Biogeochemical responses to mass coral spawning at the Great Barrier Reef: Effects on respiration and primary production

Coral mass-spawning represents a spectacular annual, short-term, fertilization event of many oligotrophic reef communities. The spawning event in 2005 at Heron Island, Great Barrier Reef, was followed by an intense bloom of benthic dinoflagellates. Within a day from the first observed spawning, the primary production of the water column and the benthic compartment increased by factors of 4 and 2.5, respectively. However, the phototrophic communities were intensively grazed by macrozoans, and after 4-5 d the net photosynthesis ( P) returned to the pre-spawning background level. The heterotrophic activity ( R) mirrored the phototrophic response: a short term of elevated activity was followed by a rapid decline. However, the net autotrophic microbial communities exhibited a marked increase in the P : R ratio just after coral mass-spawning, indicating a preferential phototrophic recycling of nutrients rather than a microbial exploitation of the release of labile organic carbon. The heterotrophic and phototrophic activity of the benthic community exceeded the pelagic activity by similar to 2- and similar to 5- fold, respectively, underlining the importance of benthic activity for coral reef ecosystem function. Mass balance calculations indicated an efficient recycling of spawn- derived nitrogen ( N) and carbon ( C) within the benthic reef community. This was presumably facilitated by advective solute transport within the coarse, permeable, carbonate sand.