Planktivore Biomass Research Articles

Local human impacts disrupt depth-dependent zonation of tropical reef fish communities

The influence of depth and associated gradients in light, nutrients and plankton on the ecological organization of tropical reef communities was first described over six decades ago but remains untested across broad geographies. During this time humans have become the dominant driver of planetary change, requiring that we revisit historic ecological paradigms to ensure they capture the dynamics of contemporary ecological systems. Analysing >5,500 in-water reef fish surveys between 0 and 30 m depth on reef slopes of 35 islands across the Pacific, we assess whether a depth gradient consistently predicts variation in reef fish biomass. We reveal predictable ecological organization at unpopulated locations, with increased biomass of planktivores and piscivores and decreased primary consumer biomass with increasing depth. Bathymetric steepness also had a striking influence on biomass patterns, primarily for planktivores, emphasizing potential links between local hydrodynamics and the upslope propagation of pelagic subsidies to the shallows. However, signals of resource-driven change in fish biomass with depth were altered or lost for populated islands, probably due to depleted fish biomass baselines. While principles of depth zonation broadly held, our findings expose limitations of the paradigm for predicting ecological dynamics where human impacts confound connections between ecological communities and their surrounding environment.

A northeast United States Atlantis marine ecosystem model with ocean reanalysis and ocean color forcing

The northeast United States Atlantis model (NEUSv2) is an end-to-end ecosystem model that can simulate biogeochemical, ecological, fishery, management, and socio-economic processes within marine ecosystems. As a major update to the original model, NEUSv2 includes changes to the model's functional group definitions and forcing data. NEUSv2 is the first Atlantis model to use a satellite-ocean-color-derived phytoplankton size class model that was tuned specifically for the region to force marine primary production. Additionally, physical ocean variables (currents, temperature, and salinity) were updated using a high-resolution global ocean reanalysis. Despite its coarse resolution, NEUSv2 was capable of reproducing the broad spatial patterns seen in the physical and biological forcing sources, with the exception of some circulation features. NEUSv2 produced plausible zooplankton and planktivore biomass, a stable lower trophic food web, and recent trends in zooplankton biomass. NEUSv2 meets calibration criteria for the persistence and long-term stability of functional group biomass. Given the success of this new Atlantis forcing approach, we detail the observations and challenges regarding spatial scale-related processes, data assimilation, and biological calibration. We also discuss possible tradeoffs with model scope, calibration, and the availability of feedback mechanisms. This NEUSv2 hindcast is well suited for exploring ecosystem-level sensitivity to lower trophic processes and for testing alternative biogeochemical forcing. Further developments will improve model performance for higher trophic levels.

The first large-scale assessment of three-spined stickleback (Gasterosteus aculeatus) biomass and spatial distribution in the Baltic Sea

Abstract Declines in predatory fish in combination with the impact of climate change and eutrophication have caused planktivores, including three-spined stickleback (Gasterosteus aculeatus), to increase dramatically in parts of the Baltic Sea. Resulting impacts of stickleback on coastal and offshore foodwebs have been observed, highlighting the need for increased knowledge on its population characteristics. In this article, we quantify abundance, biomass, size structure, and spatial distribution of stickleback using data from the Swedish and Finnish parts of the Baltic International Acoustic Survey (BIAS) during 2001–2014. Two alternative methods for biomass estimation suggest an increase in biomass of stickleback in the Baltic Proper, stable or increasing mean size over time, and larger individuals toward the north. The highest abundance was found in the central parts of the Baltic Proper and Bothnian Sea. The proportion of stickleback biomass in the total planktivore biomass increased from 4 to 10% in the Baltic Proper and averaged 6% of the total planktivore biomass in the Bothnian Sea. In some years, however, stickleback biomass has ranged from half to almost twice that of sprat (Sprattus sprattus) in both basins. Given the recent population expansion of stickleback and its potential role in the ecosystem, we recommend that stickleback should be considered in future monitoring programmes and in fisheries and environmental management of the Baltic Sea.

