Palaeoecological and biogeochemical analyses of sediments indicates long term changes associated with pearl oyster aquaculture

This study employs palaeoecological and biogeochemical methods to infer historical changes in water quality from sediment cores taken in coastal waters in the Kimberley. At each of the sites sampled we have attempted to... [ view full abstract ]

This study employs palaeoecological and biogeochemical methods to infer historical changes in water quality from sediment cores taken in coastal waters in the Kimberley. At each of the sites sampled we have attempted to compare gradients of human use or natural environmental variability in addition to looking for long term environmental changes. These sites are Cygnet Bay (comparison of pearl farm/non pearl farm sites), King George River (comparison of embayments with high/low riverine input), and Roebuck Bay (comparison of sites near/far from Broome’s anthropogenic inputs. In Cygnet Bay, pearl oyster aquaculture, with a 50-year history, has been regarded as an anthropogenic activity with low environmental risk, however we found that small disturbances accumulated over a long period can be detected. Our cores reflected an 80-year record of environmental processes and with knowledge of the changes in farm practices over 50 years we were able to reconstruct a time series of change in biogeochemical parameters which can be related to changes in water quality. Sediment cores taken inside and outside a pearl farm displayed contrasting characteristics after the start of farming in the 1960s. Total organic carbon, total nitrogen, biogenic silica, and fine-grained sediment inside the farm increased significantly over time. The ranges of C/N, δ13C and δ15N discriminated that the origin of increased organic matter were from autochthonous sources rather than allochthonous input. Our analysis detected significant changes responding to the development of pearl farming rather than variations in rainfall and temperature. Modern long line pearl culture since the late 1980s was presumed to be a dominant driver of the changes we observed, accelerating the increase of organic matter in the sediments by reducing water flow, altering sediment grain size and increasing nutrient flux. Geochemical ratios between the two sites showed pearl farming on decadal time scales, even at low density, can cause environmental change with a two to four-fold increase in organic matter, although consistent with environmental studies on surface sediments at the same sites, there is no suggestion that any environmental damage or ecological changes have occurred.