Application of the Coastal Salinity Index to Sites in Florida Bay and the Gulf of Mexico

Coastal droughts have a different dynamic from upland droughts, which are typically

characterized by agricultural, hydrologic, meteorological, and (or) socioeconomic impacts. The location of the freshwater-saltwater interface in surface-water bodies is an important factor in the ecological and socioeconomic dynamics of coastal communities. Because of the uniqueness of drought impacts on coastal ecosystems, a Coastal Salinity Index (CSI) was developed by using an approach similar to the Standardized Precipitation Index (SPI). Instead of using precipitation data, as with the SPI, the CSI utilizes salinity data. The CSI is a standardized probability index with zero indicating historical median salinity amount, and positive and negative values representing increasingly fresh and saline conditions, respectively. The CSI is computed for various time scales to capture short- and long-term conditions. Evaluation of the CSI indicates that the index can be used for different estuary types (for example: brackish, oliogohaline, or mesohaline), for regional comparison between estuaries, and as an index for wet conditions (high freshwater inflow) in addition to drought (saline) conditions.

The CSI characterizing 1- to 24-month duration salinity conditions was computed for five tributary sites in Florida Bay and for nine tributary sites in the Gulf of Mexico. Comparison of the CSIs using the same time intervals shows how the intensity of drought and freshwater conditions varies along the southwest Florida coast. Time-series plots showing the CSI index for all the computed time scales show how sites vary in response to short- and long-term conditions. To evaluate the effectiveness of the CSI as a prediction and adaptive management tool, there is a need to develop linkages between the CSI and coastal drought response variables. However, identifying potential coastal drought response datasets is challenging. Coastal drought is a relatively new concept and existing datasets may not have been collected or understood as “drought response” datasets. Potential coastal drought response datasets include tree growth and liter fall, harmful algal bloom frequency, Vibrio infection occurrence, sportfish populations, and shellfish harvesting data. The CSI computed for Florida Bay shows a strong visual correlation with the occurrence of harmful algal blooms along the coast. The presentation will describe the application of the CSI to sites along the Gulf of Mexico and Florida Bay.