, Volume 98, Issue 1, pp 143-155

Nitrogen fixation on a coral reef

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Abstract

Acetylene reduction was used to assess nitrogen fixation on all major substrates at all major areas over a period of 1 to 6 yr (1980–1986) at One Tree Reef (southern Great Barrier Reef). Experiments using 15N2 gave a ratio of 3.45:1.0 for C2H2 reduced:N2 fixed. Acetylene reduction was largely light-dependent, saturated at 0.15 ml C2H2 per ml seawater, and linear over 6 h. High fixation was associated with two emergent cyanophyte associations, Calothrix crustacea and Scytonema hofmannii, of limited distribution. Subtidally, the major contribution to nitrogen fixation came from well-grazed limestone substrates with an epilithic algal community in the reef flat and patch reefs (3 to 15 nmol C2H4 cm-2 h-1). Similar substrates from the outer reef slope showed lower rates. Nitrogen fixation on beach rock, intertidal coral rubble, reef crest and lagoon sand was relatively small (0.3 to 1.0 nmol C2H4 cm-2 h-1). Seasonal changes in light-saturated rates were small, with slight reduction only in winter. Rates are also reported for experimental coral blocks (13 to 39 nmol cm-2 h-1) and for branching coral inside and outside territories of gardening damselfish (3 to 28 nmol cm-2 h-1). This work supports the hypothesis that the high nitrogen fixation on the reef flat and patch reefs of the lagoon (34 to 68 kg N ha-1 yr-1) is because these subtidal areas support highly disturbed communities with the greatest abundance of nitrogen-fixing cyanophyte algae. It is calculated from a budget of all areas that One Tree Reef has an annual nitrogen fixation rate of 8 to 16 kg N ha-1 yr-1.

Communicated by G.F. Humphrey, Sydney