Response of Limnetic Insect Populations of Two Acidic, Fishless Lakes to Liming and Brook Trout (Salvelinus fontinalis)

Seasonal population density estimates of limnetic insects in two Adirondack (New York) lakes were obtained from horizontal and vertical net tows and benthic sweep net samples over a 3-yr period; 1 yr while the lakes were acidic and fishless, and 2 yr following addition of (calcium carbonate) CaCO3 and the introduction of brook trout (Salvelinus fontinalis). Before treatment, the limnetic insect assemblages in the study lakes resembled those reported from acidic and/or fishless lakes in Sweden and Canada. Maximum densities of dominant taxa were: Notonectidae; 1.5∙m−3; Corixidae; 1.1∙m−3, Graphoderus (Dytiscidae) larvae; 0.27∙m−3; and Chaoborus americanus; 400∙m−3. Within 3 mo after treatment, all limnetic populations were near or below the detection limit (0.01∙m−3). Limnetic densities of notonectids, corixids, and C. americanus were significantly lower (Mann–Whitney U-tests), and benthic densities of Hemiptera and Coleoptera tended to be lower (sign tests) the summer after treatment than the previous summer. Calculated trout predation levels on Hemiptera and C. americanus, and evidence from the literature, strongly suggest that predation was the major cause of reduced limnetic insect populations. The rapid reduction or elimination of these populations indicates considerable instability of the predator–prey relationships of acidic lakes which have been recently limed and stocked with fish.