Daniel E. Schindler

Food Web Structure and Phosphorus Cycling in Lakes

Abstract We assessed the effects of trophic structure on phosphorus (P) recycling by fishes, Chaoborus, and zooplankton in lakes with different food web configurations. Fishes in a piscivoredominated system recycled little P compared to that regenerated by zooplankton and Chaoborus. In a plan ktivore-dominated lake, excretion by fishes (especially young of year) dominated P recycling to algae; Chaoborus and zooplankton recycling was decreased due to intense predation by fishes. The sum of P incorporated into primary production and lost to sedimentation in the piscivore-dominated lake was only 77% of that in the planktivore-dominated lake, Our analyses suggest that the 30% greater primary production in the planktivore-dominated system may be accounted for by the greater than 40% increase in estimated P recycling that was directly due to differences in food web structure, Diel vertical migrations by Chaoborus concentrated P excretion in the epilimnion during night hours. In the plan ktivore-dominated system...

Patterns of Fish Growth along a Residential Development Gradient in North Temperate Lakes

ABSTRACT Residential development of lakeshores is expected to change a variety of key lake features that include increased nutrient loading, increased invasion rate of nonnative species, increased exploitation rates of fishes by anglers, and alteration of littoral habitats. All of these factors may alter the capacity of lakes to support productive native fish populations. Fourteen north temperate lakes were surveyed to examine how growth rates of two common fish species (bluegill sunfish, Lepomis macrochirus; largemouth bass, Micropterus salmoides) varied along a residential development gradient. Size-specific growth rates for both species were negatively correlated with the degree of lakeshore residential development, although this trend was not statistically significant for largemouth bass. On average, annual growth rates for bluegill sunfish were 2.6 times lower in heavily developed lakes than in undeveloped lakes. This effect of lakeshore development on fish growth was not size specific for bluegills between 60 and 140 mm in total length. An index of population production rate that accounted for both the size-specific growth rate and the size distribution of fishes showed that bluegill populations were approximately 2.3 times less productive in highly developed lakes than in undeveloped lakes. Our results suggest that extensive residential development of lakeshores may reduce the fish production capacity of aquatic ecosystems.

Density-dependent changes in individual foraging specialization of largemouth bass

Abstractu2002Individual foraging specializations are an important source of intraspecific variability in feeding strategies, but little is known about what ecological factors affect their intensity or development. We evaluated stomach contents in marked individual largemouth bass (Micropterus salmoides) and tested the hypothesis that diet specialization is most pronounced during periods with high conspecific densities. We collected diet data over 10 years from an unexploited population of largemouth bass that displayed a greater than threefold variation in density. Although diet composition of the aggregate bass population did not change during the study, bass body condition was inversely correlated with population size. Individual marked bass exhibited high diet consistency (diet overlap between successive captures) during years with high population densities. Diet overlap between randomly assigned pairs of bass was not correlated with population size. We did not detect the expected positive relationship between diet breadth and population size. Our analyses demonstrate that population responses to density changes may represent the sum of many unique individual foraging responses and would be obscured by pooled sampling programs. Behavioral flexibility of individuals may contribute to the ability of largemouth bass to function as a keystone predator in many aquatic communities.

Biotic feedbacks in Lake phosphorus cycles.

Limnologists are now reconsidering the role of the biota in the phosphorus (P) cycles of lakes. Changes in lake communities can have significant consequences for ecosystem P cycles. At seasonal timescales, the relative importance of nitrogen (N) and Pas limiting factors for primary production depends in part on zooplankton species composition. Phosphorus storage and recycling by fish and zooplankton can be large components of P budgets, and mobile consumers can be important vectors in P transport. Stability, resilience and resistance of lake P cycles may depend heavily on fluxes to and from upper trophic levels.

