Endemic grazer (Halocaridina rubra) forces and obscured nutrient effects on benthic autotrophy in Hawaiian anchialine pools

Abstract

Anthropogenic impacts on aquatic ecosystems alter the controls of epilithon structure and biomass by altering top-down and bottom-up forces, and their interactions. In Hawai'i, these impacts include increased nutrient loading from development and invasive nitrogen-fixing plants, as well as the continual spread of other alien invasive species. This study examined the effects of nutrient availability and the grazing effects of an endemic aytid shrimp, Halocaridina rubra , on epilithon structure and biomass in Hawaiian anchialine pools. Epilithon was sampled by deploying terra-cotta plates with a combination of nutrient enrichment and grazer exclusion treatments in anchialine pools across a background NO2- +NO3 - concentration gradient. Autotrophy and epilithon biomass significantly decreased across this background NO2- +NO 3- gradient and with grazing by H. rubra , whereas the effect of grazing treatments on relative autotrophy (ash-free dry mass chlorophyll a-1 ) was more pronounced in pools with higher background NO2 - +NO3- concentrations. However, minimal effects of nutrient enrichment treatment were found and no algal communities from pools sampled appeared to be nutrient limited over the range of background NO2- +NO3- concentrations investigated. Relatively high background NO2 - +NO3- concentrations in the pools may be responsible for the lack of nutrient treatment effects. Instead, potential effects of salinity and nutrient toxicity on epilithon may be important factors. My results suggest that grazing by H. rubra is a primary driver of epilithon structure and biomass and that this endemic grazer is a key species in maintaining the natural conditions of Hawaiian anchialine pools across a range of nutrient concentrations and abiotic conditions.