Mass coral spawning: A natural large-scale nutrient addition experiment

A mass coral spawning event on the Heron Island reef flat in 2005 provided a unique opportunity to examine the response of a coral reef ecosystem to a large episodic nutrient addition. A post-major spawning phytoplankton bloom resulted in only a small drawdown of dissolved inorganic phosphorus (DIP minimum 5 0.37 mmol L21), compared with almost complete removal of dissolved inorganic nitrogen (DIN) (minimum NO 3 5 0.01 mmol L21; NH z4 5 0.11 mmol L 21), suggesting that pelagic primary production is potentially N limited on the timescale of this study. DIN, DIP, dissolved organic nitrogen (DON), and dissolved organic phosphorus were used in the production of biomass, and mass balance calculations highlighted the importance of organic forms of N and P for benthic and pelagic production in tropical coral reef environments characterized by low inorganic N and P. The input of N and P via the deposition of coral spawn and associated phytodetritus resulted in large changes to N cycling in the sediments, but only small changes to P cycling, because of the buffering capacity provided by the large pool of bioavailable P. It is most likely that this large pool of bioavailable P in the sediments drives potential N limitation of benthic coral reef communities. For example, there was sufficient bioavailable P stored in the top 10 cm of the sediment column to sustain the prespawning rates of benthic production for over 200 d. Most of the change in benthic N cycling occurred via DON and N2 pathways, driven by changes in the quantity and quality of organic matter deposited and decomposed post-major spawning. The heterotrophic and autotrophic microbial communities within the coral reef sands were able to rapidly (6 to 7 d) process the large episodic load of N and P provided by coral mass spawning. Mass coral spawning or smaller-scale multispecific coral spawning events have been observed on coral reefs around the world (Harrison et al. 1984; Hayashibara et al. 1993; Hagman et al. 1998). In the central Great Barrier Reef (GBR) (Australia) mass spawning typically occurs on the third to sixth night after a full moon in October to December (Harrison and Wallace 1990). Synchronous multispecies coral spawning releases a large volume of eggs (1 3 106 m22) and sperm to the water column over a short period of time (Harrison and Wallace 1990). Large slicks of coral spawn products can form on the water surface and beaches of coral reef lagoons. Some of these eggs and sperm, which have a high lipid content (Arai et al. 1993), can be trapped within the coral reef lagoon (Wolanski et al. 1989; Simpson et al. 1993), resulting in a large episodic input of labile carbon (and associated nitrogen and phosphorus) to the coral reef ecosystem. For example, an estimated 310,000 kg of C and 18,000 kg of N were released from coral eggs alone (sperm not included) during a coral spawning event at the Heron Island reef in 2001 (Wild et al. 2004). As such, a coral spawning event represents a unique opportunity to examine the response of a coral reef ecosystem to a large episodic nutrient addition. Despite the potential for this large input of labile carbon to modify coral reef biogeochemistry and pelagic and benthic food webs, little work on the effects of coral spawning on the energy and nutrient cycles of coral reef ecosystems has been undertaken. Wild et al. (2004) measured a 2.5-fold increase in sediment oxygen demand after a spawning event on Heron Island in the GBR, reflecting an input of labile carbon and subsequent decomposition in permeable coral reef lagoon sediments. Similarly, Simpson et al. (1993) recorded a dramatic decrease in water column dissolved oxygen on Ningaloo Reef after mass coral spawning, resulting in mass mortality of fish and other reef animals due to the respiratory demand of the coral spawn. Glud et al. (2008) found that nutrients released after coral spawning not only stimulated the heterotrophic communities but also the autotrophic communities with 4.0and 2.5-factor increases in pelagic 1 Corresponding author ([email protected]). 2 Present address: Scottish Association for Marine Science (SAMS), Marine Laboratory Dunbeg, Oban, Argyll PA37 1QA, Scotland, United Kingdom. Acknowledgments We thank Iain Alexander and Anni Glud for assistance with the laboratory work and data compilation and analysis and Tage Dalsgaard for running the anammox samples. This work was supported by an ARC Discovery Grant (DP0342956) awarded to B.D.E., and an SCU Visiting Researcher Fellowship and NSF (Denmark) grants awarded to R.N.G. and was carried out under permit G06/18413.1. Limnol. Oceanogr., 53(3), 2008, 997–1013 E 2008, by the American Society of Limnology and Oceanography, Inc.