between nutrients, primary production, and rotifers and cyclopoids, a pattern that has not been previously detected or has been neglected. We used linear regression analysis to examine the relationships between the zooplankton community and pond smelt (Hypomesus nipponensis), the most abundant planktivore and the most important commercial fish species in Lake Kasumigaura. The relative abundance of pond smelt was significantly and positively correlated with the abundances of rotifers and cyclopoids, which were causally influenced by primary production. This finding suggests that bottom-up linkages between nutrient, primary production, and zooplankton abundance might be a key mechanism supporting high planktivore abundance in eutrophic lakes. Because increases in primary production and cyanobacteria blooms are likely to occur simultaneously in hypereutrophic lakes, our study highlights the need for ecosystem management to resolve the conflict between good water quality and high fishery production.

Macrobenthos functional groups as indicators of ecological restoration in reclaimed intertidal wetlands of China’s Yangtze Estuary

The functional group method of community assessment was used in this study to assess the ecological success of reclaimed wetland restoration in the Yangtze Estuary. Macrobenthos were collected from reclaimed intertidal wetlands at contrasting stages of succession and divided into five functional groups that comprised planktivores, herbivores, carnivores, detritivores, and omnivores. There was a comprehensive recovery in species number, abundance, and biomass in long-restored wetland, and in vegetated short-restored wetland, there was an increase in the abundance of planktivores and herbivores. In contrast, species number, abundance, and biomass of all functional groups declined in bald short-restored wetland. Cluster and NMDS analyses suggested similar community structures between the long-restored and non-reclamation wetlands, indicating effective artificial restoration of wetland habitat was possible. Ecological health was fully restored in the long-restored wetland, partially restored in vegetated short-restored wetland, but had deteriorated in the bald wetland. Successional restoration improves with time, and recruitment of vegetation is key to the initiation of macrobenthic community development.

Combined effects of urbanization and connectivity on iconic coastal fishes

AbstractAimDisturbance and connectivity shape the structure and spatial distribution of animal populations in all ecosystems, but the combined effects of these factors are rarely measured in coastal seascapes. We used surf zones of exposed sandy beaches in eastern Australia as a model seascape to test for combined effects of coastal urbanization and seascape connectivity (i.e. spatial links between surf zones, estuaries and rocky headlands) on fish assemblages.LocationFour hundred kilometres of exposed surf beaches along the eastern coastline of Australia.MethodsFish assemblages were surveyed from surf zones of 14 ocean‐exposed sandy beaches using purpose‐built surf baited remote underwater video stations.ResultsThe degree of coastal urbanization and connectivity were strongly correlated with the spatial distribution of fish species richness and abundance and were of greater importance to surf fishes than local surf conditions. Urbanization was associated with reductions in the abundance of harvested piscivores and fish species richness. Piscivore abundance and species richness were lowest on highly urbanized coastlines, and adjacent to beaches in wilderness areas where recreational fishing is intense. By contrast, seascape connectivity was correlated with the spatial distribution of omnivore and planktivore abundance. Spatial linkages between surf zones and estuaries were also important to two globally threatened guitarfish that are of international conservation significance.Main conclusionsWe show for the first time how vulnerable and iconic fish species in surf zones are affected by the presence of coastal cities and the attributes of seascapes in which these cities are embedded. It is possible that greater connectivity may lessen ‘urbanization’ impacts for species with broad diets and strong ecological links to other ecosystems – this offers new perspectives in coastal conservation, for fishes and beyond.

Arrive, survive and thrive: essential stages in the re-colonization and recovery of zooplankton in urban lakes in Sudbury, Canada

