Nitrogen fixation activity by periphytic blue-green algae in a seagrass bed on the Great Barrier Reef

Acetylene reduction activity, an index of nitrogen fixation, by epiphytic a nd epibenthic algae and coral debris was measured in seagrass beds around G reen Island, Australia. Epiphytic blue-green algae showed a high activity, ranging from 56.1 +/- 31.9 to 729 +/- 105 mmol (g chi a)(-1) d(-1) on a chl orophyll basis, or 3.9 mu mol N-2 m(-2) d(-1) to 16 mmol N-2 m(-2) d(-1) on an areal basis. The activities on an areal basis were comparable to those reported in a Fijian seagrass bed, whereas activities on a chlorophyll basi s were higher on Green Island than on Fiji. A relatively high activity was observed on the leeward side of the island, partly due to the high seagrass biomass and partly due to the high epiphyte biomass. This trend of high ac tivity on the leeward side of the island suggested that the biomass and/or activity of nitrogen fixing blue-green algae could be affected by unknown m echanism(s) related to the island environment. Algae on (or in) coral debri s, which were abundant around live corals, were able to reduce acetylene.