LINK STRENGTH IN LAKE LITTORAL FOOD WEBS - NET EFFECTS OF SMALL SUNFISH AND LARVAL DRAGONFLIES

To show how predaceous fish and dragonflies affect benthic community s tructure we conducted enclosure experiments in the littoral zone of Ba ys Mountain Lake, Tennessee. A ''natural'' benthic assemblage was subj ected to all combinations of 2 densities of 3 predator treatments-smal l sunfish, 0 or 4/m(2); large dragonfly larvae, 0 or 15/m(2); and drag onfly eggs at 2 densities, 90 or 900/m(2). Treatments were assigned ra ndomly in each of 6 spatiotemporal blocks. Net effects of predation ov er 4 mo show that small sunfish had ''strong'' effects (>50% reduction of densities) on triclads, large daphnid cladocerans, and snails, and ''moderate'' effects (50% > reduction > 25%) on small dragonflies and ostracods; all these effects were statistically significant, except t he one on ostracods. Large dragonflies had moderate non-significant ef fects on triclads and ostracods. There were no significant increases i n prey density associated with fish predation; but chydorid cladoceran s and midge larvae showed ''weak'' non-significant increases that migh t be caused by ''indirect effects'' of fish predation on invertebrate predators or grazers. There were only 2 significant interaction terms indicative of ''higher-order interactions'': Fish x Dragonfly on the l arge daphnid Simocephalus, and Fish X Egg Density on snails, which wer e associated with relatively low probabilities. In both cases, the net effect of dragonflies was to reduce prey densities more when fish wer e present.