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 m gL 21 range of experimental phosphorus concentrations. The saturation threshold for phosphorus effects occurred at 25 m gL 21 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 mmol photons m22 s21 range of experimental irradiances. Biovolume accrual was light-saturated at 100 mmol photons m22 s21 (5 mol photons m22 d21). 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 m gL 21 SRP, but the effect of enrichment is probably minimized when streambed irradiances are kept below 2 mol photons m22 d21 by riparian shading or turbidity. Primary production by algae provides a high-quality food base for a great number of heterotrophs in aquatic habitats. Variation in the rate of primary production propagates throughout freshwater ecosystems, affecting food webs, community structure, biogeochemical cycling, and even the bioaccumulation of environmental contaminants (Vannote et al. 1980; Smith 2003; Hill and Larsen 2005). When abiotic constraints on algal growth are relaxed, the rate of algal production can outpace its rate of consumption by herbivores, and the potential for habitat degradation by excessive algal biomass becomes a matter of practical concern. Abiotic variables regulating algal growth are therefore of considerable interest to aquatic ecologists and environmental managers. Among the abiotic factors likely to limit autotrophic production in freshwater ecosystems, nutrients and light stand out. The relationship between nutrients and algal production has been a focus of aquatic ecologists for decades. The role of phosphorus in pelagic freshwater ecosystems has received a particularly large research effort, resulting in highly quantitative descriptions of the effects of phosphorus on phytoplankton biomass in lakes (Vollenweider 1976). Attempts to develop predictive models of nutrient effects in streams have been much less successful: regressions of benthic chlorophyll a (Chl a) vs. phosphorus or nitrogen are characterized by large amounts of unexplained variance, only some of which can be accounted for by the effects of disturbance (Biggs 2000; Dodds et al. 2002). The failure to find reasonably precise quantitative relationships between nutrients and algal biomass has frustrated stream ecologists and impeded the development of protective nutrient standards. There are probably a number of reasons for this failure, but the confounding effects of multiple limiting factors undoubtedly contribute significantly to the problem.