Nutrient limitation of algal biomass accrual in streams: seasonal patterns and a comparison of methods

In-situ, nutrient amendment experiments (nutrient-diffusing substrata, NDS) were con- ducted in 12 New Zealand gravel-bed streams to investigate seasonality of biomass accrual and nutrient limitation of benthic algal communities. Benthic algal biomass accrual rates exhibited sig- nificant (p = 0.019, repeated measures ANOVA) seasonal differences; rates were greatest in summer and least in winter. The degree of nutrient limitation also differed (p = 0.003) seasonally; periphyton community biomass was most responsive to nutrient amendments in summer and least responsive in winter. Temperature may be the underlying cause of these patterns. The ratios of dissolved inor- ganic nitrogen to soluble reactive phosphorus (DIN:SRP) in streamwater and of streambed periphyton communities were of limited use for predicting which nutrient limited NDS bioassays; cellular nu- trient content was weakly predictive. This study demonstrates the need to consider temporal changes (i.e., seasonality) when assessing the influence of nutrients on stream ecosystems, and indicates that the use of nutrient ratios to ascertain which nutrient may limit benthic algal biomass should be validated with field experiments.