Monitoring changes in biogeochemical processes provides a mechanism for measuring early response of the Florida Bay ecosystem to environmental perturbations. We have performed seasonal measurements of productivity associated with representative benthic substrate types in Florida Bay to begin establishing baseline rates before significant changes occur during restoration. Monitoring changes in productivity during implementation of restoration plans will allow resource managers to evaluate the progress and success of South Florida restoration efforts. Carbonate environments such as Florida Bay are characterized by three primary biogeochemical processes including 1) carbonate sediment production by calcifying organisms and dissolution, 2) photosynthesis and 3) respiration (referred to collectively as productivity). These processes are sensitive to changes in water quality including salinity and nutrients, and show distinct rate changes before visual evidence of environmental disturbances such as seagrass die-off, algal blooms, and shifts in ecosystem success indicator species. Water management practices in South Florida have already been altered in an effort to restore the Everglades and Florida Bay. Resulting changes in water chemistry will first affect biogeochemical processes, and may, subsequently, result in changes in species distributions (such as seagrass, algae, etc.).

Carbonate sedimentation and organic productivity (calcification, photosynthesis, and respiration) are most effectively determined from precise, in situ measurements of alkalinity, pH, temperature, conductivity, and air:sea CO₂ and O₂ gas fluxes. Net productivity was determined on seagrass beds in basins located in the western bay near Buchanon Keys and in the central bay near Manatee Keys during March of 1999 and 2000 and during September of 1999, and on mud-bottom and hard-bottom (soft corals and small hard corals, sponges, calcareous algae) communities during March 2000. A large incubation chamber called the Submersible Habitat for Analyzing Reef Quality, or SHARQ, was used to isolate water over the substrate and to measure temporal changes in key geochemical parameters over 24 hour time periods. Productivity in the water column was determined in March 2000 by isolating a mass of water inside of the SHARQ from the substrate by placing a floor in the incubation chamber. Geochemical parameters including pH, dissolved oxygen and temperature were measured continuously using a flow-through analytical system throughout the duration of incubation periods. Water samples were removed every 4 hours from sample ports for total alkalinity measurements. Dissolved oxygen, pH and alkalinity data were used to calculate average rates of net calcification, photosynthesis, and respiration for light and dark hours. Productivity on mud-banks located at Russell Bank was measured during March and September of 1999 and March of 2000 using an upstream/downstream sampling strategy. Changes in key geochemical parameters were determined by identifying unidirectional currents across Russell Bank and establishing upstream and downstream sampling sites along 200-400 meter bank transects. Average rates of net calcification, photosynthesis and respiration were calculated from total alkalinity, pH, dissolved oxygen, air:sea CO₂ and O₂ gas fluxes, salinity, temperature, and wind measurements taken every 4 hours during 24 hour time periods at each sampling site.

Rates of calcification for Buchanan Keys Basin and Manatee Key Basin seagrass beds indicate net dissolution of carbonate sediments during March 1999 and 2000 and September 1999. The average rate of net photosynthesis during daylight hours for Buchanon Keys Basin seagrass beds is 0.071 g carbon/m²/hr with values ranging from 0.04 to 0.08 g carbon/m²/hour. Net photosynthesis values for Manatee Key Basin seagrass beds ranged from 0.005 to 0.13 g carbon/m²/hour with an average of 0.069 g carbon/m²/hour. Average rates of respiration during night hours were 2.5 x 10^-4 g carbon/m²/hour for Buchanon Keys Basin and 1.1 x 10^-4 g carbon/m²/hour for Manatee Key Basin seagrass beds.

Preliminary results of productivity measurements on representative substrate types during March 2000 indicate that the highest rates of net photosynthesis occurred on seagrass beds in Manatee Key Basin followed by seagrass and hard-bottom communities in Buchanon Keys Basin. Highest net calcification rates (day calcification - night dissolution) were associated with the hard-bottom community. Rates of calcification and photosynthesis for mud-bottom and water column in Manatee Key Basin were up to an order of magnitude less than rates for seagrass and hard-bottom communities. Generally, for all substrate types, net precipitation of carbonate sediments was observed during daylight hours, while net dissolution occurred at night. Relatively low respiration rates may result from consumption of respired CO₂ by carbonate sediment dissolution reactions.

Results of Russell Bank productivity measurements indicate net carbonate sediment production during March 1999 of 0.071 g carbon/m²/24 hours and net sediment dissolution during September 1999 of 0.014 g carbon/m²/24 hours. Average rates of photosynthesis were -0.061 g carbon/m²/hour for March and -0.042 g carbon/m²/hour for September indicating net oxygen consumption on the bank top. However, average rates of respiration were -7.0 x 10^-4 g carbon/m²/hour for March and -2.3 x 10^-3 g carbon/m²/hour for September indicating net carbon fixation. This suggests that oxygen may be consumed through inorganic oxidative processes (e.g. sulfide oxidation, etc.) and that additional nutrient measurements will be required to quantify these processes.

(This abstract was taken from "Programs and Abstracts - 2001 Florida Bay Science Conference". (PDF, 6.8 MB))

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