Shift in baseline chlorophyll a concentration following a three-year Synechococcus bloom in southeastern Florida

A picophytoplankton bloom dominated by Synechococcus formed in September 2005 in a series of shallow lagoons between Florida Bay and Biscayne Bay and lasted until May 2008. Chlorophyll a concentrations peaked at > 20 mu g L-1. The bloom coincided with a massive mortality of sponges and caused massive mortality of the seagrass. However, follow-up analysis to determine if there were any long-term impacts from the bloom on the system is lacking. We used long-term water quality data (chlorophyll a and nutrient concentrations) collected at 13 stations in the affected region over a 20-yr period to compare environmental conditions before (1995-2004) and after (2009-2014) the bloom. We found that after the bloom, baseline chlorophyll a concentration significantly increased 45%, from 0.42 (SE 0.02) to 0.77 (SE 0.04) mu g chl a L-1, at the stations most impacted by the bloom. Before-After Control-Impact paired analysis suggested these changes were related to the 3-yr bloom and not a larger, regional scale shift. The increase in chlorophyll a does not appear to be associated with additional changes in water quality, but is potentially due to a reduction in the epibenthic community (e.g., SAV and sponges). Now that the bloom has terminated and the causes of the bloom abated, the system has not returned to its original status, suggesting a lasting impact from the bloom on the ecosystem.