Abstract
This study is a baseline attempt to quantify the biogeochemical fluxes of nutrients along the Ogun River to the Lagos Lagoon for a period of five months (from February 2016 to July 2016), in order to evaluate the influences of riverine discharge as well as their current contribution to net nutrient budgets in this coastal region. This was carried out by modelling stream flow in the river-eastern box and quantifying its relative nutrient discharged. Estimated flux from River headland showed spatial variations and seasonal distribution patterns with average fluxes of the various nutrients; Nitrate (354.2mgNO3/L/m3s-1 ± 156.5), Phosphate (361.35mgPO4/L/m3s-1 ±153. 9), Ammonium (0.2254mgNH4/L/m3s-1 ± 0.106) and Silicate (0.195mg SiO2/L/m3s-1 ± 0.099) recorded alongside an average water discharge estimate of 86.548m3s-1 when measured from the river mouth. The flow rate was highest in July with a flow value of 127.32m3s-1 and lowest in the intermittent month of June (33.75m3s-1). Nutrient enrichment from water discharge exhibited ranges of Phosphate (1.69 - 6.6mg/L), Nitrate (1.69mg/L - 6.60mg/L) and Silica (0.001mg/L - 0.004mg/L). Average Phytoplankton biomass estimate (as Chlorophyll a) was 5.39 ± 2.35 mg/L and have a positive correlation (r = 0.47, 0.32, 0.67 and 0.16) with nutrient concentration (NO3, NH4, PO4, SiO2 respectively). From this study, net nutrient fluxes seem to be a major driver of chlorophyll-a dynamics. However, Silicate fluxes seem relatively discharged at relatively low concentrations compare to other nutrients and could potentially have an important influence on the health of the Lagos Lagoon ecosystem (food web and community structure) and primary productivity of this coastal water. Further works in developing computerized models for quantifying these fluxes is needed as predictive tools in evaluating the environmental ramification riverine influenced discharge on the lagoon.
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