The use of nutrient budget approach for informing eutrophication management in urbanised shallow coastal lakes: A case study from Lake Nokoué in Benin

Abstract

Lake Nokoué in Benin supports the livelihoods of more than 500.000 people through fish production and sand mining. However, its health is jeopardised by eutrophication and excessive growth of the invasive water hyacinth, which dies off and sink to sediment during dry seasons when the lake's salinity increases due to tidal intrusion. Water-hyacinth constitutes then a potential source of internal nutrient loading through the decomposition of detritus and therefore can exacerbate eutrophication problems. An important management question is whether the removal of water hyacinth before it dies would significantly reduce eutrophication problems through the removal of nutrients. To compare the amount of nutrients removed by water-hyacinth biomass to the amount of nutrients transported into the Lake, mass-balance approach has been taken. Nutrient concentrations, water hyacinth cover, and hydrological inflows were measured over a 12 month period from November 2016 to October 2017. Water-hyacinth biomass estimation was based on a calibrated NDVI and field measurement relationship. In total the hyacinth covered up to 18 km2 (10%) of the lake, corresponding to 80,000 tonnes of dry weight which sink to the bottom each year when salinity rise. The potential internal nutrient loaded from hyacinth to the system was estimated to 1021 tonnes of nitrogen (8% of external input) and 231 tonnes of phosphorus (40% of external input). Considering that the tidal nutrient output compensates almost all the external nutrient inputs (about 70%), part of the nutrient available for eutrophication must come from internal sources, likely through the decomposition of sediment detritus.