THE RELATIONSHIP BETWEEN NITROGEN-FIXATION AND TIDAL EXPORTS OF NITROGEN IN A TROPICAL MANGROVE SYSTEM

Various components (sediments, algal mats, decomposing logs and algal-covered prop roots) of a tropical mangrove forest showed low to moderate nitrogen fixation (acetylene reduction) rates. Measurements carried out in March, May and October 1989 revealed negligible or inconsistent seasonal variation in activity for all components. Prop roots with their associated algae showed much greater activity during night-time, typical of many substrates with associated cyanobacteria, whereas all other components had constant activity over 24 h periods. Extrapolation of the nitrogen fixation rates for each component to a 'whole-forest' basis, using field estimates of the densities of each component, indicated that sediments, prop roots and decomposing logs contributed approximately 3·5, 1·6 and 1·0%, respectively, of the nitrogen requirements for forest net primary production (FPPN). Blue-green algal mats on bare saltpan areas showed no significant nitrogen-fixing activity (in excess of that for the bare sediments in the same area). The total nitrogen contribution of the various components (6% of FPPN) closely matches previous esimates of the net nitrogen loss from the system through tidal fluxes of particulate and dissolved materials. The possible relative importance of other nitrogen input and loss mechanisms for this mangrove system is also discussed. © 1990.