This is an ArcGIS shapefile which depicts the seasonal salinity dynamics of 32 Gulf of Mexico estuaries. To characterize the dynamic nature of estuarine salinity gradients, a multivariate methodology (Bulger et al. 1993) was applied to derive five bio-salinity zones in four salinity seasons for 32 Gulf of Mexico estuaries (Christensen et al. 1997). This seasonal salinity zone spatial framework built upon and refined earlier studies which characterized salinity on an annual-averaged basis (NOAA 1985, Orlando et al. 1993, NOAA 2007). Precipitation, flow gage data, and monthly salinity averages were evaluated to determine which months would be used to represent the high, low, and transitional (increasing and decreasing) salinity periods. A contour modeling procedure was applied to the data to develop seasonal salinity zones for each estuary. The salinities used to define the five seasonal zones were: 1) Salinity Zone I: 0 - 0.5 ppt; 2) Salinity Zone II: 0.5 5 ppt; 3) Salinity Zone III: 5-15 ppt; 4) Salinity Zone IV: 15-25ppt; and 5) Salinity Zone IV: >25ppt. These salinity zones are two-dimensional and depth-averaged, and vertical stratification is not explicitly characterized. Therefore, they can be readily represented geographically as two-dimensional areas, which shift seasonally. The monthly periods of high, low, increasing and decreasing salinity seasons vary greatly among estuaries, primarily because of different typical periods of high and low freshwater inflow. For example, the low salinity season in Galveston Bay, Texas occurs in April - June, while in Mobile Bay, Alabama, the low salinity season occurs in February April.
