Quantifying phosphorus and light effects in stream algae

Simultaneous gradients of phosphorus and light were applied in experimental streams to develop quantitative relationships between these two important abiotic variables and the growth and composition of benthic microalgae. Algal biovolume and whole-stream metabolism responded hyperbolically to phosphorus enrichment, increasing approximately two-fold over the 5-300 mu g L-1 range of experimental phosphorus concentrations. The saturation threshold for phosphorus effects occurred at 25 mg L-1 of soluble reactive phosphorus (SRP). Light effects were much stronger than those of phosphorus, resulting in a nearly ten-fold increase in algal biovolume over the 10-400 mu mol photons m(-2) s(-1) range of experimental irradiances. Biovolume accrual was light-saturated at 100 mmol photons m(-2) s(-1) (5 mol photons m(-2) d(-1)). Light effects were diminished by low phosphorus concentrations, and phosphorus effects were diminished by low irradiances, but evidence of simultaneous limitation by both phosphorus and light at subsaturating irradiances was weak. Contrary to the light : nutrient hypothesis, algal phosphorus content was not significantly affected by light, even in the lowest SRP treatments. However, algal nitrogen content increased substantially at lower irradiances, and it was very highly correlated with algal chlorophyll a content. Phosphorus enrichment in streams is likely to have its largest effect at concentrations < 25 mu g L-1 SRP, but the effect of enrichment is probably minimized when streambed irradiances are kept below 2 mol photons m(-2) d(-1) by riparian shading or turbidity.