Potential influences of submarine groundwater discharge, nutrients, and herbivory on Hawaiian reef algae

Abstract

Phase shifts have occurred on tropical reefs throughout the world. In Hawaii, phase shifts have been attributed to eutrophication, herbivore reduction, and alien algae. The first study from this dissertation examines the environmental relationships of Submarine Groundwater Discharge (SGD), a likely source of anthropogenic nutrients to Hawaii‟s reefs. The second study analyzes the relative and simultaneous effects of nutrient enrichment and reduced herbivory on algal succession in contrast to control conditions. The third study assesses the potential of indigenous herbivorous fish to control alien algae. The fine scale salinity profiles in the first study established differences between the average salinity values of three sample events, and correlations between SGD and low tide and, SGD and depth. Water quality samples displayed correlations with SGD and concentrations of numerous chemical constituents. Nitrate from water quality samples at Waiopae were up to 3.8 times greater than samples from upland wells suggesting that anthropogenic sources of nitrate are intruding the aquifer. This study exemplifies the dynamics of SGD and nutrient intrusion. Results from the second study in Kealakekua Bay show that the effects of nutrient enrichment were minor, herbivore reduction were considerable, and nutrient enrichment combined with herbivore reduction were major in governing algal biomass and cover. Coralline crustose algae dominated surfaces under control and nutrient enrichment. In herbivore reduction treatments turf algae dominated. Nutrient enrichment had a greater effect at one site, and herbivore reduction had a greater effect at another site, demonstrating that a single solution may not be equally effective for all Hawaiian reefs. Feeding assays from the third study documented that two herbivorous fish species were potential biological control agents for managing alien algae. These fish can achieve different objectives in an effort to manage alien algae. Acanthurus triostegus is suited to control attached populations, Kyphosus vaigiensis is crucial in reducing dispersal by fragments. Results from this dissertation show that nutrient concentrations enter the near shore environment quite rapidly on Hawaii. With sustainable land use practices and increasing herbivore pressure by enhancing select herbivore species, we can improve the resilience of Hawaiian reefs and minimize the possibility of phase shifts.