Nutrient fluxes in the Bons Sinais Estuary (Mozambique) – sources and sinks

Abstract

ABSTRACT Estuaries are the main gateway of nutrients into marine coastal waters. Human activities and biodegradation processes in coastal freshwater and mangrove swamps are the major sources of nutrients. This study examines the sources, sinks and flux rates of nitrogen (N), phosphorus (P) and silicon (Si) through the interfaces: river estuary, mangrove estuary and ocean estuary. Measurements of the concentrations of nutrients and water velocity were made near the surface on an hourly basis, and then integrated over the flood and ebb flows over the cross-sectional area and a budget estimated over the tidal cycle, at the three interfaces. The results indicated that freshwater swamps, along the river basin, and mangrove swamps were the major source of nitrate in the estuary; the urban sewage effluent was the major source of phosphate; major sources of silicate were the freshwater swamps and coastal ocean waters. Mangrove swamp showed a tendency to retain phosphate, confirming the hypothesis that mangroves are effective sewage filters and pollutant retainers. The river exported nitrate into the estuary at a rate of 59 tons per tidal cycle, and then the estuary as a whole exported nitrate into the coastal waters at a rate of 68 tons per tidal cycle. The present study contributes to the explanation of the observed high coastal productivity of the Sofala Bank and to the development of prognostic and predictive models to quantify and predict the nutrient export to – and productivity of – coastal waters.