Porewater nutrient fluxes in a shallow fetch-limited estuary

The Swan River Estuary is a shallow meso-eutrophic system in SW Australia. The dominant summer wind pattern in the region includes a strong SW sea bre eze each afternoon. We hypothesised that such a regular and significant win d event may be important in causing the release of sediment porewater with subsequent effects on water quality and nutrient dynamics. To test this, we conducted a multidisciplinary field experiment in 1996 during which we mea sured temperature microstructure profiles (to yield turbulence parameters), velocity profiles, light profiles and nutrient profiles. Bottom shear stre sses were estimated via the viscous-dissipation method, which can be used i n stratified and unstratified water bodies or during convective cooling, al l of which are likely to affect sediment-water dynamics in shallow systems. We observed that, after the sea breeze began, there was a dramatic breakdo wn of temperature stratification, an increase in mean velocities and an inc rease in turbulence levels at the sediment-water interface. This coincided with a doubling of water column ammonium concentrations. However, this puls e of nutrients was shown to occur prior to conditions necessary for sedimen t resuspension. We therefore attribute the increased porewater fluxes to in teractions between increasingly energetic hydrodynamics and the benthic env ironment. The technique used here to measure porewater fluxes as a function of hydrodynamic forcing is an excellent alternative to benthic chamber exp eriments, which isolate the sediments from the overlying hydrodynamics.