<p>The recovery of lakes from severe, historical acid and metal pollution requires that colonists of extirpated species arrive, survive and subsequently thrive. We employed 40 year records from weekly to monthly crustacean zooplankton samples from Middle and Clearwater lakes near Sudbury, Canada, to identify the main mechanistic bottlenecks in this recovery process. While both lakes now have circum-neutral pH, acidity decreased more rapidly in Middle Lake because of past liming interventions, while Clearwater Lake, being larger and supporting more housing, likely receives more zooplankton colonists than Middle Lake. Community richness increased much faster in Middle Lake than in Clearwater Lake, at 1.6 <em>vs</em> 0.9 species decade<sup>-1</sup>, respectively. Richness has recovered in Middle Lake, when assessed against a target of 9-16 species collection<sup>-1</sup> determined from regional reference lakes, but it has not yet recovered in Clearwater Lake. Species accumulation curves and a metric of annual persistence show that this difference is a product not of greater rates of species introduction into Middle Lake, but rather to their greater annual persistence once introduced. Greater annual persistence was associated with better habitat quality (<em>i.e</em>., lower acid and metal toxicity) in Middle Lake, particularly early in the record, and lower planktivore abundance, more recently. These results support a growing consensus that ecological recovery of zooplankton from acidification and metal pollution does not depend strongly on propagule introduction rates which are adequate, but rather on propagule persistence, in lake-rich, suburban landscapes such as those near Sudbury. </p>

Cumulative impacts of fisheries in the California Current

AbstractEcosystem‐based fisheries management calls for the consideration of the indirect and cumulative effects of fishing, in addition to estimating direct fishing mortality. Here, we quantify such effects of fishing fleets, and their interactions, using a spatially explicit Atlantis simulation model of the food web and fisheries in the California Current. Simulations testing the effects of single fleets suggested that bottom trawl, fixed gear, and hake (Merluccius productus) trawl primarily have direct impacts on their target and bycatch species. Few indirect effects from these three fleets extended through predator–prey links to other parts of the food web. In contrast, effects of the purse seine fleet extended beyond the three groups it harvested, strongly altering the abundance of predators, planktonic prey, and benthos. In terms of nine ecosystem attributes, our experiments involving single fleets identified six fleets that caused the bulk of negative impacts. Specific fleets impacted different aspects of the ecosystem, for instance with groundfish gears causing reductions in piscivore abundance, and hake trawl and purse seine increasing krill through reducing abundance of planktivores. In terms of interactions among fleets' effects, the vast majority of effects were simply additive – the combined effect of two fleets was simply the sum of the individual fleets' effects. The analyses offer one way to sharpen the focus of ecosystem‐based fisheries management in the California Current, emphasizing impacts and interactions of particular stressors.

Reef fish structure and distribution in a south-western Atlantic Ocean tropical island

The community structure of the reef fish fauna of Trindade Island, a volcanic oceanic island located 1160 km off the coast of Brazil, is described based on intensive visual censuses. Seventy-six species were encountered in 252 censuses, with mean ± S.E. of 99 ± 3 individuals and 15.7 ± 0.3 species 40 m(-2) transect. The average fish biomass, calculated from length-class estimation, was 22.1 kg 40 m(-2) transect. The species contributing most to biomass were, in decreasing order, Melichthys niger, Cephalopholis fulva, Kyphosus spp., Holocentrus adscensionis, Sparisoma amplum, Sparisoma axillare, Acanthurus bahianus and Epinephelus adscensionis. Carnivorous fishes were the largest trophic group in terms of biomass, followed by omnivores and roving herbivores. The two predominant types of reef habitat, fringing reefs built by coralline algae and rocky reefs made of volcanic boulders, showed significant differences in the biomass and the abundance of the trophic guilds. Within each habitat type, significant differences in species richness, density and biomass were detected among crest, slope and interface zones. Although similar in overall species composition to coastal reefs in Brazil, the fish fauna of Trindade Island shares certain characteristics, such as a high abundance of planktivores, with other Brazilian oceanic islands. Despite comparatively high fish biomass, including the macro-carnivorous species habitually targeted by fisheries, signs of overfishing were evident. These findings highlight the urgency for a conservation initiative for this isolated, unique and vulnerable reef system.

