Implications of nitrogen‐rich glacial meltwater for phytoplankton diversity and productivity in alpine lakes

Abstract

We compared phytoplankton diversity and productivity over various time scales in a set of lakes in the central Rocky Mountains of North America (fed by both glacial and snowpack meltwaters [GSF] and by snowpack alone [SF]) to better understand the influence of nitrogen-rich glacial meltwater on the structure and function of phytoplankton. Nitrate concentrations in GSF lakes were on average 44 times higher than in SF, even though only 0.01-0.59% of the catchment area of GSF lakes was covered by glaciers. In three lakes of each type, we determined the vertical distribution of algae, epilimnetic primary productivity rates, and nutrient-limitation patterns. Phytoplankton richness and community structure were compared between lake types in the nutrient enrichment experiments, as well as in the tops (i.e., modern) and bottoms (circa 1850) of sediment cores from four lakes of each type. Average primary productivity rates were five times higher in GSF lakes, but vertically integrated chlorophyll concentrations did not differ. However, algal biomass was higher in the epi-and metalimnion of GSF lakes, while it was equivalent in the hypolimnion of both lake types. Nutrient-limitation patterns differed between lake types, with GSF lakes limited by phosphorus and most SF lakes co-limited by nitrogen and phosphorus. Modern diatom species richness was lower in GSF compared with SF lakes, whereas differences between lake types were not apparent during the mid-19th century. Key differences in the structure and function of GSF compared with SF lakes imply that GSF lakes will take a different ecological trajectory if glaciers disappear.

Phytoplankton responses to nitrogen enrichment in Pacific Northwest, USA Mountain Lakes

“…To classify species and biomass nutrient limitation, four treatments were established (control, N, P, N ? P) using concentrations similar to those in previous bioassays (Elser et al, 2009b;Slemmons & Saros, 2012;Bergström et al, 2013). Treatments receiving N had 112 lg N l -1 added in the form of NaNO 3 ; treatments receiving P had 31 lg P l -1 added as NaH 2 PO 4 .…”

“…Bioassays were conducted during late summer (August or September) in 2013 (NOCA) or 2014 (MORA and OLYM). Our general approach was modeled after other studies (Nanus et al, 2012;Slemmons & Saros, 2012). We collected lake water, divided it among 1 l containers to create experimental treatments, incubated containers in the lake for 7-11 days, and quantified changes in phytoplankton biomass and species densities resulting from treatment nutrient additions.…”

“…Nutrient limitation of biomass growth was classified using an approach similar to Slemmons & Saros (2012). A one-way ANOVA and Tukey HSD means separation test were applied to Chl a data from the control, N, P, and N ?…”

Climate regulates alpine lake ice cover phenology and aquatic ecosystem structure

“…The same climate factors that influence ice cover phenology also affect rates of glacial and permafrost thawing, which have been increasing in the past three decades in the Green Lakes Valley and surrounding areas [Hoffman et al, 2007;Caine, 2010]. Meltwater from these sources is a primary vector of nitrogen and rock weathering products (e.g., calcium, magnesium, and sulfate) that are transported into lakes, particularly in autumn months [Williams et al, 2006;Mast et al, 2011;Slemmons and Saros, 2012;Barnes et al, 2014]. As a result, years with early ice-off lead to greater inputs of late season ions into the lake.…”

Nitrogen Subsidies in Glacial Meltwater: Implications for High Elevation Aquatic Chains

“…While the mechanism for the source of this high N remains unclear, there are ecological implications from an increase in this subsidy. SF lakes in this region are commonly colimited by N and phosphorus (P) or limited by N alone (Morris & Lewis, ; Saros et al, a) and N‐enriched GSF lakes are P limited (Slemmons & Saros, ). The N subsidy in glacial meltwater leads to shifts in nutrient limitation, and changes in the N:P stoichiometry increase primary production rates and decrease phytoplankton species richness in GSF lakes (Slemmons & Saros, ).…”