Fine-scale Hydrodynamics around St. John, U.S. Virgin Islands. Part I: Spatial and Temporal Heterogeneity in the Coastal Environment

Weifeng (Gordon) Zhang^*Yan JiaAmy ApprillT. Aran Mooney

• Woods Hole Oceanographic Institution, Woods Hole, United States

Physical conditions in coastal ecosystems can vary dramatically in space and time, influencing marine habitats and species distribution. However, such physical variability is often overlooked in ecological research, particularly in coral reef research and conservation. This study aim to quantify fine-scale variability in the physical conditions of a coastal environment to provide critical context for coastal ecosystem conservation and coral reef restoration. By developing and analyzing a 50 m-resolution hydrodynamic model, we characterize the physical oceanographic environment around the tropical island of St. John, U.S. Virgin Islands. Model simulations reveal that tides, winds, and the Amazon and Orinoco River plumes, interacting with the complex coastline and seafloor topography, create significant spatial and temporal variability in the coastal environment. Differences in tidal characteristics between the north and south shores generate strong oscillatory tidal flows in the channels surrounding St. John. The mean flow around the island is predominantly westward, driven by prevailing easterly winds. Water temperature and salinity exhibit variability over relatively small length scales, with characteristic along-shore length scales of 3-10 km, depending on the season. Hydrodynamic conditions also vary across multiple time scales. Strong tidal flows interacting with headland geometry produce transient eddies with strong convergent/divergent flows and variability on the scale of hours. Synoptic-scale flow variations are driven by weather events, while seasonal variations are strongly influenced by the Amazon and Orinoco River plumes. During summer and full, these river plumes freshen the waters on the south shore of St. John, creating significant salinity differences between the north and south shores. These fine-scale physical variabilities can exert a strong influence on the coastal ecosystem and should be considered in the management of coastal resources. By providing a detailed understanding of the physical environment, this study supports efforts to conserve and restore coastal ecosystems, particularly coral reefs, in the face of dynamic and complex oceanographic conditions.