Fish Community and Trophic Structure at Artificial Reef Sites in the Northeastern Gulf of Mexico

Fish community and trophic structure were examined quarterly from fall 2004 through spring 2007 at 27 artificial reef sites located 15-20 mi south of Pensacola, Florida. Location of study sites was not reported to the public when reef modules were deployed in 2003, thus sites were presumed to be unfished. Community structure estimates were derived from video sampling that was performed with a micro remotely operated vehicle (ROV) equipped with a laser scale to estimate fish size. A total of 81,207 individuals from 77 different taxa was enumerated from video samples. Fish assemblages were dynamic in species richness, fish community structure, and trophic structure across time and by reef design. Total fish density, biomass, and species richness increased across time. Red snapper, Lutjanus campechanus (Poey, 1860), was by far the most abundant species (29% of individuals, 45% of total biomass), and its density and biomass increased over the course of the study. Differences in fish community structure were driven by sharp declines in grouper abundance and the proliferation of smaller fishes across time. Trophic structure was consistently dominated by invertivore/piscivores, while fluctuation in piscivore biomass and increased abundance of planktivores drove differences in trophic structure. Size distributions for many fishery species known to display low annual site fidelity were composed primarily of young, sub-legal individuals. Disappearance with increasing fish size was likely due to shifting ontogenetic habitat requirements or exposure to high regional fishing mortality for species that displayed low annual site fidelity to study sites.

Temporal trends in Mysis relicta abundance, production, and life-history characteristics in southeastern Lake Michigan

The density and life history characteristics of Mysis relicta were evaluated at a 110-m and 45-m station in southeast Lake Michigan during spring, summer, and fall for two time periods, 1995–2002 and 2007–2008. Mysis were more abundant during 1995–2002 than 2007–2008 for all seasons and depths, with average declines across seasons of 81% at 45 m and 70% at 110 m. Offshore densities of Mysis in 2007–2008 were similar to published densities within the same region during 1985–1989, but under differing ecosystem conditions (e.g. higher fish biomass and primary production in the 1980 s). Growth averaged 0.032 ± 0.002 mm/day in both 1995–2002 and 2007–2008, and the proportion of females with broods (overall 7%) did not differ between time periods. Mean brood size adjusted for length did not differ between 1995–2002 (17 ± 0.6) and 2007–2008 (15 ± 1.3). The mean length of reproductive females was higher in 2007–2008 (17 ± 0.2) than during 1995–2002 (16 ± 0.1). New recruits (≤ 4 mm) were collected during each season for each time period, but were lower in abundance in 2007–2008 than in 1995–2002. During spring, there was a significant relationship between offshore water column chlorophyll concentration and Mysis recruit abundance, but not during summer/fall. Declining spring chlorophyll levels may be altering food availability for small mysids in spring, and the decline of the benthic macroinvertebrate Diporeia may be increasing fish predation pressure on Mysis despite declining planktivore abundance.

Variation in large‐bodied fish‐community structure and abundance in relation to water‐management regime in a large regulated river

Variation in life-history traits (growth, condition, mortality and recruitment) and relative abundance of 11 large-bodied fish species was investigated among three water-management regimes (unimpounded, run-of-the-river and winter reservoirs) in the large regulated Ottawa River, Canada. If waterpower management had an effect on fishes, then (1) would be expected community structuring among water-management regimes and (2) species with similar life-history traits should be affected in a similar manner. Large-bodied fish communities were assessed using two different standard index-netting techniques, one using trap nets and the other gillnets. Community structure could be discriminated based on species caught in nets using holographic neural networks (78.8% correct overall classification rate using trap nets and 76.0% using gillnets); therefore, water-management regimes affected community structure in the Ottawa River. Littoral zone benthivores were significantly lower in abundance (P < 0.001) or absent in winter reservoirs, whereas the abundance of planktivores or species that were planktivorous at young ages were significantly greater than in unimpounded river reaches. Growth, condition and mortality did not vary among reach types except smallmouth bass Micropterus dolomieu were in better condition in winter reservoirs than unimpounded reaches. Lake sturgeon Acipenser fulvescens recruitment was impaired in run-of-the-river reaches, whereas recruitment for other species that spawn in fast water was not affected.

