Modeling water temperature effect in diatom (Stephanodiscus hantzschii) prediction in eutrophic rivers using a 2D contaminant transport model

In many eutrophic rivers, the diatom dynamics mainly depends on change of water temperature rather than nutrient concentration because phosphorus and nitrogen loadings are usually sufficient to saturate the diatom growth. In this study, a two-dimensional contaminant transport model was developed in which diatom growth was predicted by incorporating the dynamics of water temperature, dissolved inorganic phosphorus and dissolved inorganic nitrogen as well as the hydrodynamics of river flow. In this study, in order to accurately model the effect of the water temperature on the diatom growth, a new form of the water temperature limitation function, f(T) was proposed, using the field data obtained from water quality monitoring stations at the Nakdong River, one of the eutrophic rivers in South Korea. Fitting the water quality data, the skewed Gaussian function was empirically derived for f(T) with parameter estimation to calculate the growth rate of the centric diatom, Stephanodiscus hantzschii (S. hantzschii) for which the optimal water temperature was found to be 8.7 °C from winter to spring seasons. The calibration of the two-dimensional contaminant transport model with the proposed f(T) showed that the Nash Sutcliffe Efficiency (NSE) of the simulation with the observed data at each monitoring station was 0.63 and 0.85. The validation of the calibrated model also showed that the simulation results were in good agreement with the observed data with NSE larger than 0.74. The results of the parametric sensitivity analysis revealed that the optimal water temperature, Topt most influenced the temporal variation in the diatom abundance. According to results of the scenario simulation using the calibrated and validated model, the thermal effluents introduced from the tributary as well as dispersion coefficients, DL and DT significantly affected the spatial distribution of the diatom concentration in the study site of the Nakdong River.