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

In certain alpine systems, glacial meltwater is rich in dissolved inorganic nitrogen and glacially-fed lakes and streams have nitrate concentrations one to two orders of magnitude higher than lakes and streams fed by snowmelt alone. In alpine systems where glacial meltwater is rich in dissolved inorganic nitrogen, it remains unclear how these nitrogen subsidies propagate down a chain of lakes and streams. We assessed the effects of these nutrient subsidies in a set of aquatic chains in the U.S. central Rocky Mountains. Algal biomass, phytoplankton community structure and nutrient limitation via nutrient enrichment experiments were measured in a chain of lakes and streams fed by glacial meltwater and a chain fed by snowmelt alone. The high nitrogen in glacial meltwater resulted in increased algal biomass, a more homogenous phytoplankton community structure, and altered patterns in nutrient limitation in glacially-fed systems. Algal biomass was significantly higher in glacially-fed lakes compared to snowmelt-fed ones. Although not significant, species diversity appeared to be greater in snowmelt-fed lakes than glacially-fed ones. In the glacial chain, both lakes in the chain were phosphorus-limited; the strength of this limitation signal weakened down the chain, with the lake at the bottom showing secondary nitrogen and phosphorus co-limitation. In the snowmelt chain, lakes were co-limited with no change in strength down the chain. Algal biomass varied more in streams but showed an increasing trend down both glacially-fed and snowmelt-fed streams within each chain, with the average algal biomass higher in the glacially-fed system. In glacially-fed streams, periphyton community structure was again, more homogeneous than snowmelt-fed streams. Overall, aquatic chains fed by glacial meltwater had higher variability in nutrient limitation patterns and were more productive. It is important to understand the extent and magnitude of glacier meltwater influences on these interactions to better understand how the loss of this resource subsidy will affect aquatic chains as glaciers disappear