Classification, density, and spatial distribution of polychaete reefs in the Coorong, South Australia

The cosmopolitan species Ficopomatus enigmaticus is one of the most prominent providers of polychaete reefs in shallow and brackish waters. The effects of polychaete reefs on the environment can be positive or negative subject to their structural complexity and vary with local environmental conditions and over time. The Coorong is a large temperate estuarine and lagoonal system with extensive polychaete reefs built by F. enigmaticus. The aims of this study were to (1) classify polychaete reefs based on size and morphology, (2) quantify reef density, (3) assess their spatial distribution and (4) assess the correlation between reefs (e.g., density, size) with the environmental gradient. Structural morphologies of F. enigmaticus were classified as either halo, circular, irregular, platform or fringing reef types. The largest densities of polychaete reefs were recorded in the vicinity of the mouth of the estuary with 224 reefs per ha compared to the lagoon (62 reefs per ha). The most common reef morphology was circular and largest sizes in the lagoon were positively correlated with salinity, chlorophyll a, TRIX (trophic index) and bicarbonate. The largest reef diameter recorded was 11.3 m, exceeding previously known F. enigmaticus circular reef formations globally. The growth of large circular reefs may have benefitted from low flows and eutrophic conditions in the Coorong during the last two decades. This study serves as an important baseline for future assessments of reef change in a Ramsar listed wetland. Furthermore, this study highlights reef characteristics of F. enigmaticus across an environmental gradient, which can be informative for the management of flow and eutrophication in estuaries and coastal lagoons.