Topical Area: Higher Trophic Levels

Pamela J. Schofield, Department of Biological Sciences, University of Southern Mississippi, Hattiesburg, MS

Estuarine-dwelling organisms are well known for their ability to withstand small-scale environmental variation. However, chronic stress from large-scale perturbations may ultimately limit the ability of these organisms to grow and reproduce. Because northern Florida Bay exhibits an east-west gradient of environmental harshness, it is an excellent system within which habitat use patterns may be compared against physiological stress. To effectively manage such a system, we must know how habitat use shifts in response to seasonal cycles and how larger-scale environmental perturbations (e.g., freshwater inflow management) affect organisms in this environment. In this proposal, I suggest the use of an integrated approach to quantifying habitat quality, which combines laboratory and field manipulations to test for effects of environmental heterogeneity on the physiological response, feeding ecology and interspecific associations of two gobies that exhibit a gradient of resource use across northern Florida Bay.

Two gobies (Microgobius gulosus, Gobiosoma robustum) are commonly found in northern Florida Bay (Thayer et al. 1987; Sogard et al. 1987; Tabb & Manning 1961). These species are opportunistic benthic predators, feeding on similar prey types, and occur over a wide range of salinities and habitat types (Sogard et al. 1989; Thayer et al. 1987; Tabb & Manning 1961; Springer & Woodburn 1960). Although their distributions overlap to a limited degree, published reports indicate Gobiosoma robustum is more abundant in the seagrass beds of the western portion of the bay whereas Microgobius gulosus is most common in the eastern section (Sogard et al. 1987).

Although vegetation characteristics may be important in habitat selection for these species, studies of adjacent areas of dense and sparse vegetation suggest that salinity regimes may be more important in structuring patterns of habitat use (Sogard et al. 1989). Habitat selection may be an interplay between several factors including physiological and biotic components. To fully understand habitat selection, physiological tolerance to environmental variability must be documented as well as the effects of environmental heterogeneity on biological interactions such as competition and predation.

The interaction of physiological stress and competitive abilities may explain distribution patterns of these two gobies in Florida Bay. Microgobius gulosus appears to be more abundant than G. robustum in stressful environments characterized by hypersalinity and reduced vegetation (eastern FL Bay). Gobiosoma robustum was only rarely found in the eastern portion of the bay, which may represent a decreased ability to thrive in this harsh environment. However, where salinities are more moderate, M. gulosus is rare. This suggests that G. robustum may be more competitive for resources in lushly vegetated seagrass habitats or more susceptible to

predation in areas without dense seagrass cover. The elucidation of the factors structuring the abundance of these two gobies may provide insight into the effects of environmental harshness on species distributions and the interactive effects of abiotic and biotic variables.

In this proposal I plan to document environmental variables and food availability for these two species in the western, interior and eastern portion of Florida Bay. Because variations in salinity regimes appear to affect the distribution of these two species, special attention will be given to the effects of varying salinity on feeding ecology, interspecific competition and predation. To understand the individual and interactive effects of abiotic and biotic factors on the ecology of M. gulosus and G. robustum, the following critical questions will be addressed:

1. How is the survival ability of each species affected by salinity variation?
2. How do the diets of each species differ among subenvironments (western, interior and eastern) and microhabitats (mud and seagrass substrates) of north FL Bay?
3. How are feeding habits related to prey availability in these habitats?
4. How do various salinity regimes affect interspecific competition between M. gulosus and G. robustum?
5. How does susceptibility to predation differ with respect to salinity regimes and substrate types?

Goby species will be caged both separately and together in each of the three subenvironments of Florida Bay with distinct salinity regimes (eastern, interior and western) in both seagrass and mud substrates. Effects of environmental heterogeneity and interspecific competition on condition and food habits will be evaluated. To determine shifts in habitat use over seasonal scales, caging experiments will be conducted during the wet and dry seasons. Additionally, the physiological tolerance of each species to shifts in salinity will be tested under laboratory conditions. Finally, susceptibility to predation under various salinity regimes and habitat types will be tested by placing both species (separately and together) in enclosures with a model predator.

This study will provide information on the environmental sensitivity of these two species, and will delineate mechanisms involved in their distribution and abundance throughout northern Florida Bay. Studies of resident species that are both sensitive to environmental variability and independent of fisheries are vital, as they are more likely to reflect changes in habitat quality than transient species. Furthermore, studies of salinity tolerance must go beyond simple distributional surveys and document the importance of salinity in more complex interactive effects such as species interrelationships. Once gained, this knowledge may be used to characterize patterns of environmental quality in Florida Bay. For example, the shift in dominance from G. robustum to M. gulosus may represent declining habitat quality due to increased harshness caused by salinity variation.

Literature cited:

Sogard, S. M., G. V. N. Powell & J. G. Holmquist. 1987. Epibenthic fish communities on Florida Bay banks: relations with physical parameters and seagrass cover. Marine Ecology Progress Series 40:25-39.

Sogard, S. M., G. V. N. Powell & J. G. Holmquist. 1989. Spatial distribution and trends in abundance of fishes residing in seagrass meadows on Florida Bay mudbanks. Bulletin of Marine Science 44:179-199.

Springer, V. G. & K. D. Woodburn. 1960, An ecological study of the fishes of the Tampa Bay area. Professional papers of the Florida State Board of Conservation, Number 1. 104 pp.

Tabb, D. C. & R. B. Manning. 1961. A checklist of the flora and fauna of northern Florida Bay and adjacent brackish waters of the Florida mainland collected during the period July, 1957 through September, 1960. Bulletin of Marine Science of the Gulf and Caribbean 11:552-649.

Thayer, G. W., D. R. Colby & W. F. Hettler, Jr. 1987. Utilization of the red mangrove prop root habitat by fishes in south Florida. Marine Ecology Progress Series 35:25-38.