MEPS 183:39-47 (1999)  -  doi:10.3354/meps183039

Porewater nutrient fluxes in a shallow fetch-limited estuary

C. E. Oldham1,*, P. S. Lavery2

1Centre for Water Research, The University of Western Australia, Nedlands, Western Australia 6907, Australia
2Centre for Ecosystem Management and School of Natural Sciences, Edith Cowan University, Joondalup, Western Australia 6027, Australia

ABSTRACT: 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 breeze each afternoon. We hypothesised that such a regular and significant wind 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 measured temperature microstructure profiles (to yield turbulence parameters), velocity profiles, light profiles and nutrient profiles. Bottom shear stresses were estimated via the viscous-dissipation method, which can be used in stratified and unstratified water bodies or during convective cooling, all of which are likely to affect sediment-water dynamics in shallow systems. We observed that, after the sea breeze began, there was a dramatic breakdown of temperature stratification, an increase in mean velocities and an increase in turbulence levels at the sediment-water interface. This coincided with a doubling of water column ammonium concentrations. However, this pulse of nutrients was shown to occur prior to conditions necessary for sediment resuspension. We therefore attribute the increased porewater fluxes to interactions between increasingly energetic hydrodynamics and the benthic environment. The technique used here to measure porewater fluxes as a function of hydrodynamic forcing is an excellent alternative to benthic chamber experiments, which isolate the sediments from the overlying hydrodynamics.

KEY WORDS: Porewater fluxes · Resuspension · Subtidal pumping · Ammonium

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