Control of phytoplankton by oysters and the consequent impact on nitrogen cycling in a Subtropical Bay.

Although there has been growing interest in using oysters to remediate estuarine eutrophication since the 1980s, the role of oysters in remediation remains controversial. In this study, we took advantage of the intensive oyster farming in a nutrient-enriched estuary to evaluate the control of phytoplankton blooms by oysters. Observing the phytoplankton density variations in March and October, we found that despite doubled nutrient levels in March, oysters kept the phytoplankton density low. This is the first estuary-scale evidence that oysters can effectively suppress phytoplankton blooms. Measuring the isotopic signals in seawater revealed the significant impact of pelagic nitrogen cycling in the estuary. In March, oysters facilitated NO removal in the water column by enhancing the coupled phytoplankton assimilation-oyster filtration and denitrification in the sediments. While in October, the oysters-related nitrification could outcompete the removal processes, adding a significant amount of NO to the water column. Our study illustrates the influence of massive oysters on pelagic nitrogen cycling in an "oyster-remediated" estuary, providing implications for restoring oyster reefs to mitigate the symptoms of estuarine eutrophication.