Microcosm Studies of Nutrient Cycling in Bahamian Stromatolites

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I report results of field observations and experiments that examine the oxygen and nutrient fluxes for stromatolites in Highborne Cay in the Exumas, Bahamas. The aim of this study is to determine whether nutrients play a role in the transition of the community structure within the mats that is thought to be responsible for lithification and, ultimately, mat growth and structure. The research includes nutrient monitoring of the sediment and water column, and measures of rates of oxygen and inorganic nutrient exchange from stirred microcosm chamber incubations of mats with varied community structure. On the basis of mat community composition, I hypothesized that different mat types would have different fluxes, and that Highborne mats would be limited by one or more nutrients that efficient recycling within the mats might otherwise help supply. Samples of the four major mat types were sealed in stirred microcosm flux chambers, incubated in a circulating water bath, and sampled for oxygen, NH4, NO3, PO4, and Silicate. Nutrient addition, treatments of PO4 and Si were employed to investigate whether they stimulate primary productivity, signaling that mats are limited in these solutes. Nutrients in Highborne Cay were high in nitrogen relative to P, with N:P as high as 30. There was no difference in nutrient flux or productivity among mat types, and the addition of nutrients did not change mat productivity. These observations suggest that mat development in Highborne Cay is not limited by nutrients, but more likely structured by external physical factors such as the rate of turbulent flow which may limit the recruitment of competitors such as macroalgae and benthic branching diatoms.