Pelagic-benthic coupling- The importance of diatom interannual variability for population oscillations of Monoporeia afinis

The amphipod Monoporeia a&is (LindstrGm) exhibits long-term population oscillations in oligotrophic Lake V&tern. Correlation showed profundal oxygen and temperature, and intraspecific and predatory interactions to be poor explanatory variables. Conversely, the interannual variability in food availability-diatom biovolumewas highly correlated with amphipod total density at t + 1 yr and the density of 0’ amphipods and growth (amphipod body length in August) of O+, I+, and 2 b generations. Amphipod recruitment success of 0’ animals may be an important factor governing long-term population dynamics, and strong 0.’ year classes result in greater l+ and 2’ population densities. Fish predation, although tentatively not an important population-regulating factor, was negatively correlated with amphipod total body lengths of the I+ and 2.’ age classes in May. Size-selective fish predation pressure on juveniles and adults may facilitate coexistence of the 3-yr age classes. Although it is a generally accepted paradigm that pelagic detrital pulses of autochthonous (phototrophic) material drive profundal zoobenthos production (Brinkhurst 1974; Graf 1989), in temperate lakes the quantitative importance of trophic coupling between secondary profundal producers and phytodetritus (particularly Bacillariophyceae) is equivocal. Simply put, little is known regarding whether profundal macroinvertebrate growth is supported directly by nutrients in fresh or decaying algae, indirectly via microbial production, or through the utilization of accumulated refractory detrital carbon directly (e.g. Johnson et al. 1989). A further caveat in understanding the relative importance of detrital algae or bacteria as dietary sources of C for profundal communities is that the food availability varies seasonally. Diatoms, due to a relatively high sedimentation rate and C content per cell, are an important source of C for profundal macroinvertebrates following the sediment deposition of Bacillariophyceae blooms Acknowledgments This work was partially supported by a grant from the Swedish Environmental Protection Agency (No. 53 13400-3). We thank the many people involved in this sampling program. Special thanks to E. Will&n for discussions concerning the ecology of diatoms and to Lars Eriksson for measuring Monoporeia length. We also thank anonymous reviewers for their comments and suggestions. (Jonasson 1972; Johnson 1986). However, in dimictic, temperate lakes Bacillariophyceae biovolume varies temporally, both seasonally and interannually, making this an unpredictable food resource. Conversely, bacterial C, though quantitatively less important when compared with diatoms, is a more predictable source of C (Johnson et al. 1989; Goedkoop and Johnson 1993). It has been postulated that species exposed to an unpredictable food supply should store more energy than those confronted with more uniform food conditions (Slobodkin and Richman 196 1). In other words, a strong evolutionary linkage might be expected between an unpredictable food source such as pelagic (diatom) production and zoobenthos which can store energy in the form of large quantities of lipids. In Lake Michigan, the nutritional value of particulate matter reaching the profundal has been shown to vary seasonally with the highest flux, of detrital diatoms, in spring (Gardner et al. 1989). Lipid content of the amphipod Diporeia hoyi (Smith) also varied seasonally in Lake Michigan, with the highest values noted in May following the spring diatom bloom (Gardner et al. 1989; Gauvin et al. 1989). Moreover, the lipid content of profundal macroinvertebrates varies with feeding behavior. Gardner et al. (1985) showed, for example, that surficial deposit-feeding amphipods and chironomid larvae had