The influence of habitat complexity on littoral invertebrate distributions: patterns differ in shallow prairie lakes with and without fish

Relationships between littoral habitat complexity and invertebrate distributions in fishless lakes are not well understood compared with well-documented relationships in lakes with fish. We examined littoral invertebrate distributions over fine-scale gradients of weed-bed habitat complexity and contrasted these patterns in four shallow prairie lakes — two with fish and two without. The above-sediment portion of submerged macrophytes and associated invertebrates was sampled from three littoral microhabitats: weed-bed centres (highly complex), weed-bed edges (moderately complex), and single plants that grew apart from distinct weed beds (least complex). Total invertebrate densities in fishless lakes did not differ between littoral microhabitats, nor were they correlated with macrophyte biomass. In contrast, total invertebrate densities in lakes with fish increased with microhabitat complexity and were positively correlated with macrophyte biomass. Weed-bed complexity also affected littoral invertebrate community structure; in all lakes, the proportion of filter-feeders decreased with increasing microhabitat complexity, but the proportion of predatory invertebrates was greater overall in fishless lakes than in lakes with fish. Our results demonstrate that small-scale variation in littoral microhabitat complexity can lead to specific patterns of invertebrate distribution that systematically differ between lakes with and without fish, and that these systematic differences may be mediated through top-down mechanisms.