Limited Benefits of Oyster Aquaculture on Water Clarity in Two Rhode Island Salt Ponds

Shellfish restoration and aquaculture are considered as innovative methods to mitigate in-water nutrients in coastal waters. Water quality was examined at two oyster aquaculture farms in Potter (2021–2023) and Pt Judith (2023) Ponds in southern Rhode Island, USA. Twice monthly, on a flooding tide, upstream and downstream positions were established and water quality measures were made using fluorometry sondes and laboratory analysis. Significant differences in chlorophyll a, turbidity, and nutrient concentrations between upstream and downstream positions were identified; however, the differences were not consistently greater upstream or downstream. Percent Chl a removed varied from −74% to 64% at Potter Pond among years and −51% to 29% at Point Judith Pond, indicating a deficit or increase in Chl a concentration downstream as compared to upstream over each sampling period. Chlorophyll a measured inside oyster bags was higher compared to the upstream position at Potter Pond, and results from the upstream, downstream, and within farm productivity experiment in both ponds suggest oyster byproducts may facilitate localized and seasonal phytoplankton production. Natural resource managers should consider that while oyster farms in coastal ponds can provide local water clarity through removal of phytoplankton, benefits may be site specific and seasonal.