Environmental Impacts of Oyster Aquaculture on the Coastal Lagoons of Southern Rhode Island

Abstract

Coastal lagoons are shallow estuarine systems which hold significant ecological and economic value to Rhode Island and its coastal communities. As the land around these coastal lagoons has been developed, excess inputs of nitrogen (N) from anthropogenic activity have entered the ecosystems. These inputs have resulted in eutrophication, leading to loss of ecosystem services and poor water quality. Oyster aquaculture has the potential to reduce N inputs via filter-feeding, helping to maintain water quality and ecosystem services. In this study, I monitored water quality within aquaculture and control sites in three coastal lagoons located in southern Rhode Island, to assess the effectiveness of cultured-oysters to maintain water quality. I measured water temperature, pH, salinity, chlorophyll (chl) a, dissolved oxygen, total suspended solids, ammonium, nitrate, and soil pore-water sulfides at both aquaculture and control sites. With the exception of chl a and soil pore-water sulfides, oyster aquaculture had no significant effects on the water quality parameters. Aquaculture areas had significant lower chl a levels, suggesting oysters improve water quality by filtering phytoplankton from the water column. The increase in sulfides in the pore-water suggests that oyster biodeposits also alter the pore-water chemistry in the soil. To further our understanding on the impacts of biodeposits on the benthic environment, I measured changes to the soils at different magnitudes of oyster biodeposition. Oyster biodeposition rates ranged from 0.10 to 0.64 g DW oyster-1 day-1, or 68.86 to 346.47 g DW m-2 day-1, whereby higher densities and larger oysters produced more biodeposits. I applied one week’s worth of biodeposits representing a control (no oysters), an average stocking density (500 oysters m-2), and a high stocking density (2000 oysters m-2) to the soil surface to monitor changes in soil N and C levels from biodeposits over a one-week period. I found that no significant enrichment of either N or C occurred within the soils - even at the highest oyster density - suggesting that the microbial and benthic community can process considerable amounts of biodeposit-derived N (5.4 g m-2 and C (44.3 g m-2) in a short time. To assess the long-term impacts of biodeposits and aquaculture practices on the benthic environment, I inventoried resident benthic infauna and measured particle size distribution, electrical conductivity, bulk density, total N, total C, and incubation pH of soils that supported aquaculture from 0 (control) to 21 years. Significant differences were observed in soil properties among aquaculture sites and