Nutrient management and structural shifts in fish assemblages: Lessons learned from an Area of Concern in Lake Ontario

Abstract While total phosphorus (TP) is a critical determinant of freshwater ecosystem productivity, multiple stressors can induce shifts in energy pathways, with profound implications for ecosystem and fishery restoration. The Bay of Quinte (Lake Ontario, Canada) is a Great Lakes nearshore ecosystem that has been historically subjected to a variety of environmental perturbations: cultural eutrophication, low dissolved oxygen, reduced fisheries, climatic extremes, phosphorus (P) abatement, and aquatic invasive species. We used the Bay of Quinte to study how trophic state alterations affect fish assemblages in Great Lakes nearshore environments by examining the response of fish biomass to TP concentration variability in the presence of multiple stressors. Our analysis is based on a 42‐year (1972–2013) dataset from the Bay of Quinte for water quality through the food web to fishes. We employed a series of statistical tools that can offer insights into the structural changes induced by the events examined. We first used dynamic linear modelling to detect temporal trends in fish biomass, while accounting for year‐to‐year TP variability over three spatial segments of the bay. We then developed piecewise regression models to assess the extent to which specific ecological events induced distinct shifts in the fish assemblage. Multiple regression modelling was used to quantify the relative importance of TP, zooplankton, and surface water temperature on fish biomass. Based on gillnets, there were consistent fish biomass changes across the bay with increased biomass before P control (1972–1977), declines after P control followed by the establishment of a steady state or modest increase (1978–1994), and a declining trajectory during the recent period (1995–2013). Even when accounting for the role of water temperature and zooplankton, TP still had a significant effect on fish biomass. However, the strength and nature of the relationship varied among fish groups, and overall, the effect of TP on fish biomass has weakened in recent years. Our models show that fish biomass in the Bay of Quinte is shaped by the year‐to‐year TP variability. However, the relationship between P and fish abundance has been modulated by various ecological events with the consequence that the Bay of Quinte fish assemblage has changed and the food web now produces less fish biomass per unit of TP. A projected reduction of mean ambient TP levels from 30 to 25 μg/L, is expected to induce a 24% decline in total fish biomass, and further shift the fish assemblage with the biomass of planktivores and walleye declining by >60% and 30%, respectively. Recreational fishing provides important economic benefits in the Bay of Quinte, through tourism and other local business operations. Recognising the economic importance of fishing, our analysis provides critical insights regarding the on‐going management efforts to reduce external nutrient loadings (point and non‐point sources, urban storm water) and further lower ambient TP levels and primary productivity. The potential implications of already low nutrient concentrations for fish productivity represent a major challenge in effectively balancing water quality with fisheries management in the area.

Human, oceanographic and habitat drivers of central and western Pacific coral reef fish assemblages.

Coral reefs around US- and US-affiliated Pacific islands and atolls span wide oceanographic gradients and levels of human impact. Here we examine the relative influence of these factors on coral reef fish biomass, using data from a consistent large-scale ecosystem monitoring program conducted by scientific divers over the course of >2,000 hours of underwater observation at 1,934 sites, across ~40 islands and atolls. Consistent with previous smaller-scale studies, our results show sharp declines in reef fish biomass at relatively low human population density, followed by more gradual declines as human population density increased further. Adjusting for other factors, the highest levels of oceanic productivity among our study locations were associated with more than double the biomass of reef fishes (including ~4 times the biomass of planktivores and piscivores) compared to islands with lowest oceanic productivity. Our results emphasize that coral reef areas do not all have equal ability to sustain large reef fish stocks, and that what is natural varies significantly amongst locations. Comparisons of biomass estimates derived from visual surveys with predicted biomass in the absence of humans indicated that total reef fish biomass was depleted by 61% to 69% at populated islands in the Mariana Archipelago; by 20% to 78% in the Main Hawaiian islands; and by 21% to 56% in American Samoa.

Is the island biogeography model a poor predictor of biodiversity patterns in shallow lakes?

