Estuaries are sites of significant biogeochemical processes. My research focuses on how the transport and residence time of nutrients within an estuary are affected by circulation mechanisms generated by tides and river flows.

Seasonal changes in Residual Circulation

Elkhorn Slough is an estuary in a Mediterranean climate: its only source of significant freshwater input is during the rainy winter months. Summers are dry and warm and, as a result, the salinity (and density) in the estuary can be greater than in the ocean. This density difference generates a density-driven residual current called inverse estuarine circulation. In contrast, during the wet winter months, the estuary is fresher (and thus lighter) than the ocean, and classical estuarine circulation prevails.

My research in Elkhorn Slough examines how seasonal changes in circulation affect residence time and mixing mechanisms within the estuary using long-term records of water velocity profiles, salinity, and temperature collected through the LOBO project. [top]

Lateral circulation in stratified estuarine flows

Lateral circulation refers to velocities that are perpendicular to the primary current direction; these lateral velocities can redistribute salt, nutrients, and momentum across an estuarine channel faster than turbulent motions alone can, and thus their effect upon longitudinal mixing and estuarine circulation cannot be overlooked.

This area of my research is focused on determining how stratification and turbulence interact with lateral circulation around a channel bend. I am using measurements made around a sharp bend in Elkhorn Slough to quantify the terms in the lateral momentum budget. This work will ultimately improve our ability to determine the relative importance of lateral circulation as a mixing mechanism in estuarine flows. [top]

Hydrodynamic and geochemical numerical modeling

The interpretation of most geochemical measurements suffers from poor spatial and temporal resolution; lateral and vertical distributions of velocity, salinity, nutrients, and suspended sediment are rarely homogenous. In an estuarine system, the problem is further compounded by the phase of the tide in relation to the sampling time. With a hydrodynamic transport model we can resolve these distributions and use the model output to provide a context within which geochemical measurements can be interpreted.

I am using TRIM3D to model nitrate and radium transport in Elkhorn Slough, in collaboration with Ken Johnson and Chip Breier, respectively. By keeping the geochemical portions of the model relatively simple, we can investigate the possible sources and sinks of biogeochemical activity within the estuary. In addition to improving our understanding of how the estuary functions, the numerical model can be used to explore the potential impact of restoration actions presently under consideration. [top]

nidzieko at stanford.edu - stanford, ca