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As the inaugural Margaret A. Davidson Fellow at Great Bay NERR, graduate student Anna Lowien, is excited to be investigating the biogeochemistry of Great Bay Estuary. Biogeochemistry refers to the study of the chemical, physical, geological, and biological processes that influence the movement of nutrients (i.e. nitrogen and phosphorus) and carbon throughout an ecosystem or even the globe. Her research will provide new perspective regarding Great Bay Estuary’s biogeochemical role in processing inputs of nutrients, carbon, and sediments and will help identify thresholds of material flux that the Estuary can ecologically handle.

Her work utilizes a mass balance, black box approach that accounts for the inputs of nutrients, carbon, and sediment to the Estuary and the output of those same materials from the Estuary. The boundaries of this black box are defined as Great Bay, south of Adams Point, and include the mouths of the Lamprey, Squamscott, and Winnicut Rivers. By accounting for the inputs and outputs, it is possible to estimate the amount of nutrients, carbon, and/or sediments that remain stored within the Estuary (the black box). We do that by using the following equation:

Outputs – Inputs = Storage

Inputs, outputs, and storage are reported as a flux measurement, which is a unit of measure referred to as mass/time. For example, a nitrogen flux would be reported as kilograms of nitrogen per year. The output of materials from Great Bay is defined as the movement of nutrients, carbon, and/or sediments out of Great Bay via low tide. Inputs of nutrients, carbon, and sediments into the Estuary, include precipitation, river flow, groundwater, coastal runoff, and high tide flow into Great Bay (Figure 1). The difference between the output and inputs provides an estimate of the amount of nutrients or carbon remain in the estuary.

There are three broad possible outcomes from the black box analysis (Figure 2). First, inputs into Great Bay could equal output from Great Bay. In this scenario, nutrients, carbon, and sediments are said to behave conservatively and Great Bay simply transports them onward to the ocean. The second possible outcome is inputs are greater than the output, which indicates more nutrients, carbon, and/or sediments enter Great Bay than leave Great Bay. The third possible outcome is inputs are less than output, indicating another source of nutrients, carbon, and/or sediments is contributing to the flux. For example, if more nitrogen is leaving Great Bay than enters Great Bay, that indicates that Great Bay itself may be contributing additional nitrogen from within the black box. This could be do to another, unmeasured input or production of nitrogen within the Estuary.

Results of the black box modeling approach will then be compared to ecological response variables, including coverage of eelgrass, seaweed, and phytoplankton abundance and coverage. By comparing the biogeochemistry to the ecological response in the Estuary, we can look for patterns between transport/retention of material fluxes and biological prominence of important species in the ecosystem. Anna’s findings will be available in the summer of 2021 and will help inform future work to protect the biogeochemical and ecological integrity of the estuary in the face of anthropogenic stressors.

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