Responses of loggerhead sponges Spechiospongia vesparium during harmful cyanobacterial blooms in a sub-tropical lagoon

Shallow, coastal lagoons are vulnerable to eutrophication and harmful algal blooms, often due to the loss of benthic suspension feeders. Florida Bay, USA, is a sub-tropical lagoon that has suffered from a series of ecological disturbances, including cyanobacterial blooms, loss of seagrass habitat, and widespread sponge mortality. A field study was executed at sites across Florida Bay to investigate effects of cyanobacterial blooms of the genus Synechococcus on the suspensionfeeding loggerhead sponge Spheciospongia vesparium. In situ measurements of loggerhead sponge survival, water pumping rates, and particle retention were made over the course of a year under both bloom and non-bloom conditions and for naturally-occurring and transplanted sponges. The mortality of transplanted loggerhead sponges significantly increased following cyanobacterial blooms. Sponge water pumping rates, particle retention, and filtration rates were depressed in areas where cyanobacteria blooms were present. When loggerhead sponge community filtration rates were compared with cyanobacteria intrinsic growth rates, sites with low abundances of sponges had persistent, dense blooms and harbored positive net growth rates for cyanobacteria (0.78 to 1.45 d ^-1). In contrast, sites with high abundances of sponges had few or no blooms and net growth rates for cyanobacteria that were slow or negative (-0.17 to 0.24 d ^-1). This suggests that the loss of filtration from sponge mortality in Florida Bay has contributed to the persistence of algal blooms. Restoration of benthic suspension feeders such as sponges could be an important management tool to mitigate algal blooms in shallow coastal lagoons, although survival of transplanted populations will likely require concurrent improvement of water quality by other means since blooms inhibit sponge pumping, particle retention, and filtration.