Models describing the dependence oflake trophic state on external phosphorus loading are of two general types: empirical, steady state models derived from statistical treatment of data from large numbers of lakes, and theoretical, dynamic models based on more or less detailed mathematical descriptions of kinetics of nutrient and population dynamics.In the former type of models, inlake P concentrations have been found to depend mainly on inflow P concentrations and on water residence time. Efforts to explain the remaining variance by other factors such as iron, calcium, alkalinity, etc. have generally not been successful. Relations between lake P concentrations and chlorophyll may be useful for the prediction of maximum chlorophyll levels with some confidence but large variations in the actual chlorophyll concentrations are caused by, e.g. zooplankton grazing, varying chlorophyll content of the algae and other limiting nutrients.Dynamic, theoretical models are more suited for the prediction of rates of response to perturbations than the empirical steady state models. Due to the much larger demand for input data and labour investment, these models are generally more suited for the scientist as tools for formalizing and testing hypotheses, than for the water manager. Simple models with flexible structure generally have advantages over complex models.Both empirical and theoretical models are potentially powerful tools in eutrophication research and management. It is important for both scientists and water managers to select the optimum complexity with regard to the problem to be solved and to the resources available in each case.

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Lake Onega is the second largest lake in Europe after Lake Ladoga. This paper is a part of the project concerning a general plan of water protection, as expressed in the Water Framework Directive. The aim of this paper was to present an investigation of the status of Lake Onega and to present steady state and dynamic modelling approach in order to assess the impacts of different loading scenarios of water quality of Lake Onega. In the project more catchment and water quality models were used but in this paper these models were chosen. The presented steady state model was the mass balance model of Vollenweider and the dynamic model is the box-type model AQUATOX. While Lake Onega preserves a good status of water as a whole, the problems with pollution and eutrophication exist in Petrozavodsk and Kondopoga Bays where anthropogenic loading is more pronounced.

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Mass balance models for elongated lakes have been developed. In these models particular attention is paid to the hydraulic description of the lake and to the description of net sedimentation. The hydraulic description is a linear combination of CSTR and PFR, with a coefficient regulating the degree of mixing. The PFR description was modified so that also additional input along the lake can be considered. Net sedimentation is described either as a first order or a second order reaction. Both voluminal and areal sedimentation is possible in the models. The models were tested against phosphorus balance data for two Finnish lakes. Lake Haukivesi and Lake P5jiinne. In both lakes the PFR model with additional input together with second order net sedimentation gave the best fit to observations. The values of the sedimentation coefficients are not universal but vary from lake to lake. They are also dependent on the hydraulic description used in the model. There is a critical point for the monitoring of the lake at which measurements give the same sedimentation coefficient regardless of the hydraulic description.