Trophic cascades promote threshold-like shifts in pelagic marine ecosystems

Fisheries can have a large impact on marine ecosystems, because the effects of removing large predatory fish may cascade down the food web. The implications of these cascading processes on system functioning and resilience remain a source of intense scientific debate. By using field data covering a 30-year period, we show for the Baltic Sea that the underlying mechanisms of trophic cascades produced a shift in ecosystem functioning after the collapse of the top predator cod. We identified an ecological threshold, corresponding to a planktivore abundance of approximately 17 x 10(10) individuals, that separates 2 ecosystem configurations in which zooplankton dynamics are driven by either hydroclimatic forces or predation pressure. Abundances of the planktivore sprat above the threshold decouple zooplankton dynamics from hydrological circumstances. The current strong regulation by sprat of the feeding resources for larval cod may hinder cod recovery and the return of the ecosystem to a prior state. This calls for the inclusion of a food web perspective in management decisions.

FISH REINTRODUCTIONS REVEAL SMOOTH TRANSITIONS BETWEEN LAKE COMMUNITY STATES

Whether communities respond smoothly or discontinuously to changing environmental conditions has important consequences for the preservation and restoration of ecosystems. Theory shows that communities may exhibit a variety of responses to environmental change, including abrupt transitions due to the existence of alternate states. However, there have been few opportunities to look for such transitions in nature. Here, we examine the impact of a two-orders-of-magnitude decrease and then increase in planktivore abundance in Wintergreen lake (Michigan, USA), caused by the extinction and reintroduction of two dominant fish species (largemouth bass, Micropterus salmoides, and bluegill, Lepomis macrochirus). Over a 16 + yr period of slow change from high planktivory to low planktivory back to high planktivory, the zooplankton community changed smoothly and predictably between states. In years of low planktivory, the zooplankton assemblage was dominated by a single, large, cladoceran species, Daphnia pulicaria, whereas in years of high planktivory, D. pulicaria disappeared and was replaced by a suite of small-bodied cladocerans. We quantified the multivariate change in zooplankton community dissimilarity and found that community state smoothly tracked changes in planktivore density in both a forward and backward direction. Thus, there was little evidence of discontinuity in this system where transitions are strongly driven by planktivory.

ESTABLISHMENT OF A METALIMNETIC OXYGEN REFUGE FOR ZOOPLANKTON IN A PRODUCTIVE LAKE ONTARIO EMBAYMENT

Hypolimnetic oxygen injection is a management tool used to improve water quality by preventing anoxia and associated phosphorus release from the sediments. An additional benefit would be formation of a low-oxygen refuge for large-bodied herbivorous zooplankton in the metalimnion. The magnitude and timing of hypolimnetic oxygenation was deliberately manipulated during summer 1997 in Irondequoit Bay, a eutrophic Lake Ontario embayment (New York, USA), to maintain metalimnetic dissolved oxygen (DO) concentrations in the range of 1–2 mg/L. We assessed the vertical distributions of fish and zooplankton in 1997, a year with controlled oxygenation, and 1996, a year with oxygenation but without a deliberate attempt to create amenable refuge conditions. After initiation of oxygenation, June through early August metalimnetic DO in 1997 was 1.8 ± 0.5 mg/L (means ± 2 se), whereas metalimnetic DO in 1996 was 3.8 ± 1.0 mg/L. Hydroacoustic surveys in 1997 and gillnet catches in both years indicated that >96% of planktivorous fish, alewives (Alosa pseudoharengus) and emerald shiners (Notropis atherinoides), were restricted to depths with DO > 4.0 mg/L. A refuge effect in 1996 was slight and evidenced only by significantly larger zooplankton in the metalimnion while densities of Daphnia galeata medotae and D. retrocurva were similar or significantly greater in the epilimnion. In 1997, the refuge effect was strong despite an almost twofold increase in planktivorous fish abundance compared to 1996. Overall zooplankton densities were similar in the epilimnion and metalimnion while zooplankton size and densities of both daphnids were significantly greater in the metalimnion. Densities of an invertebrate predator, Mesocyclops edax, were also significantly higher in the metalimnion compared to the epilimnion. Despite high fish abundance in 1997, mean summer (late May through August) chlorophyll a decreased 29% compared to 1996, consistent with expectations from grazing by large Daphnia protected in the refuge. Presence of large-bodied zooplankton in 1997 despite high planktivore abundance demonstrated that successful creation of a low-oxygen refuge from fish predation was possible in an open Great Lakes embayment where traditional biomanipulation via fish removal was not practical.

