Distribution, photobiology, salinity tolerance and population structure of Siderastrea radians and its symbionts in Florida Bay, Florida USA

The distribution of Siderastrea radians (Pallas) Blainville and photophysiology of its symbiont in Florida Bay were determined during annual macrophyte surveys in spring 2006 and 2007. Siderastrea radians was present in five of the eleven sampled basins in areas with little sediment and low abundance of the seagrass Thalassia testudinum. The five basins are located along a northeast-to-southwest transect that also represents a salinity gradient from inshore, predominantly estuarine conditions adjacent to the Everglades, to offshore near-marine salinities adjacent to the Gulf of Mexico. Colony abundance was highest in the two basins at the extremes of this physical range. Effective quantum yields, measured in situ by PAM fluorometry, were significantly correlated between S. radians and the dominant seagrass T. testudinum among all five basins where the two species co-occurred, albeit with a 40% reduction in mean yields of S. radians. These findings indicate S. radians may function as an alternative eco-indicator species for regions in which T. testudinum is absent and/or act as an indicator of stress for other benthic invertebrate taxa. The tolerance of S. radians to hyposalinity was examined in a mesocosm study using salinity treatment levels based on historical salinity records for the basins where this species occurs. Colonies of S. radians collected from the five basins in Florida Bay (i.e., along the northeast-to-southwest salinity gradient) were used to assess effects of hyposalinity on the photophysiology of the symbiont, as an indicator of holobiont stress. Colonies from each basin were assigned four salinity treatments (30, 20, 15, and 10) and salinities were reduced 2 d-1 from ambient (30) to simulate a natural salinity decrease. Maximum (Fv/Fm) and effective (? F/Fm') quantum yields were measured using PAM-fluorometry at dawn and noon, respectively. Because of high variability in ? F/Fm' in response to varying mid-day light conditions only Fv/Fm measurements were used to assess salinity treatment effects. There was generally no decrease in Fv/Fm for 20 and 15 treatment colonies versus controls (i.e. 30) up to five days post-target salinity. This indicates a greater ability to withstand reduced salinity for relatively extended periods of time in S. radians compared to other reef species. When salinity of 10 was reached, there was a significant reduction in Fv/Fm versus 20 and 30, indicating a critical threshold for hypo-saline tolerance. At salinity of 10, colony yields from the more salinity-variable northeast basins versus the most marine southwest Twin Key basin (TWN) were significantly different, suggesting populational acclimation or adaptation in salinity tolerance corresponding with historical basin salinity ranges. Genetic tools were used to determine whether differences among basin populations in photophysiological responses to hypo-saline stress were due to unique symbiont type among basin populations (i.e. adaptation) or to acclimation. Subcladal identification of Symbiodinium for each basin was completed using PCR-DGGE analysis of the ITS2 region of the rDNA. TWN colonies possessed an ITS2 variant distinct from all other basins, correlating with differences in photobiology measured during hyposalinity experiments. These findings suggest long-term differences among basin salinities lead to differential responses in the holobiont, which are related to a shift in symbiont photophysiology and symbiont adaptation. Despite low overall cover and frequency, S. radians in Florida Bay is a valuable marginal population for investigating tolerance to physical stressors across a natural physical gradient. Coral populations residing in marginal coastal habitats may provide insights as to how these communities will respond to higher environmental variability under predicted future climate patterns. The coral S. radians occurs along a physical-environmental gradient in Florida Bay that has been variably influenced by anthropogenic impacts to its watershed that have primarily affected salinity variability across the Bay. It is unclear what effects projected increased freshwater inflow under the Comprehensive Everglades Restoration Plan (CERP) will have on this community, but the results presented here indicate a greater tolerance to hyposaline conditions than generally reported for corals.