Ecosystem linkages between lakes and the surrounding terrestrial landscape in northeast Iceland

Despite a recent emphasis on understanding cross-habitat interactions, few studies have examined the ecological linkages between lakes and surrounding terrestrial habitats. The current paradigm of land-lake interactions is typically unidirectional: the view is that nutrients and matter are transported downslope from the surrounding watershed to their ultimate lacustrine destination. Emergent aquatic insects, which spend their larval stages in lake sediments and emerge as adults to mate over land, can act as vectors of material, energy and nutrients from aquatic to terrestrial habitats. In this study, we document a gradient of midge (Diptera: Chironomidae) infall rates into terrestrial habitats (measured as g dw midges m(-2) d(-1)) surrounding eight lakes in Northern Iceland (approximate to 6 degrees N latitude). Lakes ranged from having virtually no midge infall (for example, Helluvaostjorn, 0.03 g m(-2) d(-1)) to extreme levels (for example, Myvatn, 19 g m(-2) d(-1)) with abundances of midges decreasing logarithmically with distance from shore. Annual midge input rates are estimated as high as 1200-2500 kg midges ha(-1) y(-1). As midges are approximately 9.2% total N, this can result in a significant fertilization effect of terrestrial habitats with consequences for plant quality and community structure. In addition, we used naturally-occurring delta(13)C and delta(15)N isotopes to examine food web structure and diet sources of terrestrial arthropod consumers surrounding lakes with differing amounts of midge input. Terrestrial arthropods showed increased utilization of aquatic-derived (that is, midge) C relative to terrestrial sources as midge infall increased. This pattern was particularly pronounced for predators, such as spiders and opiliones, and some detritivores (Collembola). These findings suggest that, despite being largely ignored, aquatic-to-terrestrial linkages can be large and midges can fuel terrestrial communities by directly serving as resources for predators and decomposers.