Spatial and temporal distribution of dissolved inorganic nutrients and phytoplankton in Mida Creek, Kenya

摘要

The spatial and temporal distributions of dissolved inorganic nutrients were investigated between May 1996 and April 1997 in Mida Creek, a mangrove area along the north coast region of Kenya. The nutrient levels of pore water from boreholes/wells within the surrounding area of the creek were also investigated for comparison. In addition, phytoplankton distribution in Mida Creek was assessed in three stations within the creek in order to determine the structure and succession stages of the phytoplankton community and to provide an indication of the status of primary productivity of the creek. Measurements carried out within the creek revealed that the mean concentration ranges for NH_4~+ -N, (NO_2~- +NO_3~-)-N, PO_4~(3-)-P and SiO_3~(2-)-Si were: 0.002-5.45; 0.12-5.63; 0.10-0.58 and 1.31-81.36 μM, respectively. For the case of boreholes/wells found in the surrounding area, their respective nutrient levels were found to lie in the ranges 0.4-907.0; 16.7-4897.0; 1.09-22.39 and 83.9-596.0 μM. A total of 295 species of phytoplankton belonging to 78 genera were identified with great temporal variability in abundance in all the stations sampled. The most dominant algal members in the Creek included Chaetoceros spp., Chroococcus limneticus and Oscillatoria spp. The diversity values recorded were indicative of mesotrophic conditions. The highest nutrient concentration levels within the creek were measured during the wet season as compared to dry season and this trend closely corresponded with that of the phytoplankton productivity. However, no significant variation (p > 0.05) was found in all cases with respect to the tidal cycles. On the contrary, diurnal nutrient concentrations especially in areas with high flooding duration ( > 12 h) were found to be highest during the dry season as opposed to wet season for all nutrients except for SiO_3~(2-). The relatively high nutrient laden groundwater outflow into the creek water, coupled with surface runoff events during wet season, are the two main factors responsible for the elevated nutrients in the creek waters in the absence of river inflow into the creek.