Understanding the flushing capability of Bellingham Bay and its implication on bottom water hypoxia

In this study, an unstructured-grid finite-volume coastal ocean model (FVCOM) was used to simulate hydrodynamic circulation and assess the flushing capability in Bellingham Bay, Washington, USA. The model was reasonably calibrated against field observations for water level, velocity and salinity, and was further used to calculate water residence time in the study site. The model results suggest that, despite the large tidal ranges (∼4 m during spring tide), tidal currents are relatively weak in Bellingham Bay with surface currents generally below 0.5 m/s. The local water residence time in Bellingham Bay varies from near zero to as long as 15 days, depending on the location and river flow conditions. In general, Bellingham Bay is a well-flushed coastal embayment affected by freshwater discharge, tides, wind, and density-driven circulation. The basin-wide global residence time ranges from 5 to 7 days. The model results also provide useful information on possible causes of the emerging summertime hypoxia problem in the north central region of Bellingham Bay. Model results suggest that the formation of hypoxia in bottom water results from an enhanced oxygen consumption rate in the oceanic bottom water inflow with low dissolved oxygen by organic matters accumulated at the regions characterized with relatively long residence time in summer months.