Quantitative aspects of inorganic nutrient fluxes in the Gazi Bay (Kenya): implications for coastal ecosystems

Abstract

Fluxes of dissolved inorganic nutrients: NH 4 +, NO 2 −, NO 3 −, PO 4 3− and Si(OH) 4 from nearshore sediments of Gazi Bay were measured in situ within mangrove, seagrass and coral reef biotopes using benthic flux bell-jar chambers of cross-sectional area 0.066 m 2 and volume 0.0132 m 3. The objectives were: (1) to determine the influence of benthic fluxes, fluvial discharge and seasonal variations on the nutrient budget in the Bay waters; (2) to determine the effect of tidal and spatial variations on 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. The directly measured fluxes ranged from −270 to +148 μmol NH 4 +–N/m 2/h; −60 to +63 μmol NO 2 −–N/m 2/h; −79 to +41 μmol NO 3 −–N/m 2/h; −79 to +75 μmol PO 4 3−–P/m 2/h and +30 to +350 μmol Si(OH) 4–Si/m 2/h for and respectively. It was established that benthic fluxes are the major sources of dissolved inorganic NH 4 +, NO 2 − and Si(OH) 4 while fluvial sources are important for NO 3 − and PO 4 3− into Gazi Bay waters. Seasonal variations had an appreciable effect on the PO 4 3− fluxes, N:Si ratio, river nutrient discharge, plankton productivity and important environmental factors such as salinity and temperature. 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.