Epibenthic gastropods of the middle Florida keys: the role of habitat and environmental stress on assemblage composition

Abstract

A survey of epibenthic prosobranch gastropods was undertaken in both seagrass and hard substratum (coral or old reef rock) habitats on opposite sides of the Florida Keys (Florida Bay and Hawk Channel) to compare faunal differences attributable to differences in the above two habitats and environments. Additionally, two data sets (26 continuous months) of daytime dissolved oxygen, surface salinity and water temperature from Florida Bay (Long Key) and Hawk Channel (Key Largo) environments were compared to determine differences that might constitute environmental stresses likely to affect the fauna. The above data were collected to determine if several hypotheses concerning effects of stress on organisms, assemblage, community and faunal composition were consistent with data on assemblage structure. These hypotheses were that: (1) stress should reduce the average size of organisms; (2) shorten food chains; (3) reduce predation intensity; (4) reduce species richness and diversity; and (5) increase the relative abundance of predator-susceptible ancestral species (i.e. Archaegastropoda). Water quality data suggest that the two most likely forms of stress in deeper (>1 m) areas of Florida Bay adjacent to the Keys are cold water temperatures associated with winter cold fronts and low predawn oxygen associated with warm summer temperatures, high salinity, and periodic algal and seagrass drift buildups. Seagrass sites had high population densities and low diversity due to the dominance of Astraea americana Gmelin (American star shell) in Florida Bay and Modulus modulus L. in Hawk Channel seagrass habitats. Florida Bay sites had high species richness on a small spatial scale, but Hawk Channel sites had more species and greater encounter rates of new species on a larger scale. Predawn oxygen measurements taken during July in four habitats were positively correlated with prosobranch species richness and diversity. Faunal data, analysed on a population density basis, fit the above hypotheses of body size, trophic level, and evolutionary age of the species. Attempts to measure predation on an experimental prosobranch ( A. americana) were unsuccessful but a tethering experiment with a sea urchin ( Echinometra lucunter L.) indicated higher predation in the less stressful Hawk Channel than Florida Bay hard substratum sites. Stress appears to reduce the abundance of higher trophic levels (both prosobranch and finfish predators) resulting in the dominance of ancestral forms not adapted to predation but tolerant of environmental stress. Eutrophication or increased oxygen demands in Florida Bay could result in further species richness and diversity declines.