Shrinking boreal lakes as agents of change: untangling structure and function in hydrologically-coupled lakes and wetlands

Abstract

Widespread lake shrinkage has occurred over the last 30 years throughout interior Alaska and other boreal regions. This trend has been broadly linked to climate change, via multiple proximate drivers including permafrost thaw, shifting water balance, and terrestrialization caused by peat growth. The ecological effects of shrinking boreal lakes are still poorly understood. I used space-for-time substitution based on field surveys from a spatially balanced random sample of lakes (n=130) to examine the implications of shrinking lakes in the lowland floodplain of the Yukon River within the Yukon Flats National Wildlife Refuge in northern Alaska. Historical lake shrinkage over the last 30 years increased plant functional diversity, woodiness and aboveground biomass in lake-margin wetlands, despite a significant loss of wetland and lake area. Shrinking lakes appeared to have decreased hydrological connectivity with surrounding wetlands, and reduced organic carbon and nitrogen inputs from the surrounding landscape. However, land cover and bathymetry were better predictors of water chemistry than lake shrinkage. Continued reductions in lake surface area, combined with terrestrial succession, may reduce wetland area and increase the relative abundance of woody wetland vegetation compared to herbaceous plants. Lake shrinkage could also reduce below-ground C stocks, because lake sediments contain more organic C per m² than terrestrial soils, and lake sediment C appears vulnerable to aerobic decomposition. Overall, lake shrinkage will most likely affect plant and animal biodiversity, waterfowl and wildlife habitat quality, and C storage in contrasting ways, and management of drying landscapes may require difficult tradeoffs to be made as a result. These decisions would be aided by process-based modeling that accounts for the role of plant functional traits and explicitly represents hydrological interaction between terrestrial and freshwater ecosystems.