The ability of planktivorous fish to regulate the population dynamics of Daphnia gale&a in Lake Mendota, Wisconsin, was examined during spring and summer 1987. Low rates of consumption by yellow perch (Percaflavescens) and cisco (Coregonus artedii) allowed populations of D. galeata to expand rapidly during spring, resulting in a distinct clear-water period that lasted from midMay until early June. A subsequent decline in D. guleatu abundance resulted from low food availability brought about by its exploitation of algal resources. Once populations of D. guleuta were reduced, predation by perch and cisco was sufficient to keep populations of D. g,lleata at low densities during July and August in spite of more abundant algal food resources, Extreme seasonal variation in the degree to which planktivorous fish can regulate daphnid population dynamics may have allowed daphnids to persist in Lake Mendota during the past century in the presence of abundant zooplanktivores.A distinct clear-water phase characterized by high Secchi disk transparency and low phytoplankton biomass occurs each spring in many meso-and eutrophic lakes (Lampert 1978;Shapiro and Wright 1984; Lampert et al. 1986). In most instances this clearwater phase occurs because herbivorous zooplankton become abundant and graze phytoplankton populations to low levels. Lampert et al. (1986)

Using a series of field enclosure experiments, we investigated the trophic dynamics of Leptodora kindtii in Lake Mendota. Leptodora fed on both small and large Daphnia as well as on Diaphanosoma, Conochilus, and copepod nauplii. These results were combined with field data and a computer simulation model to examine how various densities ofLeptodora might affect the densities of Daphnia and other zooplankton in Lake Mendota. Leptodora could account for all of the mortality calculated for Daphnia galeata mendotae in late June to early July. Reduced numbers minimized Leptodora's potential, however, for direct control of Daphnia earlier in the year. The simulation model demonstrated that, because the dominant planktivorous fish in the lake fed on both Leptodora and daphnids, a decrease in planktivorous fish could result in higher Daphnia mortalities due to increased predation by Leptodora. Although invertebrate predators are found in virtually all aquatic systems, their roles in shaping the community structure of lakes are poorly understood. Field researchers have seen dramatic temporal correlations between abundances of invertebrate predators and their prey (Edmondson and Litt 1982;Wright 1965). Feeding experiments with Chaoborus (Fedorenko 1975; Vanni 198 8) and Mysis relicta (Nero and Sprules 1986) confirm their potential for consuming large quantities of zooplankton. Despite this potential impact, invertebrate predators are not included in many models of aquatic food webs (Bartell et al. 1988).A research project is being conducted in Lake Mendota (Univ. Wisconsin-Madison, Center for Limnology, and the Wisconsin Dep. Nat. Resour.) to examine the potential for controlling the intensity of nuisance algal blooms by increasing the abundance of I Present address and address for reprints:

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We tested the hypotheses that benthic macroinvertebrates, especially chironomid larvae in the Tribe Chironomini, from small oligotrophic arctic lakes are only weakly linked to pelagic food sources, and that they are trophically linked to biogenic methane. All offshore benthic macroinvertebrates sampled from 20 lakes were depleted in 13 C compared with nearshore benthic macroinvertebrates. Nearshore macroinvertebrates had ␦ 13 C consistent with feeding on periphyton and/or terrestrial detritus. However, ␦ 13 C of offshore Chironomini, the dominant macroinvertebrate group, and oligochaetes, was more depleted than that of other offshore macroinvertebrates to a degree that could only be explained by a diet that included carbon derived from biogenic methane. Seston ␦ 13 C showed a small shift toward greater depletion in deeper lakes, but ␦ 13 C of Chironomini, oligochaetes, and predatory chironomids varied as a function of dissolved oxygen concentration (DO) at the sediment-water interface, which was generally lower in shallow lakes that were deep enough to stratify. In lakes with lower DO in bottom waters, the importance of methane-based carbon to Chironomini was greater. Experimental 15 N enrichment of phytoplankton in small-and mid-sized arctic lakes resulted in limited 15 N enrichment of most benthic macroinvertebrates, indicating that their productivity was not closely tied to pelagic production. It is likely that aerobic benthic metabolism of dissolved organic carbon (DOC) also is an important energy source for benthic macroinvertebrates in sediments because the link between benthic macroinvertebrate production and contemporary pelagic primary production was weak regardless of dependence on biogenic methane.