Geological Controls on the Distribution of Oyster Reefs and Substrates in Copano Bay, Texas

Copano Bay is a shallow (< 2-3 m), microtidal estuary in south central Texas. At ~180 km², it is a secondary bay of the Aransas Bay system. In an effort to both determine the distribution as well as investigate the controls on the distribution of oyster reefs and other habitats and substrates, Copano Bay was surveyed using sidescan sonar, CHIRP sub-bottom profiler and single beam bathymetry in June and July of 2007. Over 200 survey lines with a spacing interval of 150 m provided maximum coverage of the seafloor. Side scan sonar data were groundtruthed using 45 surface sediment grab samples and vibra-cores collected from various locations across the bay. Samples were analyzed to obtain grain size distribution and shallow stratigraphy. Thematic ArcGIS layers of the sidescan sonar mosaic, surface sediment grain size distribution along with CHIRP profiles are being used to delineate substrate types, including oyster reefs. CHIRP data reveal that in many cases, there appear to be antecedent geological controls or influences on the position of the oyster reefs. These controls include buried channel levees, tidal deltas and other geotechnically stabilized features that provided subsurface foundations which were more capable of supporting reefs than the surrounding, unstabilized bay bottoms. A better understanding of the geological controls on oyster reefs will allow for better management of existing reefs, as well as provide better selection criteria for the placement of new reefs for restoration purposes.