Drained lakes in Bering Land Bridge National Preserve: Vegetation succession and impacts on loon habitat

Swanson, David; Hampton-Miller, Celia

doi:10.36967/2296593

The northern coastal plain of Bering Land Bridge National Preserve (BELA) lost lakes at an alarming rate over the first two decades of this century, including four lakes over 100 ha in size in 2018-2019 alone. To understand the effects of these lake drainages, we sampled vegetation of these lakes in 2019 (a reconnaissance visit) and 2021 (for the installation of permanent vegetation monitoring plots). We used these data to summarize the changes that occurred in the first three years after drainage, and to create vegetation maps from 3-m resolution satellite images coinciding with the visit dates. We used time series of these satellite images to study the rate of drainage and vegetation colonization on the lakes. We analyzed our existing data from older drained lake basins (estimated to be more than 200 years since drainage) and reviewed the literature on vegetation change in drained lakes to understand the vegetation changes that are likely in the future. Finally, we used a model of lake occupancy by loons developed by Mizel et al. (2021) to predict the effect of the 2018-2019 lake drainages on available loon habitat, using both our detailed maps of the four sampled drained lakes, and also data on all drained lakes over most of northern BELA derived from Landsat satellite images. Our results show that the four study lakes drained early in the summer, before the end of June, in 2018 (3 lakes) and 2019 (one lake). A combination of record warm weather and heavy snowfall made 2018 and 2019 especially favorable for lake drainage: thaw subsidence probably enlarged existing drainage outlet channels from the lakes, and large amounts of spring snowmelt runoff deepened the outlet channels by thermal erosion (the combination of thaw and erosion). Drainage exposed moist loamy sediment on the lake bottoms that was rapidly colonized by plants. Substantial vegetation cover developed by late summer in the same year as lake drainage in one lake, in the first post-drainage summer in a second lake, and during the 2nd year after drainage in the remaining two lakes. The first vegetation communities to develop consisted of just one or two dominant species, notably Eleocharis acicularis (spike rush), Equisetum arvense (horsetail), and/or Tephroseris palustris (mastodon flower). Other important early species were Arctophila fulva (pendant grass) and Rorippa palustris (yellow cress). By year 3, the communities had become more diverse, with significant cover by taller wetland graminoid species, including A. fulva, Eriophorum scheuchzeri, and Carex aquatilis. Frozen soil was observed in most locations on the lakes in July of 2021, suggesting that permafrost was forming on the lake bottoms. Comparison of the three-year trends in vegetation change with data from older lake basins suggest that ultimately most lake basins will develop wet tundra communities dominated by Carex aquatilis and mosses, with various low shrub species on acid, peat-dominated soils and permafrost; however, this process should take several centuries. The loon habitat model suggests that drainage essentially eliminated the potential habitat for Yellow-billed Loons on the four study lakes, because the residuals ponds were too small for Yellow-billed Loons to take flight from. A total of 17 lakes drained in northern BELA in 2018-2019. As a result, the potential Yellow-billed Loon nesting habitat in northern BELA probably decreased by approximately 2%, while habitat for Pacific Loons decreased less, by about 0.6%. Habitat for the more abundant Red-throated Loons probably increased slightly as a result of lake drainage, because of their ability to use the small residual ponds created by lake drainage.

Drained lakes in Bering Land Bridge National Preserve: Vegetation succession and impacts on loon habitat

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