Are alpine lake outlets less harsh than other alpine streams?

Abstract

We investigated the intra-annual chemical and physical properties of 16 lotic systems situated in the Swiss Alps that comprised alpine lake outlets and streams of kryal and rhithral origin. A primary goal of the study was to examine whether alpine lake outlets are less harsh lotic habitats than alpine non-outlet streams, as expected from studies of lowland lake outlets. The presence of a glacier and associated seasonality in glacial melt were primary determinants of environmental conditions in alpine lotic systems through the influence on flow regime, water temperature, and seasonal changes in chemical variables. Lake outlets exhibited higher maximum water temperatures and lower daily temperature fluctuations compared to non-outlet streams. Channel stability of the lotic systems was analyzed using estimates of shear stress, the Pfankuch score (PSI) and a multivariate habitat index (MHI). Indices using shear stress characterized channels with stream slopes ≥15 % as least stable and lake outlets with slopes ≤4 % as most stable. PSI and MHI mainly separated rhithral sites as stable and kryal sites as unstable, resulting from differences in discharge patterns. None of the indices resulted in distinct differentiation between the habitat stability of lake outlets and respective streams. Assessment of these indices suggested that it is necessary to incorporate both spatial and temporal changes in habitat parameters to provide a valid measure of the stability of a particular alpine stream type. Our results indicated that the habitat characteristics of alpine lotic systems are mainly controlled by the presence of a glacier and its seasonality, and on a lower hierarchical level by the presence of an upstream lake.