Bioavailability and Sources of Nutrients and the Linkages to Nuisance Red Drift Algae

Eutrophication of estuaries with nutrients from urban and agricultural sources is both a local problem for the Caloosahatchee estuary and for most estuaries worldwide. Beginning in the winter of 2003-2004, unusually large masses of drift red macroalgae accumulated on Sanibel Island and then later in Bonita Springs and Fort Myers Beach. Enriched nutrient concentrations (~10 fold) were found in 2005 associated with large freshwater discharge events. Concentrations at offshore reefs, however, were only 2-3 times enriched, suggesting that there was substantial absorption and cycling of nutrients en route to offshore locations. The sources of available nutrients and their role in producing large-scale drift algae blooms on Sanibel Island and the waters of Lee County, Florida are the focus of this study, which integrates hydroacoustic surveys with bioassays, in situ and drift algae surveys, stable isotope analyses, and a hydrologic model. Sampling is being conducted from the C-43 canal downstream of Lake Okeechobee, the upper, middle and lower tidal Caloosahatchee River, and the coastal waters (25 km) from Redfish pass to Wiggins pass. This broad geographic area represents a hypothesized gradient of nutrients and algal biomass downstream and away from the mouth of the Caloosahatchee Estuary.

Specific project objectives include the following:

• To determine whether Lake Okeechobee or basin sources are more important to algal biomass and productivity.
• To determine if benthic nutrient flux derived from degradation of organic matter is a significant contributor to harmful algal blooms.
• To determine the submarine groundwater flux of nutrients to coastal waters.
• To determine the distribution of macroalgal biomass using underwater video and hydroacoustic surveys.
• To determine the distribution and productivity of attached macroalgal biomass and the conditions favorable for uncontrolled growth, including substrate attachment.
• To determine the ecological consequences of the accumulation of nuisance algal biomass by measuring decomposition rates and nutrient recycling on area beaches.
• To verify and calibrate the CH3D-IMS hydrodynamic model for the system to address the potential fate of nutrients generated and exported from the Caloosahatchee River and the fate of detached macroalgae as they enter nearshore waters around Sanibel Island.
• To develop a sediment transport model and implement the coupled sediment and hydrodynamic model (above) for the same time periods.
• To develop a water-quality computer model of the West Florida Shelf in the Ft Myers region that can be used to guide nutrient reduction strategies for the management of harmful algal blooms.
• To provide critical management tools to more accurately predict the fate and transport of nuisance algal blooms on area beaches.

The results of this study will provide the data necessary to determine the location of extensive/ excessive algal growth, the role of riverine and benthic nutrient fluxes in supporting/stimulating this algal growth, the vectors linking excessive growth to drift algae accumulation on Sanibel beaches, and the ecological impacts on benthic nutrient cycling and benthic food webs.

This project is a collaboration between FGCU, the University of Miami, and the Sanibel Captiva Conservation Foundation Marine Laboratory and is funded by the City of Sanibel in partnership with Lee County, Florida.