Summary The classic island biogeography model (IBM) predicts highest species richness in large, connected habitats due to colonisation and reduced risk of extinction. Promoting large, connected habitats has subsequently become a common theme in conservation biology. However, the IBM does not account for direct and indirect interactions among species. For example, planktivorous and benthivorous fish may reduce biodiversity in shallow lakes by inducing shifts to a turbid‐water lake state with low habitat complexity. We assessed relationships between species richness, landscape features, fish biomass and lake state in 104 shallow lakes in Minnesota, U.S.A. First, we tested whether lake size and connectivity influenced species richness of fish and biomass of planktivores and benthivores (fish biomass). We subsequently tested whether fish biomass affected the probability that lakes were in turbid versus clear‐water states. Finally, we tested whether species richness of macrophytes and taxon richness of aquatic invertebrates showed stronger relationships with lake size and connectivity or with fish biomass and lake state. Fish richness and biomass both increased with lake size and were higher in connected basins. Fish biomass, in turn, increased the probability that lakes would be turbid. In contrast, macrophyte and invertebrate richness were unrelated to lake size or connectivity. Instead, macrophyte richness was best predicted by lake state, while invertebrate richness was predicted by lake state and fish biomass. Richness of both macrophytes and invertebrates was higher in clear lakes, and invertebrate richness was inversely related to fish biomass. Our results indicate the IBM poorly explains the diversity of macrophytes and invertebrates in shallow lakes, with diversity more strongly driven by biotic interactions and influences associated with fish. We suggest that ecological implications of increased connectivity and lake size should be considered in future conservation strategies for shallow lakes.

Relative importance of phosphorus, fish biomass, and watershed land use as drivers of phytoplankton abundance in shallow lakes

Phytoplankton abundance in shallow lakes is potentially influenced by ambient phosphorus concentrations, nutrient loading accentuated by human activities in lake watersheds, and abundance of planktivorous and benthivorous fish. However, few studies have simultaneously assessed the relative importance of these factors influencing phytoplankton abundance over large spatial scales. We assessed relative influences of watershed characteristics, total phosphorus concentrations, and fish biomass on phytoplankton abundance in 70 shallow lakes in western Minnesota (USA) during summer 2005 and 2006. Our independent variables included total phosphorus (TP), benthivore biomass, planktivore biomass, summed planktivore and benthivore biomass (summed fish), areal extent of agriculture in the watershed, region (prairie versus parkland lakes), and year. Predictive models containing from one to three independent variables were compared using an information theoretic approach. The most parsimonious model consisted of TP and summed fish, and had over 10,000-fold greater support compared to models using just TP or summed fish, or models comprised of other variables. We also found no evidence that relative importance of predictor variables differed between regions or years, and parameter estimates of TP and summed fish were temporally and spatially consistent. TP and summed fish were only weakly correlated, and the model using both variables was a large improvement over using either variable alone. This indicates these two variables can independently increase phytoplankton abundance, which emphasizes the importance of managing both nutrients and fish when trying to control phytoplankton abundance in shallow lakes.

Incorporating biogeography into evaluations of the Channel Islands marine reserve network

Networks of marine reserves are increasingly a major component of many ecosystem-based management plans designed to conserve biodiversity, protect the structure and function of ecosystems, and rebuild and sustain fisheries. There is a growing need for scientific guidance in the design of network-wide monitoring programs to evaluate the efficacy of reserves at meeting their conservation and management goals. Here, we present an evaluation of the Channel Islands reserve network, which was established in 2003 off the coast of southern California. This reserve network spans a major environmental and biogeographic gradient, making it a challenge to assess network-wide responses of many species. Using fish community structure data from a long-term, large-scale monitoring program, we first identified persistent geographic patterns of community structure and the scale at which sites should be grouped for analysis. Fish communities differed most among islands with densities of individual species varying from 3- to 250-fold. Habitat structure differed among islands but not based on reserve status. Across the network, we found that, after 5 years, species targeted by fishing had higher densities (1.5×) and biomass (1.8×) inside reserves, whereas nontargeted species showed no significant differences. Examining trophic groups, piscivore and carnivore biomass was significantly greater inside reserves (1.8× and 1.3× more, respectively), whereas the biomass of planktivores and herbivores was similar inside and out. A framework for incorporating biogeographic variation into reserve network assessments is critical as we move from the evaluation of single reserves to networks of reserves.

Thresholds and Stability of Alternative Regimes in Shallow Prairie–Parkland Lakes of Central North America

