Sources Of Dissolved Inorganic Nutrient Fluxes In The Gazi Bay And Implications For Coastal Ecosystems

Abstract

Nutrient fluxes of dissolved NI-I/, N02-, N03-, P043- and Si(OH)4 in the water column and nearshore sediments were measured in situ in the Mangrove, Seagrass and Coral reef biotopes of the Gazi Bay. The measurements were made using benthic flux bell-jar chambers each of cross-sectional area 0.066 m2 and volume 0.0132 m'. The objectives were: (1) to determine the influence of benthic fluxes, external sources (fluvial discharge) and seasonal variations on the nutrient budget in the Bay waters; (2) to determine the effect of tidal and spatial variations on the nutrient loads in the water column and (3) to establish the relative importance of the nutrient sources with regard to total community production of the Bay. Measured benthic flux rates varied between -270 and + 148; -60 and +63; -79 and +41; -79 and +75; and +30 and +350 micromoles/mvh for NH/-N; N02--N; N03--N; PO/-p and Si(OH)4-Si respectively. Assessment of these values showed that they were comparable to those theoretically calculated from interstitial sediment-water nutrient concentrations. However, differences in flux direction (+ve or -ve) and magnitude were attributed to external sources as well as interactive effects of the three biotopes. It was established that benthic fluxes are the major sources of dissolved NH/; N02- and Si(OH)4 species, while fluvial sources are important for N03- and P04 - loading in the Gazi Bay. Seasonal variations had a profound influence on the benthic pol- fluxes, N :Si ratios, fluvial nutrient discharge, plankton productivity, as well as fluctuations in temperature and salinity of the nearshore areas. By contrast, tidal and spatial variations had no significant effect on nutrient concentrations and net fluxes within the water column. The results imply that benthic fluxes are largely responsible for the nutrient dynamics of the nearshore coastal ecosystems especially where direct terrestrial inputs do not contribute significantly to the nutrient budget