Are the deep chlorophyll maxima in alpine lakes primarily induced by nutrient availability, not UV avoidance?

Abstract

Alpine lakes are often highly transparent to ultraviolet (UV) wavelengths, which has led to the suggestion that a deep chlorophyll maximum (DCM) results in these systems from UV avoidance by phytoplankton. However, an alternative explanation is that the formation of the DCM is primarily driven by greater nutrient availability below the thermocline in these oligotrophic systems. We investigated the location of the chlorophyll maximum over spatial and temporal scales in a set of high-elevation lakes in the Beartooth Mountains (Montana/Wyoming). The position of the DCM was compared to a suite of physical and chemical variables across systems. Chlorophyll was strongly correlated to a suite of nitrogen variables, whereas correlations with UV parameters were not consistently observed. We also conducted an experiment with the natural phytoplankton assemblage from the DCM in Beartooth Lake; both UV exposure and nutrient additions were tested in a factorial design. The UV-exposed treatment and the control had the same final total phytoplankton biovolume, while the nutrient addition treatment had a final biovolume ten times as great. These results suggest that, as in other oligotrophic aquatic systems, greater nutrient availability in the hypolimnion leads to the development of the DCM in alpine lakes. © 2005 Regents of the University of Colorado.