Methods for sampling free-living Symbiodinium (zooxanthellae) and their distribution and abundance at Lizard Island (Great Barrier Reef)

The abundance and distribution of free-living dinoflagellates in the genus Symbiodinium have important implications for the ecology of coral reefs, determining both the symbionts available to newly recruited corals and symbiont types available for uptake by adult corals during environmental stress. However, little is known about where symbiotic dinoflagellates reside outside the host, due to the difficulty of capturing and detecting unicellular organisms in the marine environment. This study presents a successful protocol for sampling Symbiodinium from both the benthos and the water column. Comparisons of two detection methods for enumerating Symbiodinium indicated that conventional microscope analysis is accurate and more efficient when estimating Symbiodinium densities in sediment samples, while an automated particle counter (FlowCAM) was more efficient in detecting cells in the water column where densities are low. Symbiodinium densities were found to be relatively high (1000–4000 cells/mL) in sediment samples and much lower (up to 80 cells/mL) in the water column, indicating that the free-living form resides mainly in the benthos. Symbiodinium densities were found to be highly variable spatially, differing significantly between two reef locations. Within sites, elevated densities of Symbiodinium along reef margins combined with significant decreases in densities one meter away from the reef, suggest that cells aggregate within the reef habitat.