Numerous studies have demonstrated alternative regimes in shallow lake ecosystems around the world, with one state dominated by submerged macrophytes and the other by phytoplankton. However, the stability of each regime, and thresholds at which lakes shift to the alternative regime, are poorly known. We used a cross-sectional analysis of 72 shallow lakes located in prairie and parkland areas of Minnesota, USA, during 2005 and 2006 to assess the occurrence of alternative regimes and shifts between them. Cluster analysis revealed two distinct groups of lakes characterized not only by different macrophyte abundance and chlorophyll a levels but also by different total phosphorus–chlorophyll a relationships. Thirtynine lakes were macrophyte- and 23 lakes phytoplankton-dominated in both years, whereas 10 sites shifted sharply between those regimes. We failed to detect a universal shifting threshold in terms of chlorophyll a or total phosphorus. However, 95% of the lakes with chlorophyll a concentrations less than 22 l gl -1 were in a clearwater regime, whereas 95% of the lakes with chlorophyll a higher than 31 l gl -1 were in a turbid regime. Total phosphorus less than 62 l gl -1 was an accurate predictor of lakes in a stable clearwater regime, whereas a large change in biomass of planktivores and benthivores between years was the only variable weakly related to regime shifts. Our results support the theoretical prediction that regime thresholds vary among lakes. We recommend that lake managers focus on improving resilience of clear regimes in shallow lakes by reducing nutrient loading, rather than attempting to identify and manage complex triggers of regime shifts.

Seasonal and distributional patterns of seabirds along the Aleutian Archipelago

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 357:301-311 (2008) - DOI: https://doi.org/10.3354/meps07325 Seasonal and distributional patterns of seabirds along the Aleutian Archipelago Martin Renner1,*, George L. Hunt Jr.1, John F. Piatt2, G. Vernon Byrd3 1School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, Washington 98195, USA 2Alaska Science Center, U. S. Geological Survey, 1011 E. Tudor Rd, Anchorage, Alaska 99503, USA 3Alaska Maritime National Wildlife Refuge, 95 Sterling Highway, Suite 1, Homer, Alaska 99603, USA *Email: auklet@bigfoot.com ABSTRACT: The Aleutian Archipelago is of global importance to seabirds during the northern summer, but little is known about seabird use of these waters during winter. We compare summer and winter abundances of seabirds around 3 islands: Buldir in the western, Kasatochi in the central, and Aiktak in the eastern Aleutians. The density of combined seabird biomass in nearshore marine waters was higher in summer than in winter at Buldir and Kasatochi, but was higher in winter at Aiktak, despite the departure of abundant migratory species. Comparing foraging guilds, we found that only piscivores increased at the western and central sites in winter, whereas at the eastern site several planktivorous species increased as well. The only planktivore remaining in winter at the central and western sites in densities comparable to summer densities was whiskered auklet Aethia pygmaea. Crested auklet Aethia cristatella and thick-billed murre Uria lomvia showed the greatest proportional winter increase at the eastern site. The seasonal patterns of the seabird communities suggest a winter breakdown of the copepod-based food web in the central and western parts of the archipelago, and a system that remains rich in euphausiids in the eastern Aleutians. We suggest that in winter crested auklets take the trophic role that short-tailed shearwaters Puffinus tenuirostris occupy during summer. We hypothesize that advection of euphausiids in the Aleutian North Slope Current is important for supporting the high biomass of planktivores that occupy the Unimak Pass region on a year-round basis. KEY WORDS: Aleutian North Slope Current · Euphausiids · Indicator species · Seabird biomass Full text in pdf format PreviousCite this article as: Renner M, Hunt GL Jr, Piatt JF, Vernon Byrd G (2008) Seasonal and distributional patterns of seabirds along the Aleutian Archipelago. Mar Ecol Prog Ser 357:301-311. https://doi.org/10.3354/meps07325 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 357. Online publication date: April 07, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Stocking piscivores to improve fishing and water clarity: a synthesis of the Lake Mendota biomanipulation project

