Benthic macro invertebrate communities of high conservation value Thirsty and Little Thirsty Lagoons, Cape Barren Island, Tasmania.

Abstract

This study represents the first account of the invertebrate ecology and biology of the estuarine Thirsty and Little Thirsty coastal lagoons on Cape Barren Island, Tasmania. Due to its remoteness, Thirsty Lagoon is one of the most pristine coastal lagoon systems in Tasmania and is, therefore, an important reference point against which to measure future human impacts in coastal lagoons in the Bass Strait islands, and in south-eastern Australia generally. The system comprises two interconnected lagoons. The lower of the two lagoons, Thirsty Lagoon, is connected to the sea by an open channel allowing tidal exchange. This exchange maintains salinities in the lower reaches at or near seawater concentrations. As the basin is shallow, rates of evaporation are high, particularly in summer, elevating salinity levels and resulting in periodic drying-out of sections of the lagoonal system. At the time of our visit in late summer, freshwater input from feeder streams was minimal and there was little tidal exchange between Thirsty and, the upper lagoon, Little Thirsty. As a consequence salinities in Little Thirsty were very high (ca. 60). These coastal lagoons, and one other sampled, supported a low diversity of invertebrate fauna that is typical of coastal lagoons elsewhere in Tasmania. The fauna included marine polychaete worms, molluscs, small crustaceans and high densities of a dipteran larvae in Little Thirsty Lagoon. The fauna found in the lower reaches of Thirsty Lagoon include a number of invertebrate species that are typically marine in origin, while the upper reaches were dominated by species that commonly occur in estuaries elsewhere, albeit in low salinity or brackish waters. Despite very high salinities and periodic evaporation, Little Thirsty and Thirsty lagoons supported high densities of invertebrates that may constitute an important food source for visiting migratory and wading birds.