Vulnerability and resilience of seagrasses to hurricane and runoff impacts along Florida's west coast

Many climate change models predict increasing frequency and severity of tropical cyclones (hurricanes) in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. To assess this potential threat to seagrass communities in Florida's Big Bend region, we performed a habitat change analysis based on aerial seagrass surveys performed prior to, and after, the extremely active Atlantic cyclone seasons of 2004 and 2005. To provide a regional context for changes in the Big Bend region, we also compared impacts there with changes in three other West Florida estuaries. Our analysis showed that storm impacts on seagrasses varied along Florida's west coast. Physical disturbance caused minor losses in parts of Charlotte Harbor and the Big Bend region. However, heavy rainfall in Florida and Georgia associated with Frances and Jeanne combined with winter rains to cause complete loss of 1,500 ha of seagrasses and thinning of another 1,700 ha in the vicinity of the Suwannee River mouth. In Tampa Bay, Sarasota Bay, and Charlotte Harbor, despite localized losses, total seagrass area actually increased between 2004 and 2006. On the other hand, Tampa Bay, Sarasota Bay, and Charlotte Harbor all showed significant, and more pronounced, declines in seagrass cover as the result of another major rainfall and runoff event: the 1997-1998 El Nino event. Our results indicate that light stress, likely caused by suspended sediments, phytoplankton blooms, and dissolved organic matter, resulted in seagrass losses extending up to 40 km from the mouth of the Suwannee River. We conclude that water quality impacts, especially if they are persistent, can be more damaging than physical impacts of moderate (Category 1-3) tropical cyclones. We also conclude that runoff-related impacts on seagrasses vary depending on the timing, volume, and persistence of storm runoff in relation to normal seasonal runoff patterns and seagrass growth in each estuary.