SUMMARYY 1. A total of 2.7 × 106 walleye fingerlings and 1.7 × 105 northern pike fingerlings were stocked during 1987–99 in eutrophic Lake Mendota. The objectives of the biomanipulation were to improve sport fishing and to increase piscivory to levels that would reduce planktivore biomass, increase Daphnia grazing and ultimately reduce algal densities in the lake. The combined biomass of the two piscivore species in the lake increased rapidly from < 1 kg ha−1 and stabilised at 4–6 kg ha−1 throughout the evaluation period.2. Restrictive harvest regulations (i.e. increase in minimum size limit and reduction in bag limit) were implemented in 1988 to protect the stocked piscivores. Further restrictions were added in 1991 and 1996 for walleye and northern pike, respectively. These restrictions were essential because fishing pressure on both species (especially walleye) increased dramatically during biomanipulation.3. Commencing in 1987 with a massive natural die‐off of cisco and declining yellow perch populations, total planktivore biomass dropped from about 300–600 kg ha−1 prior to the die‐off and the fish stocking, to about 20–40 kg ha−1 in subsequent years. These low planktivore biomasses lasted until a resurgence in the perch population in 1999.4. During the period prior to biomanipulation when cisco were very abundant, the dominant Daphnia species was the smaller‐bodied D. galeata mendotae, which usually reached a biomass maximum in June and then crashed shortly thereafter. Beginning in 1988, the larger‐bodied D. pulicaria dominated, with relatively high biomasses occurring earlier in the spring and lasting well past mid‐summer of many years.5. In many years dominated by D. pulicaria, Secchi disc readings were greater during the spring and summer months when compared with years dominated by D. galeata mendotae. During the biomanipulation evaluation period, phosphorus (P) levels also changed dramatically thus complicating our analysis. Earlier research on Lake Mendota had shown that Daphnia grazing increased summer Secchi disc readings, but P concentrations linked to agricultural and urban runoff and to climate‐controlled internal mixing processes were also important factors affecting summer readings.6. The Lake Mendota biomanipulation project has been a success given that high densities of the large‐bodied D. pulicaria have continued to dominate for over a decade, and the diversity of fishing opportunities have improved for walleye, northern pike and, more recently, yellow perch.7. Massive stocking coupled with very restrictive fishing regulations produced moderate increases in piscivore densities. Larger increases could be realised by more drastic restrictions on sport fishing, but these regulations would be very controversial to anglers.8. If the lake's food web remains in a favourable biomanipulation state (i.e. high herbivory), further improvements in water clarity are possible with future reductions in P loadings from a recently initiated non‐point pollution abatement programme in the lake's drainage basin.

Growth, morphological variation and ontogenetic niche shifts in perch (Perca fluviatilis) in relation to resource availability.

Despite the common occurrence of ontogenetic niche shifts, their consequences for morphological adaptations have been little studied. To address this question, we studied morphological adaptations related to ontogenetic niche shifts in Eurasian perch (Perca fluviatilis) in eight lakes that varied in density of benthic resources and planktivorous fish biomass. Perch start to feed on pelagic zooplankton, then shift to benthic resources at intermediate sizes, and finally, when large enough, mainly feed on fish. These three functional niches over ontogeny are expected to set constraints on the morphology and size-specific growth of perch. The growth of perch was negatively related to planktivorous fish biomass in the zooplanktivorous niche, but positively related to planktivorous fish biomass in the piscivorous niche. The number of gill rakers of perch was negatively related to the biomass of planktivorous fish, providing evidence for the occurrence of character displacement as a consequence of competition in the zooplanktivorous niche. Perch in lakes with low densities of predator-sensitive macroinvertebrates had greater body height measurements and a larger mouth early during ontogeny. This pattern is suggested to be a result of a selection for increased efficiency in the benthic niche when the availability of benthic resources is low. Perch in lakes with a high biomass of planktivorous fishes had fusiform body morphology, a thicker tail and a larger mouth then the average piscivorous perch. The different responses of perch morphology to variation in the availability of benthic resources compared to variation in planktivore biomass are suggested to be partly because the availability of the former resource to a larger extent is set by abiotic conditions (humic content). We suggest that the key factors affecting size-specific growth and body morphology of perch in the system studied are the availability of resources in the benthivorous and piscivorous niches. We also provide evidence for morphological trade-offs, especially between the benthivorous and the piscivorous ontogenetic niches.

Juvenile fish expansion following biomanipulation and its effect on zooplankton

In Lake Ringsjon, southern Sweden, we estimated yearly variations in both 0+ and older fish abundances, following the fish reductions in 1988–1992. Between 1987 and 1994, the total planktivore biomass decreased fourfold whereas the abundance of 0+ fish increased twenty times. The resulting change in predation pressure from planktivorous fish on zooplankton increased up to 2–3 times, after the fish reduction. We conclude that after an initial decrease, the predation pressure on zooplankton increased considerably due to the increase in 0+ fish abundance. The expansion in juvenile fish abundance may therefore have been an important factor behind the apparent lack of top-down effect on the zooplankton community following biomanipulation of Lake Ringsjon. The horizontal distribution of 0+ fish in Lake Ringsjon is also discussed.

