ECOLOGICAL CONTROLS ON STROMATOLITE DEVELOPMENT IN A MODERN REEF ENVIRONMENT - A TEST OF THE ECOLOGICAL REFUGE PARADIGM

Paleontological and neontologicaI studies suggest that stromatolite de velopment may be limited to environments where ecological processes su ch as colonization (recruitment), growth competition, and predation (h erbivory) are low. We quantitatively tested this ''ecological refuge p aradigm'' at st unique stromatolite-reef complex at Stocking Island, B ahamas where three contiguous zones grade in dominance from stromatoli tes in the back reef to a macroalgal turf in the reef flat to reef bui lding corals and coralline algae and reef fishes in the fore reef. At two independent transacts along this gradient, we quantified the distr ibution and abundance of the dominant organisms and measured the rates of the above-listed ecological processes and determined survival and growth of stromatolite, turf, and coralline algae transplanted to each of the three zones. We also measured both the sediment dynamics and t emperature profiles along this gradient to determine if these may be c ontrolling the ecological processes. Stromatolites dominate the back r eef zone where species diversity (especially among eukaryotes) and ass ociated ecological pressures were low. Measured rates of colonization from calcified (i.e., coralline) and noncalcified macroalgae and filam entous turf algae were lowest in the back reef (i.e., <5% substrate co lonization/y) and highest in the fore reef (95% substrate colonization /y). This suggests that competition for space was lowest in the stroma tolite zone. Growth and survival rates of transplanted coralline algae were low in both the back reef (3 mm/y and 20-40% annual survival rat es) and reef flat (<2 mm/y, 0% surviving) and highest in the fore reef (about 10 mm/y, 80% surviving). Foraging activity from all fishes was lowest in the back reef (total 0.4 bites/m(2)/h) and reef flat (0 bite s/m(2)/h) and highest in fore reef (900 -2200 bites/m(2)/h mostly from omnivorous wrasses). Herbivory from all potential sources was below d etectable levels in the stromatolite zone and was highest in the fore reef. The ecological refuge found in the back reef stromatolite zone r esulted primarily from periodic sediment inundation. Most eukaryotic r eef-dwelling organisms cannot persist under as much as 700 mm of sedim ent for periods up to 100 days. Although the reef flat suffered both d esiccation and thermal stress (averaging more than 5 degrees C greater daily temperature fluctuation than the fore reef), natural and transp lanted eukaryotic turf algae thrived there. Although transplanted stro matolites survived best in the back reef (about 80% of the original tr ansplant surviving/y), they also persisted in the fore reef (about 40% surviving per year). Therefore, Schizothrix-dominated stromatolites c an persist in fore reef environments but because of the presence of ot her organisms and associated ecological pressures, their laminated mic robial-produced structure is lost or obscured While the sediment and t he organisms necessary for stromatolite formation exist in this and pr obably in other modem reef environments, their reef-building contribut ion will be low except in unusual habitats where abiotic stresses main tain an ecological refuge.