Multivariate analyses link macrophyte and water quality indicators to seagrass die-off in Florida Bay

Multivariate analyses of macrophyte and water quality data at permanent transect locations in thirteen basins from January 2013 to December 2016 were used to investigate interactions before and after the seagrass die-off in Florida Bay in 2015. Water quality data were collected monthly/bimonthly; macrophyte communities were assessed along 50 m permanent transects at the same station locations twice per year. Transect locations in the five basins that underwent die-off had significantly different water quality and macrophyte communities prior to die-off compared to the eight basins that did not undergo die-off. Only basins that had sediment depths greater than 65 cm, averaged pH greater than 8.24 and greater than 45% Thalassia testudinum cover prior to the summer of 2015 underwent die-off. In the year following die-off, macrophyte communities and several water quality factors in die-off affected basins became similar to those in non-die-off basins, but phosphate levels increased to several times greater than those observed previous to die-off. Contrary to previous die-off reports, Thalassia testudinum was not the only seagrass to exhibit significant losses in 2015; Syringodium filiforme and Halodule wrightii cover also dropped by about 50% in the year following die-off. Extremely high temperature (35.9 degrees C) and salinity (66.3) were recorded immediately preceding the 2015 die-off that were not detected by the bimonthly sampling, indicating the value of maintaining the continuous monitoring stations. Our results support recent suggestions that diverting freshwater to central and western portions of the bay to decrease salinity may reduce the likelihood of die-off by favoring a lower-density mixed estuarine macrophyte community rather than very-dense beds dominated by the more marine T. testudinum and by decreasing the probability of stressful hypersaline events.