Physicochemical characteristics, community assemblages, and food web structure in anchialine pools along the Kona Coast on the Island of Hawaii, USA

Abstract

Anchialine pools are unique localized brackish water ecosystems, comprised of fresh groundwater and saltwater from the ocean mixing through subterranean pathways. This study characterized physicochemical properties, biological communities, and food web structures in anchialine pools on Hawaii Island, hypothesizing pronounced differences between pools with and without introduced fishes. Pools varied greatly in size, temperature, salinity, and dissolved oxygen saturation. On average, 46% of observed species were introduced. Native species of high conservation concern such as the shrimp Metabetaeus lohena and Halocaridina rubra were not observed in pools with introduced fishes, where periphytic algal biomass was lower. δ13C and δ15N analyses revealed an unusual food web structure where all animal species (except Gambusia affinis) occupied the same trophic niche, independent of species, size, and sex. These findings illustrate that simple food web structures with opportunistic feeding patterns in all species can occur in ecosystems that have greatly variable physicochemical properties and diverse biological communities. The wide variation in biological communities even in adjacent pools indicates that individual anchialine pools are unique. The high proportion of introduced species and their observed effects on these fragile ecosystems suggest that they pose a serious threat to native biota in Hawaiian anchialine pools.