Effect of nutrient enrichment on turf algae productivity

Abstract

Filamentous turf algae have become a dominant group in coastal marine ecosystems as coastal urbanization increases nutrient levels. These algae are multispecies assemblages composed of small macrophytes that are invariably coupled with a suite of epifaunal and microbial consumers. While photosynthetic productivity is generally assumed to be greater than turf assemblage respiration, turf assemblages may shift from being carbon sinks to carbon sources in urbanized, nutrient rich conditions. In San Juan Island, Washington, we examined the effect of nutrient enrichment and epifauna exclusion on turf assemblage productivity with a fully factorial design of nutrient enrichment and epifaunal exclusion treatments. Results suggested that fertilizer treatments increased the nitrogen content of turf assemblages. In turn, high nitrogen conditions coincided with lower overall productivity, but only when meiofauna were present. This suggested that meiofauna were partly responsible for decreasing productivity in nutrient rich turfs. Epifauna exclusions lowered respiration rates slightly and did not have a significant effect on productivity. The exclusions were performed with Carbaryl, an insecticide that targets arthropods, but Carbaryl treatments did not appear to influence densities of macrofauna. Thus, reduced respiration rates from Carbaryl were likely due to meiofaunal or microbial response. While our results from this experiment suggest that carbon absorption may be negatively affected in urban areas due to the expansion of turf algae, future studies repeating this approach are needed and should be performed at low flow-sites.