are considered quantitatively and qualitatively and within awiderangeofspatialand time scales. The hydrological studies in the interdisciplinary The present hydrological situation in the area workgroup WQL (Water Quality research Loos- is the result of a complex sequence of events of drecht lakes) on restoration of the Loosdrecht natural and man-made origin. The present distri- Lakes have been carried from 1982-1988 (Kal bution of nutrients in the water systems of the et al. , 1984; Engelen & Kal, 1985; Kal, 1986). The area can be understood only when the hydrolog- aim of the study was to provide an integrated ical history is traced back. One has to start at the description of the abiotic part of the water sys- beginning of the Holocene period and to follow tems and its contribution to thefunctioningofthe up closely the increasing interference of man. aquatic ecosystems in the Loosdrecht Lakes (En- Overthelastcenturies and decades man has been gelen, 1986). changing the flow systems around the lakes by Theresults hadto servethefollowingpurposes: digging peat, draining lakes, extracting ground- water, maintaining fixed surface water levels in General framework. The integrated hydrological polders and transferring water from adjacent riv- studies had to provide a general frame of refer- ers and canals to supply the lakes to maintain ence for the other project participants represent- their water levels.
