Midges (Chironomidae) act as mobile links between aquatic and terrestrial food webs

Aquatic insects are an important subsidy to terrestrial systems and can influence terrestrial food web structure. We conducted a study at Lake Mývatn, a shallow eutrophic lake in northern Iceland containing large populations of aquatic insects, primarily midges (Chironomidae). The Mývatn lake-to-land subsidy is quite substantial, annually depositing tens of thousands of midges m-2. Midges emerge, mate and die in two main pulses, once in early June and again in August. In order to quantify the effect of midge input on terrestrial arthropod food webs, we conducted an experiment where we manipulated midge infall on land. Using mesh cages, we excluded all (full exclusion cages) or some (partial exclusion cages) midges from 2 x 2 m plots for three consecutive summers. We sampled arthropod food webs in June, July and August each year and also collected arthropods for C and N stable isotope analysis. Our cages effectively excluded midges and resulted in lower predator densities. Specifically, wolf spiders (Lycosidae) were more abundant in control plots than in midge-exclusion plots. Predators may have exited the cages to feed on abundant midges outside the cages. Detritivore responses to midge exclusion were mixed because both top-down (predators) and bottom-up (midges) effects were reduced. Detritivore density may also be influenced by the legacy of midge inputs from previous years, thereby buffering detritivores from midge exclusion. Isotopic signatures of arthropods in cages are less enriched, supporting our assertion that arthropod consumers in cages have less access to aquatic-derived resources.