Biological and nutrient responses to catchment disturbance and warming in small lakes near the Alaskan tundra–taiga boundary

Abstract

Understanding effects of recent climate warming and changes in catchment conditions on nutrient cycling and the biology of shallow subarctic lakes is necessary to predict their evolution. Here, we use multiple analytical methods on sediment cores to identify effects of change in catchment conditions on nutrient availability and biotic assemblages in two subarctic lakes on the Seward Peninsula (Alaska, USA). We compare limnological and biotic responses to flooding and expansion of a thermokarst lake basin (late 1950s), increased shrub growth in the catchment of another lake (since the mid-1980s), and regional warming (since the late 1970s). Among these three environmental drivers, the largest biotic responses occurred because of flooding and expansion of the thermokarst lake. An increase in the nitrogen isotope composition and decline in organic carbon isotope composition in sediments are interpreted to reflect an elevated supply of dissolved inorganic carbon and nitrogen. This was associated with significant shifts in composition of chironomid and diatom assemblages. In contrast, increases in particulate organic carbon and nitrogen from enhanced shrub growth had less influence on the biota. Declines in cold-water biotic indicators typical of warming lakes in Arctic regions occurred several decades after catchment-induced changes to the nutrient supply in both systems. This indicates that initial lake catchment condition may mediate lake-specific changes in nutrient cycling and aquatic productivity within regions undergoing warming.

Paleolimnology of thermokarst lakes: a window into permafrost landscape evolution

“…This is consistent with Jones et al (2011a), who noted that a majority of thermokarst lakes in the northern Seward Peninsula have expanded since the 1950s and that elevated nitrogen export occurs from thawing permafrost (Jones et al 2011b). Medeiros et al (2014) contrasted this thermokarst-driven change in nutrient supply and biotic response with that of a lake whose catchment has experienced substantial shrub development since the 1980s. Nutrient input to the shrub-dominated lake in the early part of the record highly contrasted that of the thermokarst lake, reflected by low C/N ratios throughout the record until the 1980s, indicating ample supply of nitrogen to support aquatic production (Fig.…”

“…Rising sea levels associated with decreased summer insolation, and a greater maritime influence, led to a reduction in seasonality and temperatures and an increase in moisture. However, there has been pronounced recent warming in the SSP, with an increase of~2°C in mean annual temperature since 1979 (Medeiros et al 2014). As the SSP is exposed to prevailing winds from the south during the ice-free season, summer temperatures are warmer than in the northern Seward.…”

“…In the study by Medeiros et al (2014), an increase in supply of dissolved inorganic carbon and nitrogen related to shoreline erosion appeared to be associated with enhanced productivity in an otherwise nutrient-limited thermokarst lake. In contrast, increase in supply of particulate organic matter following an increase in shrub growth in the catchment of another lake had less apparent influence on aquatic biota, whereas more direct responses were linked to warming and hydrological changes.…”

“…6a. Note that x-axis scaling varies as a percentage of the assemblage (adapted from Medeiros et al 2014). increase in cellulose-inferred lakewater δ 18 O (Fig. 7b).…”

“…This shift in trophic structure, despite top-down controls of established endemic keystone predators, may signal the threshold at which a tundra-to-boreal metacommunity occurs (Taylor et al 2016). Medeiros et al (2014) compared the influence of catchment condition, specifically thermokarst development and shrub growth, in lakes of the SSP in the context of recent warming using a multiproxy paleolimnological approach. The sediment record of a thermokarst lake examined indicated a shift from a high input of terrestrial organic matter (i.e., high C/N ratios), yet nitrogen-limiting conditions (δ 15 N values~0‰) (Fig.…”