Three-dimensional water-quality:Transport model compared with field observations

An ecological model, coupled with a hydrodynamic one, is applied to Lake Näsiselkä, the southernmost sub-basin of Lake Näsijärvi, Central Finland. Its surface area is 93 km2, mean depth 15 m, maximum depth 61 m, and through-flow 64 m3/s. The transport velocities to the ecological model are computed with a three-dimensional hydrodynamic sub-model. Its results are compared with the flow velocities measured in 1981. In the ecological model the transport velocities change according to the weather conditions, through-flow and temperature stratification.The ecological mdoel computes the dissolved oxygen, total phosphorus, phytoplankton biomass, biological oxygen demand and sodium lignosulphonate taking into account their decay, settling, mutual reactions, loading and transport with water flow. The model validity is tested with extensive field observation data of the year 1982 and more sparse data of the summer 1983. The results show reasonable agreement with the measured values.The model is used to test the effects of human activities (industrial loading and through-flow regulation) on the water quality. Industrial loading affects the oxygen and algal distributions in the lake whereas the executed regulation has not appreciably changed the natural conditions in the water body.