Pollution of estuaries from human activities such as agriculture and urbanization has disrupted the global cycling of elements including carbon and nitrogen (N) and reduced stocks of coastal fisheries worldwide. Enhanced water quality in New York City has stimulated interest in reviving the previously abundant eastern oyster to the Hudson-Raritan estuary (HRE). Oysters filter the overlying water for food particles, and then deposit their waste to the sediments below. The delivery of N-rich waste to sediments may stimulate the microbial transformation of biologically active N into inert N gas. This process is the only pathway through which N is permanently removed from aquatic ecosystems; however, the environmental conditions under which oysters could enhance the activity of N-cycling microbes have not been previously examined. This study will document how two important factors, oyster density and water column N concentration, control the influence of oysters on sediment N cycling. The PI?s will quantify rates of oyster filtration, excretion, and sediment N transformations seasonally over a two-year period.
This experiment will document the potential for the re-introduction of eastern oysters to enhance N removal in urbanized ecosystems. The results will directly inform the application of limited management funds towards oyster restoration projects, which can provide the maximum benefits for management of N pollution. To ensure the utility of the findings towards ongoing restoration activity, the PI?s will work in close cooperation with multiple stakeholders including government agencies and non-for-profit groups. Finally, this research supports the participation of students from high school through graduate school levels. The integration of public outreach, education, and robust experimental design are critical to the long-term conservation of urbanized aquatic environments.
Pollution of rivers, lakes, and estuaries from common human activities such as agriculture and urban sprawl has disrupted the global cycling of elements including carbon and nitrogen. Nitrogen is an important element for regulating plant and algal growth at normal levels, but under enriched conditions, can lead to oxygen depletion, thereby reducing fisheries stocks. In New York City, advances in wastewater management over the past decades have improved water quality, stimulating interest in reviving the previously abundant oyster, Crassostrea virginica, to the Hudson River estuary. Restoration of oysters may have multiple positive effects on ecosystem health, and may also help reduce problems related to nitrogen pollution including excess algae. Oysters filter the overlying water for food particles, and then deposit their nutrient-rich waste to the sediments below. The direct transfer of nutrients from the water to sediments as a result of oyster feeding may stimulate the transformation of biologically active nitrogen into inert nitrogen gas by microbes. This is a frequently cited benefit of oyster restoration, however, there have been very few direct measurements of these processes taken under natural conditions. The research study led by Dr. Timothy Hoellein (Loyola University Chicago) and Dr. Chester Zarnoch (Baruch College, City University of New York), documented how two important variables, oyster density and nitrogen concentration, controls the influence of oysters on sediment nitrogen cycling in NYC. In Jamaica Bay, we deployed oysters in mesh bags attached to trays filled with sand. The trays were attached to each other with rope, and were left sitting on the sediment surface in shallow waters. We repeated this experiment at 4 locations in the bay, and deployed oysters at 4 different densities (none, 40/m2, 85/m2, and 150/m2). For 2 years, we measured rates of oyster survivorship, feeding, and nitrogen transformations in the sediment. We found that oyster feeding behaviors were variable across sites and seasons, but in general they were influenced most by the concentration and characteristics of the food particles in the water column. The oyster clearance rates, which describe the volume of water filtered per hour, measured on oysters in Jamaica Bay were low as compared to other previously published values. This was likely due to the high concentration of particles often found in Jamaica Bay. The quality of food particles, such as organic content, was also an important factor influencing oyster clearance rates. Our results suggest that the oyster filtration may be limited in highly urbanized coastal waters and that higher filtration may be more likely realized in less-impacted waters. Oyster survivorship was high in the winter, spring, and summer seasons of our study, however, we did observe a decrease in survivorship in autumn. This mortality was likely due to disease that impacts mature oysters. Therefore, restoration efforts in the Hudson River estuary will have to address disease in their management plans. One of our important findings was that oysters increased the amount of organic material in the sediment at 3 of the 4 study locations. This organic matter is an important food source for microbes which can convert nitrogen from biologically active forms to inert nitrogen gas. However, we found limited evidence that oysters affect how sediment microbes carry out nitrogen cycling. This could be because of 1) high organic matter in sediment without oysters, and/or 2) high amounts of available nitrogen in the water. Both high organic matter and high nitrogen are common in urban-influenced waters. If either factor were lower, it is more likely that oysters would affect transformation of nitrogen from its active to inert form by providing organic fuel for sediment microbes. Our major conclusion for this portion of the research is that oysters may have a limited ability to increase nitrogen transformations in waters which are already affected by urbanization, and may be better suited to this task in ‘cleaner’ waters. While our evidence for an oyster effect was minimal, these measurements did shed a great deal of light on how nitrogen is processed in this urbanized ecosystem, and provides support for their use as enhancing nitrogen removal in less urbanized locations. Our results will help direct the selection of oyster restoration locations which can provide the maximum benefits for enhancement of water filtration and nitrogen removal. To ensure the utility of our findings towards ongoing restoration activity, we worked in close cooperation with multiple stakeholders including government agencies and non-for-profit groups. This research facilitated follow up projects measuring oysters’ influence on restored reefs in the region, and using labeled forms of nitrogen to follow how it moves from the oysters to the sediment, and then into nitrogen gas. This research supported the participation of students from high school through graduate school levels. The integration public outreach, education, and robust experimental design in this study will inform the long-term conservation priorities for aquatic environments in the region.
