Investigating the importance of micro-scale physical processes on macro-nutrient release from suspended estuarine sediments
Emma Pidduck1, Andrew J Manning2,3, Mark F Fitzsimons2, Paul J Worsfold4 and Alejandro Jose Souza5, (1)Plymouth University, Plymouth, PL4, United Kingdom, (2)Plymouth University, Plymouth, United Kingdom, (3)HR Wallingford, Wallingford, United Kingdom, (4)Plymouth University, School of Geography, Earth and Environmental Sciences, Plymouth, United Kingdom, (5)CINVESTAV-IPN, Marine Resources, Merida, Mexico
Abstract:
Long-term temporal variations and the horizontal advection of both SPM and macro-nutrient concentration are well constrained, but the vertical fluxes associated with micro-scale processes, such as turbulence and flocculation, are poorly constrained. This interdisciplinary study investigates the importance of micro-scale physical processes on water column inorganic macro-nutrient concentrations. Micro-scale physical processes were examined in a short term study in the Seine estuary, France, and a seasonal field campaign conducted in the Tamar Estuary, U.K. Parameters recorded were current velocity and TKE from an ADCP; salinity, temperature and turbidity from a CTD; particle size data from a LISST and LabSFLOC apparatus and N and P concentrations from surface and bottom water samples. Three hypotheses were tested regarding the effects of turbidity (flocculation), turbulence and salinity on the release and uptake of nitrogen and phosphorus in the water column. Nitrate, phosphate and ammonium were collected in water samples and measured spectrophotometrically (nitrate and phosphate) and fluorimetrically (ammonium).
The results from the field campaign revealed the multiple implications of salinity on the transfer and transport of ammonium in the water column; stratified waters prevented the vertical transfer of all three nutrients, but also played a significant role in the release of ammonium from the sediment via a cation exchange mechanism. This exchange mechanism was enhanced by an increase in suspended sediment associated with increased turbulence at the bed. The implications of turbulence are discussed in relation to increased SPM concentration and phosphate. The release of phosphate from the sediments was synonymous with SPM concentration increase but the levels of phosphate released during periods of high SPM concentration were also related to the level of turbulence. This study demonstrates the importance of an interdisciplinary approach to studying N and P dynamics in important anthropogenically impacted estuaries.
