Date of Award
Master of Science (MS)
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 control sites, but none of the differences were clearly associated with the number of years the soils supported aquaculture. Total abundance of infauna, deposit feeder populations, interface feeder populations, and parasite populations were significantly different across sites, with the 8- and 12-year aquaculture sites having significantly higher abundance of infauna compared to the control. All aquaculture sites > 5 years-old had higher abundance of deposit feeders than the control sites. The majority of infauna at aquaculture sites were opportunistic species (Capitella capitata and Corophium volutator), which are indicative of disturbed areas. There was no significant correlation between total abundance of infauna and N and C pools, bulk density, or change in incubation pH (soil sulfides levels). These results suggest the effects of oyster aquaculture on soil properties and infauna are likely driven by site specific impacts of aquaculture, rather than being directly related to time in aquaculture.
My findings show that oyster aquaculture has a significant impact on both the water column and benthic environments. While oysters help to maintain water quality by controlling phytoplankton levels, this activity increases biodepositional inputs, rich in N and C, to the benthic environment. Our biodeposit application study suggested that the microbial and benthic communities within the upper 2 cm of soil could process high amounts of biodeposits over a short time frame. Our long-term study suggested increased levels of N and C in the soil were not proportional to the age of aquaculture use, however, a majority of aquaculture sites had higher N and C levels between 5-20 cm, compared to the surface soils from 0-5 cm. Additionally, total infauna, deposit feeder, and interface feeder populations (dominated by opportunistic species) increased at aquaculture sites, regardless of age of aquaculture use. Results suggest there are minimal impacts to the soil properties, aside from the presence of hydrogen sulfides and N and C sequestration. While soil properties had no statistically significant effect on infauna, it is apparent that disturbances from aquaculture practices may lower the trophic quality of organisms, favoring high abundances of opportunistic species indicative of disturbance. Other ecological interactions that were not apparent in our analysis could help to explain the shift in trophic community structure; these include successional dynamics of specific species, predator-prey interactions, and sulfide tolerance levels. Together, the effects of biodeposition and aquaculture practices, increase total abundance of infauna, especially burrowing infauna, which could help to increase translocation of N and C deeper in the soil profile and enhance effects of bioturbation to the soil environment.
Duball, Chelsea Elizabeth, "Environmental Impacts of Oyster Aquaculture on the Coastal Lagoons of Southern Rhode Island" (2017). Open Access Master's Theses. Paper 1031.