Ecological controls on stromatolite development in a modern reef environment: A test of the ecological refuge paradigm

Paleontological and neontological studies suggest that stromatolite development may be limited to environments where ecological processes such as colonization (recruitment), growth, competition, and predation (herbivory) are low. We quantitatively tested this “ecological refuge paradigm” at a unique stromatolite-reef complex at Stocking Island, Bahamas where three contiguous zones grade in dominance from stromatolites in the back reef to a macroalgal turf in the reef flat to reef building corals and coralline algae and reef fishes in the fore reef. At two independent transects along this gradient, we quantified the distribution 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 temperature profiles along this gradient to determine if these may be controlling the ecological processes. Stromatolites dominate the back reef zone where species diversity (especially among eukaryotes) and associated ecological pressures were low. Measured rates of colonization from calcified (i.e., coralline) and noncalcified macroalgae and filamentous turf algae were lowest in the back reef (i.e., <5% substrate colonization/y) and highest in the fore reef (95% substrate colonization/y). This suggests that competition for space was lowest in the stromatolite zone. Growth and survival rates of transplanted coralline algae were low in both the back reef (3 mm/y and 20–40% annual survival rates) 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/m2/h) and reef flat (0 bites/m2/h) and highest in fore reef (900–2200 bites/m2/h, mostly from omnivorous wrasses). Herbivory from all potential sources was below detectable levels in the stromatolite zone and was highest in the fore reef. The ecological refuge found in the back reef stromatolite zone resulted primarily from periodic sediment inundation. Most eukaryotic reef-dwelling organisms cannot persist under as much as 700 mm of sediment for periods up to 100 days. Although the reef flat suffered both desiccation and thermal stress (averaging more than 5°C greater daily temperature fluctuation than the fore reef), natural and transplanted eukaryotic turf algae thrived there. Although transplanted stromatolites survived best in the back reef (about 80% of the original transplant surviving/y), they also persisted in the fore reef (about 40% surviving per year). Therefore,Schizothrix-dominated stromatolites can persist in fore reef environments but because of the presence of other organisms and associated ecological pressures, their laminated microbial-produced structure is lost or obscured. While the sediment and the organisms necessary for stromatolite formation exist in this and probably in other modern reef environments, their reef-building contribution will be low except in unusual habitats where abiotic stresses maintain an ecological refuge.