Effect of benthic fluxes on nutrient dynamics of bottom water during stratification in a brackish lake

Abstract

We performed in-situ investigations using a benthic lander to examine changes in the sediment oxygen demand (SOD) and benthic nutrient fluxes during development of stratification in the downstream area of an artificially constructed brackish lake, Saemangeum Lake. During each measurement period, the water temperature and salinity in the water column showed a vertically stable stratified structure. The potential energy anomaly (PEA) was 88.1–125.7 J/m3, and the intensity of the stratification gradually strengthened during the entire measurement period. The DO concentration in the bottom water gradually decreased corresponding to the intensified stratification of water with the increase in temperature; thereby, establishing hypoxic conditions. As the stratification intensified, the SOD decreased with the decrease in DO concentration in the bottom water, and the benthic fluxes of NH4-N and PO4-P exhibited the opposite effect. When the effect of offshore water (through the sluice gate of the dike) was insignificant, the SOD contributed 33% of the net DO loss below the pycnocline. During this period, the benthic NH4-N and PO4-P fluxes were estimated to contribute 55% and 87% to the net fluxes of NH4-N and PO4-P, respectively, in the water column below the pycnocline. Additionally, the benthic NH4-N and PO4-P fluxes resulted in excess phosphorus in the bottom water. When the inflow of seawater through the sluice gate is sufficient, the flow in the upstream direction of the bottom layer will move the phosphorus-rich bottom water in the downstream, which is important for algae growth in the middle-upstream.