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Effects of Experimental Fertilization on the Benthic Macroinvertebrate Community of an Arctic Lake

The benthic macroinvertebrate community of experimentally divided Lake N-2 (67°38′27″N, 149°37′30″W) was studied during six years of experimental fertilization. The dominant snail, Lymnaea elodes, increased on the fertilized side of the lake. Another snail, Valvata lewisi, previously shown to be competitively inferior to Lymnaea, did not increase in the fertilized sector until the fifth year of the study, but then increased dramatically. Chironomids were more abundant and had higher biomass on the reference side of the lake, an initial condition, and did not increase in the fertilized sector. Slimy sculpin, previously shown to control chironomids on bare sediments in nearby Toolik Lake, were the only fish predators on chironomids in Lake N-2; they did not grow faster in the fertilized sector. These results suggest that the dominant benthic invertebrates in N-2, chironomids and snails, were limited by different factors; chironomids were not food limited and appeared to be limited by predators, but snails, free from predation in N-2, appeared food limited. The planktonic model of cascading trophic interactions, if applied to benthic communities, would predict that, at low (zero) piscivore density, sculpin density and/or growth would increase as increased primary productivity was passed up the food web; they did not. The physical structure of the benthic zone is such that edible food can accumulate, whereas this cannot occur over extended time scales in planktonic systems. I suggest that this structural difference leads to functionally different constraints on planktonic and benthic communities.