The Effects of Fish Assemblage Composition on Lake Water Quality

ABSTRACT Current trophic cascade theories predict that changes in the piscivore to planktivore biomass ratio will cascade down the food web, and will cause both chlorophyll and water clarity to deviate from nutrient – predicted values in individual lakes. An extensive survey of mid summer observations for nutrients, phytoplankton, zooplankton, and fish in Argentinean lakes and reservoirs, were used to study the effects of planktivorous and piscivorous fishes on chlorophyll concentrations and Secchi disk transparency. Regression analysis for different subsets of the data was used in this study. Regressions of chlorophyll on total phosphorus and Secchi disk on total phosphorus differed significantly among lakes with different fish assemblages. At comparable total phosphorus concentrations, lakes with planktivores but without piscivores had the highest phytoplankton biomass and the lowest water clarity. In conclusion, the results show that the fish assemblage composition has a dramatic effect on chlorophyll ...

Planktivores and Plankton Dynamics: Effects of Fish Biomass and Planktivore Type

We quantified the effects of planktivore biomass and planktivore type in an experimental mesocosm study of factorial design in which five levels of fish biomass (0–75 g/m3) were cross-classified with two plantivore types: filter-feeding gizzard shad (Dorosoma cepedianum) and visual-feeding bluegill (Lepomis macrochims). As fish biomass increased, cladocerans, cyclopoids, particulate phosphorus (PP) > 200 μm, and chironomids declined; conversely, rotifers, primary productivity, chlorophyll a, turbidity, unicellular flagellates, colonial and unicellular green algae, pennate diatoms, total phosphorus, and 20–200 and 12–20 μm PP were enhanced. In the presence of gizzard shad, as compared with bluegill, cyclopoids, turbidity, unicellular green algae, pennate diatoms, > 200 μm PP, and chironomid tubes were higher whereas colonial green algae and < 0.2 μm PP were lower. Fish biomass operated independently of planktivore type for most variables, except copepods, colonial green algae, turbidity, and 20–200 μm PP. Although gizzard shad and bluegill have different trophic cascade pathways, fish biomass was more important than planktivore type as a regulator of plankton communities and water quality.

Biomass Size Spectrum of the Lake Michigan Pelagic Food Web

Biomass size spectra for the complete Lake Michigan pelagic food web from picoplankton to salmonids were constructed for nine sampling transects around the lake in May and in September 1987. Size spectra were typical for freshwaters, having distinct peaks corresponding to major size groups. Biomass concentration of algae, zooplankton, and planktivores conformed to particle-size model predictions, but piscivore biomass was lower than predicted because these species are stocked. Mean annual total pelagic biomass was 72.3 g∙m−2 compared with a predicted range of 78.8–85.3 g∙m−2. Potential production of piscivores, Mysis, and Pontoporeia was in agreement with model predictions. No estimates of zooplankton or planktivore production were available, but we calculated that these could be 72.1–91.6 and 2.5–4.1 g∙m−2∙yr−1, respectively. Our analyses suggest that piscivore production is constrained by food web structure. Bloater, which comprise 72% of planktivore biomass, make up less than 20% of salmon diets. We estimate that piscivore production could be double the current value of 0.27 g∙m−2∙yr−1 if the forage fish community changed to include species more available to salmon.

OPINION Manipulating lake community structure: where do we go from here?

SUMMARY. 1 More than 10 years experience with whole lake pelagic manipulation has suggested some general trends applicable to all freshwater pelagic communities and some specific trends related to lake depth.2 Among the general trends is the observation that the trophic cascade is strongly damped. This means that changes in phytoplankton biomass can be assured only when the fish community is strongly manipulated.3 Among the depth related trends is the observation that in shallow lakes, changes in fish community structure are more likely to have cascading impacts on phytoplankton than are changes in deep lakes.4 In shallow lakes, fish removal frequently results in decreased turbidity which is associated with the development of dense macrophyte populations and significant reductions of algal standing stocks. The mechanisms involve: increased grazing by zooplankton, the removal of fish induced bioturbation and nutrient recycling, and direct and indirect macrophyte effects (shading, zooplankton refuges and competition for nutrients).5 In shallow lakes, where planktivore biomass can be regulated and macrophyte development is acceptable, fish biomanipulalions are likely to result in reduced algal populations and improved water quality.6 In deep lakes, where macrophytes are not as important, long‐term effects of fish manipulations are strongly dependent upon the probability of non‐grazable algal bloom development. This is determined by many factors (chemical, physical and grazer related) which modify the impact that grazers have on phytoplankton biomass.7 In deep lakes, successful fish biomanipulations may only be effective when chemical and physical factors are altered to produce algal species compositions that permit strong top‐down control of prey by predators.

