Spatial patterns of ecosystem subsidies from lake to land in northern Iceland

Background/Question/Methods

It has long been observed that Lake Mývatn, a shallow eutrophic lake in Northern Iceland (65°N), experiences periodic pulses in midge production (Chironomidae, especially Tanytarsus gracilentus) that result in population fluctuations on a ≈5-7 year cycle of 3-4 orders of magnitude. This results in a considerable transfer of nutrients (nitrogen, phosphorus) from lake to land, providing a subsidy to the terrestrial vegetation community of circumboreal heathland plants dominated by small shrubs (Salix lanata, Betula nana, and Vaccinium spp.) and grasses. The objective of this study is threefold: first, we use remote sensing data from Landsat and Aster to determine the spatial pattern of this lake-to-land subsidy using proxy measures of vegetation vigor derived from imagery. Second, we use the relationships derived from remote sensing for Lake Mývatn to identify other lakes in Northern Iceland that experience similar vegetation patterns likely related to midge subsidies. Finally, we link the remote sensing data to field-based measures of nutrient inputs to estimate total nitrogen flux (fertilization) from lake to land.

Results/Conclusions

Remotely sensed normalized difference vegetation index (NDVI) values at Lake Mývatn show increases in vegetation vigor extending 250m from the lake edge. NDVI values are very high in the first 100m from the lake (0.65-0.7) and decline to background levels (0.45-0.5) around 300m. These distances correspond to elevated midge infall rates measured in cup traps up to 200m from the lake. Analyses of remote sensing imagery covering a larger area of Northern Iceland reveal at least three additional lakes showing evidence of the midge subsidy to the terrestrial vegetation adjacent to those lakes, though not nearly at the rate of Mývatn. The majority of lakes in the study area do not exhibit a signal of increased NDVI in the adjacent vegetation, thus facilitating a related analysis of the lake and landscape conditions that lead to periodic midge outbreaks. Extrapolations of field measurements from a concurrent study at Mývatn yield estimates of a midge contribution to the terrestrial system of 1,200 kg dry mass ha^-1 y^-1 in 2006 (the first year of the current outbreak) and 118-250 kg N ha^-1 yr^-1. We use our remote sensing measurements to map the spatial patterns of those contributions, which at locations within 75m of the lake exceed 200 kg N ha^-1 yr^-1. Ongoing efforts use remote sensing measures to directly estimate the effects of the midge subsidy on woody plant production.