Relationships among nutrients, algae, and land use in urbanized southern California streams

Abstract

We surveyed algal cover, algal biomass, and physical and chemical factors at 14 sites representing a range of land use types in the Malibu Creek watershed in southern California, USA. We also conducted nutrient diffuser substrate experiments to identify the nutrient limiting algal growth. Algal biomass increased with urbanization, reaching very high levels in the most urbanized streams (up to 322.4 mg chlorophyll a·m-2). Total nitrogen, total phosphorus, and benthic and total chlorophyll concentrations were positively correlated with the proportion of upstream land covered by impervious surfaces. Relationships between land use and algal biomass or nutrient concentrations were evaluated at 100 m, 500 m, and whole subwatershed scales. The closest relationships were found at the 500 m scale, where 56% of the variation in total chlorophyll could be explained by the proportion of land within a 500 m radius upstream covered by impervious surfaces. Floating macroalgae were observed in the summer in pools with high nutrient and light levels, whereas benthic algal biomass was positively related to total phosphorus concentrations and current speed. Other methods of determining nutrient limitation (nutrient diffuser substrate experiment, molar N/P ratios in stream water) produced conflicting results, possibly because algal growth was saturated by high nutrient levels at some of the study sites.