In Situ Absorbance and Fluorescence as Surrogate Data to Continuously Measure Nitrogen, Phosphorus and Carbon Concentrations in a Brackish Tidal Marsh

Rapid changes in flow and nutrient concentrations make it difficult to accurately quantify the flux of nutrients entering and leaving tidal marshes. Much effort has been put into monitoring these highly dynamic systems, but the ability to collect high temporal resolution concentration data for long periods of time has only recently become possible through linking in situ optical property measurements to nutrient and carbon concentrations. In this study, optical properties were measured in situ using both UV-Visual spectrometers and FDOM sensors in a constructed brackish tidal marsh in eastern North Carolina. Through statistical models, the absorbance and fluorescence were found to be a surrogate measurement of dissolved organic carbon, total suspended solids, nitrate, phosphorus, and other nutrients. Combining flow data with 15-min concentration data allowed us to calculate a nutrient balance for the studied marsh. The mass balances from over one year of data indicate that the marsh was a sink for nitrate, but a source of phosphorus, other forms of nitrogen, and dissolved organic carbon. The use of optical properties as a surrogate measure of concentrations greatly reduces the error in mass balance studies in tidal marshes due to infrequent sampling. This study examines the potential uncertainties in the mass balances that are associated with the models that relate optical properties and nutrient concentrations.