Episodic loadings of phosphorus influence growth and composition of benthic algae communities in artificial stream mesocosms

Abstract

Phosphorus (P) is an essential macronutrient for algal communities, but in excess can exacerbate stream eutrophication. However, P loadings to streams vary temporally from continuous to episodic as a result of inputs from point and non-point sources, respectively. P loading pattern can thus alter the temporal availability of P and may influence effects of P enrichment on algal communities. We assessed how P loading pattern influences algal biomass and composition by conducting a 29-day P enrichment experiment in nine artificial streams exposed to either: (1) continuous P enrichment; (2) episodic P enrichment, or; (3) no P enrichment. P enrichment increased algal biomass accrual, but peak biomass did not differ between continuously and episodically enriched treatments. Maximum absolute growth rates were also comparable between P enriched treatments. However, episodic P additions sustained elevated rates of biomass accrual, whereas absolute growth rates in the continuously enriched communities declined towards the end of the experiment. P enrichment resulted in comparable increases in relative abundance of chlorophytes and decreased proportions of bacillariophytes and charophytes in algal communities for continuously and episodically enriched treatments. However, composition of bacillariophyte (diatom) assemblages differed significantly among all P enrichment treatments in accordance with species autecological attributes for P. Our results demonstrate that episodic and continuous P enrichment may augment algal biomass similarly. Yet, P loading pattern regulated the composition of algal communities. Thus, remedial management strategies for the control of nuisance algae production may require focus on the predominant source of P to streams. Finally, species specific responses of diatom assemblages to P enrichment and associated loading patterns suggests this taxonomic group may have potential as diagnostic indicators for identifying the presence of key nutrient sources associated with eutrophication of stream ecosystems.