Denitrification within the Macroalgal Canopy in a Eutrophic Embayment

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The loss of nitrogen from coastal ecosystems through denitrification is believed to be a major sink for nitrogen, particularly in eutrophic environments. The focus of this project is a thick macroalgal canopy that carpets the bottom of Waquoit Bay, a eutrophic embayment on Cape Cod, MA. The macroalgal canopy frequently exceeds a thickness of 15 cm. Within the upper third of the dense mat, O2 concentrations are high during the day due to photosynthesis and NH4+ concentrations are low (several uM). In contrast, the lower portion of the canopy is anaerobic even during the day and NH4+ concentrations are very high (hundreds of uM). Therefore, within a zone about 5 cm from the algal surface conditions are favorable for nitrification with sufficient O2 from photosynthesis and diffusion from above and NH4+ diffusing from below. It is hypothesized that high rates of denitrification may occur at the high NH+4/low O2 interface within the algal canopy due to the existence of denitrification-nitrification coupling in this zone. The hypothesized interaction is in fact a triple coupling of photosynthesis-nitrification-denitrification. This hypothesis will be examined in a series of experiments to be conducted at the Marine Biological Lab in Woods Hole, MA, a facility close by to Waquoit Bay. The project will evaluate nitrification and denitrification in specially constructed chambers containing sediment, algal canopy material, and overlying water. These questions relate directly to ongoing work at Waquoit Bay which is one of the current sites under investigation through the Land Margin Ecosystem Research Program (LMER) and the measurements obtained will be an important input into the nitrogen budget of this system.