Bottom‐Up and Top‐Down Impacts on Freshwater Pelagic Community Structure

For freshwater pelagic ecosystems, the biodiversity and cascading trophic interaction theories both predict that decreased piscivore populations will result in direct, short—term (a few years) increases in planktivore biomass, reductions in crustacean herbivore biomass, and increases in chlorophyll a concentration and phytoplankton biomass. An Alternate view is offered by the bottom—up:top—down theory, which predicts that in eutrophic lakes changes in piscivore biomasses will have strong impacts on planktivore numbers, weaker but observable impacts on zooplankton biomass, and little or no long—term effects on phytoplankton biomass. A partial winterkill at Lake St. George, Ontario, Canada allowed us to test these predictions. The data set comprised measures of: (1) piscivore and planktivore numbers, (2) zooplankton species composition, size structure, and biomass, (3) chlorophyll a concentration and Secchi depth, and (4) water chemistry from 1980 through 1986. Prior to the winterkill of 1981—1982, the piscivore population was high (1000—2000 piscivores/ha), the planktivore population was intermediate (8000—10 000 planktivores/ha), zooplankton biomass was intermediate (2400 µg/L), and chlorophyll a concentration was high (5—12 µg/L). In the year following the winterkill (1982), piscivore and planktivore numbers were low, and zooplankton biomass and chlorophyll a concentration were high. During the next 2 yr (1983—1984) the planktivore population increased rapidly to densities >20 000 individuals/ha, zooplankton biomass density decreased to <1600 µg/L and chlorophyll a concentration decreased. During the final 2 yr of the study, piscivores recruited to near prewinterkill levels, planktivores were reduced to <8000 individuals/ha, zooplankton biomass increased, and chlorophyll a concentration decreased. Over the 7 yr data set, we found a strong negative correlation between numbers of piscivores and planktivores, a weaker correlation between numbers of planktivores and zooplankton biomass, and no between—year correlation between zooplankton biomass and chlorophyll a concentration. There was, however, a positive correlation between total epilimnetic phosphorus and chlorophyll a concentration. These data are consistent with predictions made by the bottom—up:top—down model, and the implication is that at Lake St. George, the trophic cascade uncouples at the zooplankton → phytoplankton link. We speculate that this may be due to the combined effects of lake trophy and Daphnia species composition and size.

ENCLOSURE EXPERIMENTS: THE EFFECTS OF PLANKTIVOROUS FISH

ABSTRACT Limnologists manipulating segments of freshwater ecosystems with enclosures frequently exclude or ignore fish populations. Both courses of action can seriously bias results because a strong negative correlation (r = .77) exists between fish and zooplankton biomass, and failure to account for planktivore effects will jeopardize an accurate interpretation of zooplankton results. These effects are also transmitted to the phytoplankton community. In eutrophic lakes, enclosures containing planktivore bio-masses of less than 20 kg/ha often support populations of large Daphnia, which can have strong negative impacts (r = .81) on phytoplankton biomass. When planktivore biomass is more than 20 kg/ha, large Daphnia cannot survive and zooplanktonic regulation of phytoplankton biomass is not observed. We conclude that even though enclosure experiments may not include the effects of planktivores, the latter cannot be ignored.

BIOLOGICAL MANAGEMENT OF A POND ECOSYSTEM TO MEET WATER USE OBJECTIVES

ABSTRACT The biological structure of a 1.4 ha (3.5 acre) pond in Oldwick, N.J., was assessed during 1983 to facilitate improving swimming and fishing conditions. A biological management strategy, including reducing planktivore populations and restructuring of fish community size distribution, was employed in the spring of 1984. Although fish biomass was reduced, proportional stock density values indicated that fishing quality was substantially improved. Growth and recruitment of gamefish appeared to be minimally affected after one year of post-implementation monitoring. Reduced adult planktivore biomass lessened predation pressure on zooplankton stocks, which increased in both abundance and mean individual size. A resultant increase in grazing activity yielded reduced phytoplankton biomass through the summer and increased water clarity. Appeal and safety for contact recreation were noticably enhanced. The results suggest that managing fish populations can have cascading effects on other trophic levels to ...