LTREB: Nutrients and climate as drivers of carbon sequestration and ecosystem metabolism in a shallow coastal ecosystem during recovery from nitrogen pollution

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Nitrogen pollution is one of the largest threats to the ecological integrity of coastal bays, leading to the degradation of two-thirds of these systems in the US. Shallow coastal bays are found along much of the east coast, and are particularly vulnerable to the effects of excess nitrogen, which can lead to algal blooms, low dissolved oxygen, and loss of high-habitat quality seagrasses. This research will examine the recovery of a shallow bay as nitrogen inputs are greatly reduced without any concomitant change in inputs of other nutrients and pollutants, making it perhaps the first such study in the US. The site, West Falmouth Harbor on Cape Cod (MA), has received high levels of nitrogen inputs for the past 15 years, due to contamination of incoming groundwater by a sewage treatment plant, causing poor water quality and habitat degradation. The nitrogen contamination has now been cleaned up, and so nitrogen inputs entering the harbor will be 3-fold less than in the past. To date, relatively few coastal bays have recovered from excessive nitrogen inputs, so this study will provide information on the rate of recovery and extent of restoration of habitat quality that is critical to better manage and protect these valuable coastal resources. While most coastal scientists believe nitrogen is the primary cause of coastal eutrophication, this remains debated. This study will provide important insights as to not only whether nitrogen is the controlling nutrient, but if so, why. One hypothesis is that a large net import of phosphorus from deeper coastal water helps maintain N limitation. If so, net phosphorus import should decline as primary production (and resultant demand for phosphorus) declines. The research will also explore the relationship of carbon storage to net ecosystem production, testing the hypothesis that lower nitrogen inputs will shift the ecosystem towards greater heterotrophy. Will the harbor continue to store carbon, by importing more carbon than the net amount respired? These questions will be addressed by measuring rates of primary production and ecosystem respiration from diel changes in dissolved oxygen and pH, seagrass extent and biomass elemental composition, and by measuring the exchanges of nitrogen, phosphorus, and carbon between the harbor and the deeper adjacent coastal waters of Buzzards Bay, MA. Results will be compared with 12 years of data taken during the high nitrogen loading period, and will thus provide a strong picture of the recovery trajectory of the system.