Predicting mercury levels in yellow perch: use of water chemistry, trophic ecology, and spatial traits

Recent research suggests that wetland abundance surrounding lakes, fish trophic position, and fish community composition may influence the bioavailability of mercury (Hg) to fish. To compare the importance of these spatial and biological factors to chemical factors known to influence bioavailability, we determined the relationship between 24 lake traits and Hg concentrations in yellow perch (Perca flavescens; whole fish samples) for 43 northern Wisconsin lakes. Independent variables included biological traits such as fish trophic position and body condition, spatial traits such as lake hydrologic position and surrounding wetland abundance, and chemical traits such as pH and water color. The strongest predictor of fish Hg levels was pH (R2 = 0.42; p &lt; 0.002). Of the biological traits measured, yellow perch body condition explained significant additional variation (final R2 = 0.54; p = 0.024). Trophic position explained limited variability and population abundance of planktivores and piscivores were not correlated to perch Hg levels. Regression tree models indicated that small lakes with greater than 6% wetland in their watershed have moderately elevated fish Hg levels. Our results indicate that within-lake chemistry and fish growth patterns are stronger correlates of Hg levels in yellow perch than spatial traits, trophic position, or fish community attributes.

Population Biology of Longfin Smelt and Aspects of the Ecology of Other Major Planktivorous Fishes in Lake Washington

ABSTRACT I assessed the relationship between Cedar River flow (a major input to Lake Washington) during egg incubation and fry emergence period and longfin smelt (Spirinchus thaleichthys) recruitment to Lake Washington. There was a significant inverse relationship between an index of abundance of odd-year class smelt and Cedar River mean flow during peak spawning period (February). There was no significant relationship between smelt abundance and peak flow during spawning period (March) of the even-year class smelt. Cedar River discharge greater than 28.33 m3/s occurred more frequently during odd years than even years. This, in addition to other factors, may be responsible for the relatively low recruitment of the odd-year spawning smelt and thus the population cycle observed in the Lake Washington smelt. The positive relationship between smelt recruitment and flow during spring, when smelt fry are migrating into the lake, supports other studies that suggest that an increase in turbidity at high flows reduces juvenile fish susceptibility to predation. There have been significant changes in the planktivorous fish assemblage in Lake Washington since the 1960s. Notably, longfin smelt, which was < 12% of the planktivorous fish abundance, now accounts for 58 to 84 % of the planktivore abundance. Recent increases in abundance of smelt may be related to reduced Cedar River mean flow that has enabled a larger proportion of spawned eggs to hatch and survive.

Effects of base addition and brook trout (Salvelinus fontinalis) introduction on the plankton community of an acidic Adirondack lake

Calcite was added to Woods Lake in 1985 and 1987, and to its watershed in 1990, to restore and maintain water quality conditions suitable for reestablishing brook trout (Salvelinus fontinalis). Juvenile and adult brook trout were introduced following the first chemical treatment and were stocked annually thereafter from 1985 to 1989. In response to the combined chemical and biological manipulation, phytoplankton abundance increased whereas production per unit of biomass decreased. We attribute the increase in phytoplankton abundance and the reduction in biomass turnover rates to a decrease in zooplankton grazing. Zooplankton densities and estimated community grazing rates were lower during the posttreatment period despite a shift from small-bodied taxa (Diaptomus minutus, Bosmina longirostris) to a more diverse assemblage that included four species of daphnids. We attribute reductions in zooplankton abundance to planktivory of introduced brook trout. The changes we observed in phytoplankton and zooplankton species composition were consistent with trends observed during whole-lake acidification experiments, but functional responses in our experiment (decreased zooplankton abundance, increased phytoplankton abundance) were not. Our functional responses were consistent, however, with those observed previously in food webs manipulated to increase planktivore abundance.

