The River Buriganga, which runs past Dhaka City, is at present one of the most polluted rivers in Bangladesh. Dhaka City is very densely populated and will be one of the ten ‘Mega Cities’ by the year 2000. However, only a small fraction of the total wastewater being generated in the City is treated. Consequently, the amount of untreated wastes, both domestic and industrial, being released into the Buriganga is tremendous and is increasing day by day. Therefore, the objectives of this study were to investigate the status of the river water quality in terms of some cardinal water quality parameters, and to simulate the dissolved oxygen (DO) level using a water quality model. In order to fulfil the objectives, a comprehensive data acquisition programme – from both in situ and laboratory testing – was carried out. Then, a one-dimensional water quality model was developed for the Buriganga River system for a dry period of 1994-95. Different scenarios were tested to predict the most likely condition of the river. The results of the model simulations have replicated the alarming low DO level in the Buriganga. Option runs show that an integrated approach would be required to restore the river water quality with regard to biodegradable pollutants.

The relative importance of nutrient loadings and hydrographical and meteorological conditions for oxygen conditions in the Thisted Bredning, Denmark were evaluated by intensive survey and model calculations. The influence of wind speed on the vertical mixing in the unstratified shallow waters was quantified and tested in an oxygen model. The significance of oxygen consuming biological processes on the oxygen conditions were tested. By the use of a eutrophication model, the influence of local discharges of sewage on the size of the oxygen producing and oxygen consuming processes in the Thisted Bredning was calculated. Calculations showed that only a minor increase in rates of the oxygen consuming processes result in a drastic rise in the probability of oxygen depletion.

A modular approach for generating an ecosystem model for the North Sea is presented. The model structure consists of modules describing physical, chemical and biological processes. The modular approach is selected to facilitate stepwise improvements in the total ecosystem model by replacing existing modules with improved modules being developed. The modules constituting the pelagic ecosystem describe the biological and chemical dynamics of particle production and dissolution simulating the flux of carbon and nutrients (N, P, Si) through the food web. This might be done as a suite of submodules based on functional groups of organisms or as modules describing the different trophic levels based on size distributions. Algal growth is dependent on cellular content of the limiting nutrient, or on net photosynthesis, whichever is the most restricting, allowing for “luxury uptake” of nutrients which may be stored for subsequent periods of shortage. The role of bacteria and other microorganisms is emphasized in recognition of the importance of the “microbial loop”. The Zooplankton module describes prey ingestion in terms of feeding behaviour and the partitioning of ingested carbon, nitrogen and phosphorus into growth and reproduction and the losses through respiration, excretion and defecation. The benthic modules concentrate on describing the small food web, since benthic biological activity in terms of carbon flow, as well as mineralization, are often dominated by micro- and meiofauna in the generally soft sediments of the North Sea. Macrobenthos are described as being a major link between the benthic small food web and higher trophic levels in the ecosystem. Higher trophic levels are described in separate modules taking into account in principle the role of fish, mammals and seabirds. Nutrient cycling is described, focusing on the identification and conceptual modelling of the chemical processes and mechanisms in order to describe the relationships between the biology of the North Sea and the nutrient chemistry. This represents a first step towards forecasting the response of the system to long-term changes due to e.g. eutrophication. The modelling of the nutrient sediment-water interaction emphasizes the role of sedimentation of particulates and the regeneration of inorganic components to the water column. This totally modular concept of the North Sea ecosystem model reflects the authors' view on the present state of the North Sea, the basic knowledge about ecosystem behaviour and a way of creating models as a tool for better understanding of the ecosystem and how man affects the North Sea environment.

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