THE NILE PERCH IN LAKE VICTORIA: INTERACTIONS BETWEEN PREDATION AND FISHERIES

Introduction of the exotic Nile perch (Lates niloticus) into Africas Lake Victoria accelerated decline of the diverse, endemic ichthyofauna, altered food web struc- ture, and created valuable fisheries. As the Nile perch population expanded and predation rates increased, many of the endemic fish species disappeared, total fishery yield increased nearly fourfold, and fishery-related employment approximately doubled. Ecological changes in this system now occur rapidly, are due largely to human actions, and have profound socioeconomic effects. We used a bioenergetics model of Nile perch predation rates to evaluate the consequences of previous, current, and future fishery exploitation patterns and their ecological implica- tions. The analysis produced three main conclusions: (1) Development of fisheries based on large-mesh gill nets reduced total predation by Nile perch to -40% of that estimated during the late 1970s, when Nile perch densities were greatest. (2) Expansion of recent intensive beach seine and small-mesh gill net fisheries for juvenile Nile perch could reduce total predation to -25%. (3) The combination of fishing methods could reduce total pre- dation to 10% of previous levels. Reduction in estimated predatory impact corresponds with recent reports of recovery by haplochromine populations and increased fish species diversity in regions (e.g., Napoleon Gulf, Mwanza Gulf, and Winam Gulf) where fisheries are locally intense.

Effects of aquatic insect predators on zooplankton in fishless ponds

We removed the surface-orienting aquatic insects from a fishless pond to determine their predation effects on zooplankton behavior and size structure. A second fishless pond served as the unmanipulated reference system in this two year study. In the reference pond and the treatment pond prior to manipulation, daphnids exhibited pronounced diel vertical migrations. Following the removal of surface-orienting aquatic insects from the treatment pond, daphnid migration changed to a reverse migration strategy that was significantly different from that observed in the reference system. Average daphnid body size increased significantly following predator removal in the treatment system. Our data indicate that predation by aquatic insect predators, such as notonectids and dytiscids, may affect daphnid migration behavior in fishless systems. Vertical migration by daphnids may allow coexistence with surface-orienting insects in ponds that are deep enough to provide a spatial refuge from these predators.

Food Web Structure and Long-Term Phosphorus Recycling: A Simulation Model Evaluation

Abstract We used models to examine the effects of different food web structures on the longterm dynamics of phosphorus recycling in lakes. We also assessed food web responses to periodic fish recruitment. These responses were compared for models with and without a typical invertebrate planktivore represented by Chaoborus. Nutrient flux was estimated based on bioenergetics models offish growth. Predator–prey interactions were developed as type II functional responses whereby feeding rate per predator increases with prey density up to some intrinsic limit on consumption. The model with Chaoborus produced higher biomass of both piscivorous and planktivorous fish and higher phosphorus recycling rates than the model without Chaoborus. When periodic piscivore recruitment was introduced into the model without Chaoborus, all nonpiscivore components in the model showed the same periodicity as the piscivore recruitment. With Chaoborus, the responses were more complex and generally less sensitive to piscivore recrui...

Homing Tendency of Three Piscivorous Fishes in a North Temperate Lake

Abstract Establishment and use of home ranges is one important component of the way fishes use resources. We evaluated the homing tendencies of largemouth bass Micropterus salmoides, smallmouth bass M. dolomieu, and yellow perch Perca flavescens in a small unexploited lake in Michigan between 1988 and 1990. Homing tendency was estimated as the probability of an individual fish being recaptured at its original site of capture after being moved to a common release site in the lake. All three species demonstrated significant homing tendencies. Yellow perch demonstrated the greatest homing tendency, followed by largemouth bass and then smallmouth bass. For all species, the probability of a recapture at the original site of capture increased with the number of times an individual was caught. Our analysis demonstrates that co-occurring species show differing tendencies to seek out home ranges, and therefore, exploit resources in different ways in a freshwater piscivore community.

Biotic Manipulations of Aquatic Ecosystems

Large-scale biotic manipulations have been performed in aquatic habitats to evaluate how ecosystem structure alters processes and dynamics. These experiments have involved either direct manipulation of species or the habitat structure that mediates species interactions. Manipulations at smaller experimental scales can not encompass the natural biology of wide-ranging species or the system-level heterogeneity that has critical effects on the interactions between community and ecosystem dynamics (e.g., Schindler et al. 1997). Manipulations of aquatic ecosystems are often used to explore the responses of possible management actions or, in fact, are large management experiments (e.g., Kitchell 1992; Hansson et al. 1998; Olson et al. 1998). In this chapter we review briefly some of the techniques used in controlled experimental settings and in natural resource management that have involved biotic manipulations of aquatic ecosystems. We also discuss some considerations for design of ecosystem experiments, and refer to important directions for future ecosystem research. We have limited our review to experiments in freshwater systems, and cite only selected examples of biotic manipulations. For more detailed descriptions of some of the methods we discuss, readers should consult Nielsen and Johnson (1983), Downing and Rigler (1984), and Cooke et al. (1993).