Water quality and the health of remnant leaf oyster (Isognomon ephippium) populations in four Australian estuaries

Abstract

Leaf oysters (Isognomon ephippium) are large intertidal bivalves that form shellfish reefs. They have a patchy and restricted distribution in estuaries in northern New South Wales, Australia, where the water quality is impacted by a range of anthropogenic stressors from coastal agriculture, urbanisation, industry and recreational activities, along with natural stochastic events such as flooding. Little, however, is currently known about the tolerance of leaf oysters to poor water quality. This study investigated the condition of leaf oyster populations in four estuaries, by assessing the density, size-frequency and condition index. These biological parameters were modelled against habitat water quality parameters including nutrient loads, organic and inorganic carbon, algal biomass, sedimentation and total alkalinity. The water quality assessments were replicated during dry conditions and after rain in two estuaries. Leaf oyster density, size, and body condition significantly differed among the four estuaries. Density ranged from as few as 0.04 oysters per m² in the heavily impacted Richmond River, up to 267 oysters per m² in the Tweed River. Overall water quality was also significantly different among the four estuaries, and significant declines in water quality were observed after rain. Distance based linear models revealed significant correlations between biological indicators of leaf oyster reef health and a range of water quality parameters. Leaf oysters with higher condition occur in locations with high dissolved oxygen and salinity levels, whereas low pH and high nutrient run-off were associated with low density and poorer oyster condition. Nevertheless, dense populations were found to persist in areas with pH below 7.9, in high turbidity (over 27 NTU) and areas dominated by fine silt. Their ability to form dense reefs on estuarine soft muddy bottom habitats with low water flow, suggests that leaf oysters could be used to complement rock oysters in future intertidal shellfish reef restoration programs.

Keywords: Condition index; Northern NSW; Pteriidae; Restoration; Shellfish reef; Water quality.