Longitudinal dynamics of seston concentration and composition in a lake outlet stream

Concentrations of suspended particulate organic carbon (POC), dissolved organic carbon, bacteria, and chlorophyll a were measured in a lake outlet in western Montana, USA. Seven sites within the first 3 km downstream from the lake were sampled over 14 mo. Downstream change in concentration of each variable was tested for fit to a power function (C=aD ^b ). Downstream changes in POC, bacteria, and chlorophyll a, sometimes fit a power function, and the value of the exponent b was positively correlated with discharge. At low discharge, a downstream decline in lake algae was accompanied by an increase in stream algae. At high discharge, concentrations of lake algae did not change downstream, and some stream algae increased. These patterns suggest that although initial concentrations of suspended organic carbon (seston) are determined by the lake, within a short distance, concentrations become regulated by stream processes. As discharge increases, lake products are transported farther downstream. Thus, the extent of the influence of the lake on seston composition expands and contracts longitudinally with increases and decreases in discharge.