Nitrogen limitation in a Sonoran Desert stream

Four nutrient enrichment bioassay experiments were conducted in Sycamore Creek, Arizona, during summer and autumn 1983. In two experiments, nitrogen and phosphorus were added alone and in combination while in the other experiments nitrogen was added singly. In experiments involving enrichment of both nutrient-diffusing substrates (clay flowerpots) and streamwater overlying tileigravel artificial substrates, nitrogen enrichment significantly enhanced rates of chlorophyll a accrual, primary production, and nitrogen uptake. Addition of phosphorus either singly or in combination with nitrogen did not result in significant responses of these parameters; thus ambient concentrations of phosphorus were above limiting levels, even when excess nitrogen was supplied. Nitrogen additions stimulated periphyton growth when background nitrate-N concentrations were 50.055 mg/L. We propose that nitrogen limitation is common in the desert Southwest since concentrations lower than this and atomic nitrogen to phosphorus ratios <16 occur in most (82% and 87%, respectively) previously surveyed southwestern streams (n=92). Temporal patterns of chlorophyll a accrual suggest that availability of nitrogen limited the rate of algal increase, but not the ultimate periphyton standing crop. If true, this hypothesis predicts that algal recolonization rates should vary depending on nitrogen supply. In desert streams, flood disturbances reduce algal standing crops to near zero, but postflood recovery periods may be quite long. Nitrogen limitation in desert streams thus may exert strong influence on rates and patterns of algal recolonization following floods.