SEMINAR: Oceanographic ecology of coral reefs: the role of oceanographic processes in reef-level biogeochemistry and trophic ecology

Coral reefs worldwide are under increasing pressure from climate change, acidification, habitat destruction and over-fishing. Given that reefs support some of the highest biodiversity and provide a large range of ecosystem goods and services, improved understanding of the factors controlling their productivity and function at an ecosystem level should be a high priority. I will present research demonstrating how reef-level biogeochemistry and the trophic ecology of some reef organisms may be influenced by oceanographic processes over broad temporal and spatial scales.

The high productivity of coral reefs in an oligotrophic environment was historicallyattributed to ‘tight recycling’ of nutrients within the reef ecosystem, with reefs appearing to be neither a net source nor sink of nutrients. However, there is now evidence that reef ecosystems and organisms may depend on the input of oceanic nutrients, as well as releasing nutrients into the water column at appreciable rates. At Ningaloo Reef, Western Australia, the rate of phytoplankton-derived nitrogen (N) uptake we have observed (2 – 5 mmol N m-2 day-1) confirms that oceanic particulate organic matter (POM) may supply most of the N ‘missing’ in previous reef nitrogen budgets. Further, the estimated gross release of dissolved N over the reef, as high as 18 mmol NOx m-2 d-1, increased significantly with increasing uptake of phytoplankton across the reef, but only when the supply of oceanic POM was high. This suggests that recycling may indeed be a significant processes, albeit driven by the supply of oceanic POM and its remineralisation by reef organisms. As well as dissolved N, the reef produced POM in significant quantities, possibly associated with coral photosynthesis and mucus release, leading to marked increases in carbon (C) isotope and C:N ratios across the reef. Additional stable isotope data from Ningaloo and experimental Hawaiian reef communities provides further insight into the relative importance of oceanic and reef-derived nutrients for a number of reef organisms.

The simultaneous uptake of oceanic nutrients and the release of reef-derived nutrients has several broad ecological implications. Firstly, Ningaloo is particularly likely to depend on oceanic nutrient supply, which was highly variable at daily-to-seasonal time scales in response to the dynamics of Western Australia’s regional current system. Acceleration of the downwelling favourable Leeuwin Current (LC) in the autumn doubled phytoplankton supply to the reef, while sporadic upwelling associated with the Ningaloo Current more than tripled dissolved nitrogen supply in the summer. Secondly, a functional dependence by reef organisms on remotely produced ocean productivity increases the potential scale at which human- and climatically-induced changes may affect reef communities and suggests that processes such as changes in offshore currents and plankton communities require greater consideration in reef-level biogeochemistry. Finally, the production and export of reef-derived nutrients is likely to play an as yet unquantified role in thebiogeochemistry and productivity of the surrounding ocean. For instance, at Ningaloo, reef-level nutrient production driven by enhanced phytoplankton supply from the accelerating LC may ultimately support the seasonal aggregation of whale sharks (Rhincodon typus) that attracts tourists to Ningaloo every autumn. The close functional association between Ningaloo Reef and Western Australia’s unique current system is further evidence of the region’s World Heritage value and emphasises the need to consider reefs in the context of broader-scale oceanographic processes in order to understand the fundamental controls of reef productivity and function.