An ecological comparison of turf algae between two sites on West Maui that differ in anthropogenic impacts

Abstract

Kahekili Beach, on the northwest coast of Maui, Hawaiʻi has experienced multiple blooms of native and non-native algae over the past 30 years. These blooms have been associated with run-off from agriculture, coastal development, golf courses, and the Lahaina Wastewater Reclamation Facility injection wells which are now known as the source for substantial wastewater delivery to the coast. Multiple studies have examined the macro-algae, fish and invertebrate communities associated with the wastewater seeps in the nearshore water at Kahekili. However, the turf algae community has never been characterized. Turfs are a functional group made up of small multi-species assemblages of algae that are hard to identify in the field. They are important as a major source of primary production and they can also trap sediments, smother corals and compete with other benthic species for space. For this study, plugs with turf algae were taken from dead Porites spp. coral heads at Kahekili Beach and Olowalu as a comparison site with relatively oligotrophic water. Plugs were examined for turf height, sediment load, percent cover and identification to the lowest possible taxon. Herbivore fish data were obtained from the State of Hawaiʻi Division of Aquatic Resources to assess whether herbivore biomass would influence differences in the turf community between sites. At both sites turf height was positively correlated with higher sediment loads, however at Kahekili, turf was taller near-shore at the seep area compared to Olowalu, where turf was higher in deeper water off shore. Olowalu had higher species richness and near the seeps at Kahekili, many of the plugs had only cyanobacteria. Herbivore biomass for grazers and browsers was similar at both sites but scraper biomass was higher at Kahekili. These results indicated that the submarine groundwater coming from the LWRF is impacting the turf community at Kahekili by reducing species richness, promoting growth of cyanobacteria, and allowing turf to grow taller and trap more sediment.