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 = 0.37 mmol L −1 ), compared with almost complete removal of dissolved inorganic nitrogen (DIN) (minimum NO 3 − = 0.01 mmol L −1 ; NH 4 + = 0.11 mmol L −1 ), 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.