Endemic grazers control benthic microalgal growth in a eutrophic tropical brackish ecosystem

Abstract

Anthropogenic changes to nutrient supply, numbers and behavior of grazers and interactions of these factors are known to change epilithon composition and biomass. In brackish waters, these changes occur across wide-ranging abiotic conditions (e.g. nutrient concentrations and salinity), which may alter their relative impacts on microphytobenthic communities. Such mediating processes are poorly understood, particularly in tropical brackish ecosystems. We examined the separate and interactive effects of nutrient additions and grazer exclusion on epilithon composition and biomass in eutrophic Hawaiian anchialine pools between contrasting levels of salinity and concomitant nutrient-rich groundwater influence (2.3 to 22.0 ppt; nitrite + nitrate, 6.3 to 102.6 μM; soluble reactive phosphorus, <0.5 to 5.18 μM). Across these conditions, we found no significant effects of nitrogen (N) and phosphorus (P) enrichment treatments on chlorophyll a, biomass (ash-free dry mass) or autotrophic index (i.e. autotrophy) of epilithon communities, and nutrient addition did not alter the effects of grazing pressure. However, autotrophy and epilithon biomass were lower in low salinity, high groundwater influenced pools, and both were strongly reduced by grazing in all pools. Furthermore, effects from grazing pressure on relative autotrophy were more pronounced in low salinity, high groundwater influenced pools. Our results suggest that (1) grazing is the primary driver of epilithon composition and biomass across these nutrient-enriched systems, (2) microphytobenthic communities in these systems are not N or P limited irrespective of co-varying salinity and background nutrients and (3) selective feeding by endemic grazers mitigates potential bottom-up forces from salinity or increased nutrient-enriched groundwater influence on autotrophy in epilithon communities.