Effects of Benthic Microalgae on Eutrophication Processes — A Laboratory Experiment and the Model Simulation

The shallow water region (SWR) of coastal marine ecosystems, with depths less than 3–5 meters, encompasses the land-water margin, and serves as the buffer zone for the transport of nutrients between land and water. When light can penetrate through the water column and reach the bottom, it triggers benthic microalgae (BMA) to perform photosynthesis, resulting in oxygen and nutrient benthic-pelagic exchange fluxes that are different from those occurring in deeper water. In this study, a controlled laboratory experiment was conducted to examine the eutrophication process in the SWR, with special emphasis on the role played by the BMA. Under controlled temperature and mixing conditions, nutrients and dissolved oxygen concentrations were measured in the light and dark in cores with sediment (that contained BMA) and without sediment (blank samples). Analysis of the laboratory results demonstrated that BMA performed photosynthesis under light conditions in the surficial sediments, resulting in the release of oxygen and the net uptake of nutrients. The laboratory-scale CE-QUAL-ICM model was utilized to simulate oxygen, ammonia, nitrite/nitrate, and phosphorus fluxes. The results compared quite satisfactorily with laboratory measurements, underpinning the proper formulation for the coupled water column and sediment flux model. It was found that the sediment type and the oxygen produced by BMA could influence the biochemical processes in sediments and thus affect the nutrient benthic flux.