The effects of natural and anthropogenic perturbations on stream primary producer communities in southern California

These studies were conducted to determine the impacts of natural and anthropogenic perturbations on plants and algae in southern California streams. I examined correlations between algae and nitrogen, phosphorus, and land for 24 streams in southern California, and conducted nutrient diffusing substrata experiments to determine the nutrient limiting growth of benthic algae. Algal biomass increased with urbanization, reaching high levels in the most urbanized streams (up to 408 mg chla m-2). Total nitrogen, total phosphorus, and chlorophyll concentrations were tightly positively correlated with the proportion of upstream land covered by impervious surfaces. At lower nutrient concentrations, algal growth appeared to be limited by nitrogen availability, whereas phosphorus appeared to limit growth in streams subject to greater human influence. Biomass of floating mats of macroalgae (Enteromorpha sp.) responded positively to experimentally increased light and nutrient availability, increasing by up to 800% over four weeks.;I also measured impacts of plant dominance in southern California streams. Ludwigia hexapetala reached high biomass (0.98+/-0.03 kg m -2) downstream of a wastewater treatment plant on the Ventura River, California. The abundance of green macroalgae was reduced, and that of diatoms increased, in the presence of Ludwigia. Ludwigia's rates of growth and nitrogen uptake increased with increasing nutrient availability, accounting for considerable decreases in dissolved nitrate. The dominance of this plant on the river was highly variable over multiple years, with biomass greatly reduced by scouring during winters with high rainfall, then requiring several consecutive years of low flows to re-establish dominance.;A conceptual model is presented for predicting primary producer community and biomass changes in streams in southern California in response to land use changes, particularly urbanization. Increases in light availability result in changes in algal community, from benthic diatoms to green macroalgae, and the inclusion of vascular plants. Increases in nutrient concentrations alone typically result in increases in algal biomass without shifts in type of algae present. However, streams draining extremely urbanized catchments may show a decrease in plant and algal biomass as increases in resource availability are offset by increases in temperature or toxins which depress or inhibit algal growth.