Scientific Review of Proposed Minimum Flows and Levels for Dona Bay/shakett Creek below Cow Pen Slough

EXECUTIVE SUMMARY These studies were conducted by the Southwest Florida Water Management District (the District) because Florida Statutes (§373.042) mandate the District's evaluation of minimum flows and levels (MFLs) for the purpose of protecting the water resources and the ecology of Shakett Creek and the Donna Bay estuary from " significant harm " that might result from potential future freshwater diversions from the contributing watersheds. With appropriate water management, including science-based MFL rules for environmentally safe operation of water supply impoundments and diversions, the District can ensure that Shakett Creek, Dona Bay and their associated tidal (estuarine) marshes and brackish wetlands will continue to provide essential food and cover for the myriad of marine and estuarine-dependent fish and wildlife that need them for survival, growth and reproduction. Historically, the Dona Bay watershed included approximately 10,000 acres, consisting primarily of native upland habitats such as pine flatwoods, cabbage palm hammocks and wetlands. Most significantly, the original Cow Pen Slough was, primarily, a wetland drainage that conveyed runoff to the Myakka River. It consisted of large, slow flowing marshes that ultimately discharged into the Myakka River. Conversion of the historical watershed included excavation of Cow Pen Slough by a series of deeply incised canals that more efficiently drained and significantly altered the character, function and values of the natural wetlands. Furthermore, the construction of the Intracoastal Waterway and Venice Inlet has resulted in an increased reach of Gulf marine waters into the Dona and Roberts Bay (DARB) estuarine region. The more efficient connection to the Gulf of Mexico has influenced water levels and circulation, 3 sedimentation, salinity, and the numbers and kinds of plants and animals inhabiting the study area. On the other hand, the diversion of Cow Pen Slough from the Myakka River watershed into Shakett Creek and the Dona Bay watershed, the connection of Curry Creek and Roberts Bay to the Myakka River via the Blackburn Canal, and the transformation of the region's natural land cover to agricultural uses (e.g., improved pastures, citrus and row crops) have increased freshwater inflows tremendously, especially during the " wet " season. Taken together, the combination of increased inflows and marine influences has created a strong salinity gradient over a relatively short distance in the DARB area, resulting in rapid, high-amplitude salinity oscillations that are not well tolerated by much of the marine life of interest here. In the Dona Bay estuary, the major …