Biological nitrogen fixation (acetylene reduction) associated with blue-green algal (cyanobacterial) communities in the Beachwood Mangrove Nature Reserve II. Seasonal variation in acetylene reduction activity

Seasonal variation in nitrogen fixation of blue-green algae associated with Avicennia marina (Forssk.) Vierh. pneumatophores and wet and dry surface sediments were investigated in the Beachwood Mangrove Nature Reserve by means of the acetylene reduction technique. The blue-green algae identified were all non-heterocystous and have been reported to fix nitrogen. The dominant blue-green algae were Lyngbya confervoides C.Ag. ex Gomont, Oscillatoria limosa Ag. ex Gomont and Microcoleus chthonoplastes Thuret ex Gomont in the pneumatophore, wet and dry surface habitats, respectively. A marked seasonal variation in ARA (acetylene reduction activity) was apparent, with summer maxima being recorded. This coincided with maximum nitrogen-fixing blue-green algal numbers, temperature, light intensity and day length. Seasonal variations in organic carbon, inorganic nitrogen, salinity, percentage moisture and bacterial numbers are discussed. It was estimated that nitrogen fixation by blue-green algal communities supplies 24.3% of the annual nitrogen requirements of the mangrove swamp