| Abstract: |
A number of physical models (e.g., Regional Ocean Modeling System (ROMS), Advanced Circulation Model (ADCIRC), have been used to describe the circulation within Chesapeake Bay and its tributaries. One goal of these models is to predict the transport and fate of particles introduced to the bay, including planktonic larvae of fish and invertebrates. While ROMS and ADCIRC may be driven by observed parameters (i.e., wind, buoyant discharge, sea surface height, tides), relatively few studies have been conducted to ground-truth the models\' simulated particle trajectories with Lagrangian field observations. Lagrangian observations can provide key information for understanding dispersal of larvae among habitats. Advection may be initially assessed with repeated releases of single drifters under varying environmental conditions, but quantifying dispersion of plankton patches requires deploying multiple drifters, and tracking them over time-scales of hours to weeks. Therefore, one goal of this study was to begin to quantify both advection and dispersion of a cluster of surface drifters in a Chesapeake Bay tributary. To this end, deployments of 2-4 drifters were made from the mouth of Weems Creek (Severn River, Annapolis, Maryland) in late summer and fall 2008. Drifters drogued from 0-1 m or from 1-3 m were deployed in an initial square-shaped pattern (diameter ~100m). The drifters were tracked via the ARGOS satellite system or by radio transmission for periods of hours to days. Two advection regimes were observed: rapid export (hours) to Chesapeake Bay, or retention within the Severn for the duration of the deployments (days). The short deployment periods precluded estimates of diffusion, but divergence, stretching, and changes in the aspect ratio of the cluster were calculated. Trajectories of the drifters deployed from 13-15 August, 2008 were then compared to output from an ADCIRC model forced by observed sea level and tides throughout the same period. The model simulation reflected a similar pattern of transport (retention within the Severn River) as was observed in the drifter trajectories, yet the model underestimated the magnitude of transport. This discrepancy is likely due to forcing factors missing from the model, in particular, vertical current shear and winds. Ground-truthing models with Lagrangian observations at the mouths of tributaries will ultimately refine current models of Chesapeake Bay circulation, and provide the tools for managers to make well-informed decisions about coastal resources. Relevance of this work to current efforts to effectively place oyster restoration bars in the Severn River is discussed. |
