The effect of aquatic subsidies on lakeshore arthropod communtities

Insects can serve as important links between aquatic and terrestrial habitats. During the summers of 2008 and 2009 (May-August) we used across season pitfall traps and once monthly vacuum sampling to assess the terrestrial arthropod communities at four lakes in Northeast Iceland. These lakes vary in midge production with midge density also a function of distance from shore, decreasing logarithmically with increasing distance from the lake. Dry mass midge deposition averaged >1.5g•m2•day-1 within the first 50m from shore tapering off to <0.2g•m2•day-1 at 500m from shore at the study lakes during this time. One lake, Mývatn (literally “Midge Lake” in Icelandic), produced hundreds of billions of midges per year, averaging >3.0g•m2•day-1 within 50m from shore. In contrast the nearby “non-midge” lake Helluvaðstjörn averaged <0.10g•m2•day-1 within the first 50m from shore. Together these four lakes provide us with a “natural experiment” in which to examine the changes in community composition across a midge input gradient. Freshly emerged live midges are prey to shoreline predators, while dead midges provide significant amounts of decaying material to detritivores and nutrients to plants. Midge lakes are characterized by greater numbers of spiders (Lycosidae), beetles (Carabidae, Staphylinidae), detritivores (Acari), and herbivores (Cicadellidae). Many taxa also are more abundant close to shore, including spiders (Linyphiidae), beetles (Staphylinidae), detritivores (Collembola, Acari), and herbivores (Cidadellidae). The causes and consequences of these differences have great impact on the food web structure of these shoreline arthropod communities including increased top-down and bottom-up effects.