A comparative study of the feeding habits of herring (clupea harengus, clupeidae, 1.) and capelin (mallotus villosus, osmeridae, müller) in the barents sea

Abstract Capelin (Mallotus villosus) and adolescent Norwegian spring spawning herring (Clupea harengus) cooccur in the southern Barents Sea during early summer. The diets of both species were dominated by calanoid copepods, and the overlap in diet was large. Both for herring and capelin the proportion of copepods in the diet decreased with increasing fish size, while euphausiids and appendicularians increased in importance. The ontogenetic shift in diet was thus similar for the two species. Herring showed an increasingly deeper distribution with increasing body size. Capelin were found deeper than herring in areas of spatial overlap. Based on the similarities in diet it is concluded that the two species are potential competitors for food in times of high abundance ofplanktivores or oflow food availability in the Barents Sea.

Multiple Trophic Level Structuring in Salvelinus–Coregonus Assemblages in Boreal Forest Lakes

Hypolimnetic trophic interactions were examined among lake trout (Salvelinus namaycush), cisco (Coregonus artedi), zooplankton, and macroinvertebrates in six northwestern Ontario lakes varying sevenfold in conductivity. Faster growth and larger body size of lake trout in high-conductivity lakes were associated with larger meal sizes, earlier entry of cisco into their diet, and consumption of relatively few, large-bodied prey. Cisco preyed on zooplankton and macroinvertebrates. Cisco were abundant, large bodied, and fast growing in high-conductivity lakes. In one low-conductivity lake (Greenwich Lake), lake trout were 12 times as abundant as cisco whereas in all other lakes, lake trout to cisco ratios were ~ 1:1 or less regardless of conductivity. In Greenwich Lake, lake trout consumed large quantities of age-0 cisco and Mysis relicta, which was associated with "top-down" trophic structuring. Food web analyses strongly suggest that in Greenwich Lake the high abundance of piscivores resulted directly in a low abundance of planktivores and indirectly in a large-bodied zooplankton community and planktivores with rapid growth rates.

Differential zooplankton feeding behaviors, selectivities, and community impacts of two planktivorous fishes

We examined the feeding behaviors and selectives of two common planktivorous fishes, pumpkinseeds Lepomis gibbosa and fathead minnows Pimephales promelas in the laboratory. Ingestion rates for both pumpkinseeds and fathead minnows feeding on zooplankton increased as a function of fish length. Pumpkinseeds fed on zooplankton strictly as particulate feeders, with preferences increasing as a function of zooplankton body size regardless of taxonomic identity. Preferences were highest for large Daphnia, intermediate for intermediate-sized copepods, and lowest for small Ceriodaphnia. Fathead minnows displayed the ability to use both particulate-feeding and filter-feeding behaviors. Differential preferences tended to reflect both zooplankton size and taxon, being highest for large, slow-swimming Daphnia, intermediate for small Ceriodaphnia, and lowest for faster-swimming copepods. These differences in prey capture behaviors and preferences of the two fishes are reflected in the zooplankton taxonomic composition of small ponds containing each fish type. The crustacean zooplankton assemblages in ponds containing both pumpkinseeds and fathead minnows were dominated by copepods. Cladocerans were rare. In ponds containing pumpkinseeds, but no fathead minnows, cladocerans were abundant, generally accounting for up to 80% of total crustacean zooplankton biomass. These results suggest that the type of planktivore, and not simply the presence or abundance of planktivores in a system, can determine zooplankton community structure.

Effects of planktivorous fish mass mortality on the plankton community of Lake Mendota, Wisconsin: implications for biomanipulation

Massive mortality of planktivorous fish had a dramatic impact on plankton community dynamics of Lake Mendota, Wisconsin, USA. After fish mortality, the larger Daphnia pulicaria replaced the smaller Daphnia galeata mendotae, resulting in greater grazing pressure on phytoplankton. This was accompanied by a much longer spring clear-water period and lower summer phytoplankton biomass compared to years before the fish mortality. Analysis of historical data (from the mid-1970’s) showed that previous fluctuations in planktivorous abundance had similar effects on Daphnia abundance and species composition, and on spring phytoplankton biomass. However, the mid-1970’s fish fluctuations had no detectable effect on summer phytoplankton. Concentrations of phosphorus were much higher in the 1970’s (spring P 80–135 μg 1−1 ’) than in the 1980’s (spring P 19–36 μg 1−1) and it is possible that high P concentrations may reduce trophic cascade effects on summer phytoplankton communities. This suggests that the success of biomanipulation programs may be dependent on lake nutrient status.