Biogeochemical responses to coral mass-spawning on 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 ~2- and ~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.