Coral reefs: nitrogen fixation

Coral reefs function as nitrogen fixers in nutrient poor environments. Two processes support this. Firstly atmospheric nitrogen is fixed by cyanobacteria and bacteria living within the skeletons of corals, in association with algae in soft sediments between colonies, as epiphtyes on dead rock, seagrasses and other macrophytes, in the periphyton and free in the water column. Rates of fixation are between 40 and 100 mg m-2 year-1 and the process is enhanced by optimal light conditions and relatively elevated local concentrations of organic substances. Reefs can be net exporters of nitrogen to adjacent waters and thus the activity of reef nitrogen fixers may support enhanced productivity in adjacent pelagic ecosystems. Where this is utilised by humans, for example in increased off- reef fisheries production, this constitutes an important non-consumptive use of coral reefs.

 

The second process is the accumulation on reefs of elevated concentrations of wind blown nitrogen fixing blue green algae, predominantly Trichodesmium rubrum. Usually growing in oligotrphic water at depths of 50-70m starving algae float to the surface and accumulate in masses which release large amounts of nutrients as they decay in coastal or lagoonal waters.

 

Nitrogen fixation is likely to be more important on and around isolated reefs such as oceanic atolls because coastal reefs receive more than enough nutrients from terrestrial run off. Indeed many suffer the opposite problem to nitrogen availability through coastal eutrophication.