Award: OCE-1916989

Tropical disturbances have the potential to alter the physical and biological composition and produce landscape-level changes in coastal waters. Barrier Islands can breach, opening semi-enclosed bays and lagoons to direct connection with adjacent oceanic waters. This can alter salinities and water temperatures and bring additional larval and adult recruits to nearshore waters. Hurricanes have been documented to move species large distances and while these can be only short-lived range shifts, there is potential for enhanced establishment of tropical species in locations where tropicalization, an increase in the ratio of tropical to temperate taxa in a given region, is already occurring owing to the decreasing frequency of cold winter temperatures. In October 2018, Category 5 Hurricane Michael produced numerous overwash areas along Cape San Blas and opened a new pass from the Gulf of Mexico to St. Joseph Bay, FL. These breaches were likely to have altered salinity, water temperatures and brought additional larval and adult recruits to the Bay. The major goals of this projects were to investigate to what extent these newly formed breaches allowed elevated immigration of tropically-associated species, such as the seagrass consuming emerald parrotfish (Nicholsina usta); altered the existing and growing population of seagrass-consuming juvenile green turtles (Chelonia mydas); and whether changes in these seagrass herbivores, resulted in altered consumption rates of the dominant foundation seagrass species Thalassia testudinum (turtlegrass). Our regular sampling during 2019 showed no evidence of increased numbers of tropically-associated fishes, including seagrass herbivores such as the emerald parrotfish. Thus, the breaches did not produce measurable changes in the fish fauna susceptible to our otter trawling methods. However, in contrast to our original concern that green turtle numbers might be reduced by Hurricane Michael?s impacts, the abundance of green turtles increased by approximately 2x during 2019, compared to abundances recorded during 2016 and 2017. Our best explanation for this increase is that green turtle nesting was very successful in 2017 and by 2019 some of the young turtles that hatched in 2017 had become resident in St. Joseph Bay. Our final prediction, that seagrass consumption rates could be increased as a result of the colonization of tropical herbivorous fishes, did not occur. Instead, the abundance of resident sea urchin (Lytechinus variegatus), consumers of seagrass in St. Joseph Bay, increased near our study locations and greatly increased their consumption of turtlegrass. This was verified by identifying the type of scars on seagrass leaves which are easily determined to be made by urchins, fish or turtles. In summary, we found no evidence for the enhanced immigration of tropical herbivorous fish or decreases in the number of green turtles. Instead, we recorded an increase in juvenile green turtle numbers, most likely a result of very successful reproduction during 2017. We also found enhanced consumption of turtlegrass by resident sea urchins, and not by immigrating herbivorous fish as originally hypothesized. Overall then, we found little evidence for changes in the species composition of trawlable fishes, but we found increases in the green turtle population and increased consumption of turtlegrass, but by resident consumers, not tropically-associated immigrants. Last Modified: 01/04/2021 Submitted by: Dorothy Byron