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

In-situ, nutrient amendment experiments (nutrient-diffusing substrata, NDS) were conducted in 12 New Zealand gravel-bed streams to investigate seasona lity of biomass accrual and nutrient limitation of benthic algal communitie s. Benthic algal biomass accrual rates exhibited significant (p = 0.019, re peated 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 nutri ent amendments in summer and least responsive in winter. Temperature may be the underlying cause of these patterns. The ratios of dissolved inorganic nitrogen to soluble reactive phosphorus (DIN:SRP) in streamwater and of str eambed periphyton communities were of limited use for predicting which nutr ient limited NDS bioassays; cellular nutrient content was weakly predictive This study demonstrates the need to consider temporal changes (i.e, season ality) when assessing the influence of nutrients on stream ecosystems, and indicates that the use of nutrient ratios to ascertain which nutrient may l imit benthic algal biomass should be validated with field experiments.