Potential of Mussel Habitat Enhancement to Alleviate Eutrophication in Nutrient-Enriched Estuaries

Abstract

Through altered freshwater flow regimes and excessive anthropogenic nutrient input, many estuaries around the world are showing signs of eutrophication. As shellfish can alleviate some of these issues through their water filtration capacity, shellfish habitat restoration efforts have increased markedly in the past decade. This study quantifies, for the first time, the water filtration capacity of the Black Pygmy Mussel Xenostrobus securis and the potential for habitat enhancement to alleviate eutrophication issues in a hypereutrophic estuary in south Western Australia. Substrate, comprising coir matting, was deployed by community volunteers in four-panel arrangements in the rivers of the Swan-Canning Estuary onto which X. securis recruited naturally. In the Swan River, average mussel densities were 3377 individuals m⁻², based on 10% mat coverage. River water comprised relatively high particulate organic matter (POM) concentrations, particularly in spring (up to 9.2 mg L⁻¹). Standardised clearance rates (CR; g⁻¹ mussel tissue) were typically greater (> 5.0 L h⁻¹) in summer when chlorophyll a concentrations, salinities and water temperature were elevated, whereas CR was often < 2.0 L h⁻¹ in early spring. In the Swan River, it was estimated that for every square metre of habitat enhanced, 9.2 × 10⁵ L of water could be potentially cleared during spring and 1.7 × 10⁶ L over summer, the latter incorporating 5.3 kg of organic matter into mussel biomass. On a larger scale, 1000 m² of deployed habitat over the course of summer has the potential to clear 24.5% of the volume of the tidal portion of the Swan River and 64.4% of the volume of the smaller Canning River. The results thus demonstrate the efficacy of using cost-effective soft substrates deployed by community volunteers to enhance habitat for mussels and its potential to assist in alleviating eutrophication issues.