Effects of flow on filamentous algae and nutrient limitation in lotic systems

This research examined the dual role of flow as a control on filamentous algae in lotic systems and its effect on nutrient limitation, with application to Florida spring-fed rivers. The primary goal was to understand how flow influences both algal growth by regulating nutrient supply and algal abundance by dictating drag forces. Four different approaches were used to address the research objectives: statistical analysis of North American stream datasets, a field survey and in situ experiment, laboratory stream channel experiments, and an ecological simulation model. The statistical analysis of North American streams utilized the LINX (Lotic Intersite Nitrogen eXperiment) datasets, and resulted in the development of a new metric, the autotrophic uptake length. The autotrophic uptake length accounts for nutrient concentration, discharge rate, and autotrophic metabolism and was a better predictor of nitrogen limitation than nitrogen concentration alone. A field survey and experiment at the Gum Slough spring system determined that increased filamentous algal abundance was related to declining discharge, and identified a flow velocity threshold of 35 cm/s above which algal abundance was minimal. The laboratory stream channel experiments tested the effect of flow velocity and nutrient concentration on the metabolism of the filamentous alga Lyngbya wollei , whose response was measured by diel changes in pH. At low nitrate concentration, metabolism was stimulated by an increase in flow velocity from 1 to 5 cm/s, whereas at high nitrate concentration metabolic rates were similar at these two velocities, suggesting that the increase in velocity decreased nutrient limitation at low concentrations. Algal metabolism was lower at higher velocity (10 cm/s) regardless of nitrate concentration. Finally, a simulation model was created to help explain the results of the laboratory experiment. The model indicated that filamentous algae and flow velocity display a subsidy-stress relationship when nutrients are limiting, but velocity is only a stress if nutrients are readily available. In combination, these approaches demonstrate that flow has significant effects on algal growth, abundance, and autotrophic nutrient limitation in lotic systems.