The Netherlands, like the other industrialized countries in the northern hemisphere, is faced with increas ing environmental pro blems. Being a country rich in water - at least in terms of surface area - and low-lying, problems related to water are as old as the nation itself. The problems in regard to water quality are becoming more acute and complicated due to the progressive rise in air and ground pollution. Glaring examples are the acid rains and contamination of ground-water resources by toxic, and often illegal, waste dumps. Being a delta country, the Netherlands is also very vulnerable to pollution from the·water sources outside the country's national boundaries, i.e. the rivers Rhine, Meuse and ScheIdt. Not only the national but also the international authorities are active in developing measures to both prevent and combat water pollution, and to restore as far as possible the natural balance or a 'reasonable' water quality. The pollution abatement programmes are based on ecological theories; accepting waterbodies as aquatic ecosystems, the authorities concerned try to influence and lead the progresses in the systems to reach or maintain the desired ecosystem structure. Suter II (1981) mentioned a number of reasons why the attempts to use ecosystem theories to assess the impacts on the environment have been disappointing to date. It is clear that one of the reasons for the failure is lack of proper knowledge.

Inhalt
The twenty-fifth anniversary of the Limnological Institute, The Netherlands (19571982).- One: Lake Vechten.- Lake Vechten: aspects of its morphometry, climate, hydrology and physico-chemical characteristics.- The phytoplankton species composition and the seasonal periodicity in Lake Vechten from 1956 to 1979.- The primary production of phytoplankton in Lake Vechten.- Contribution of photosynthetic sulphur bacteria to primary production in Lake Vechten.- The aquatic macrophytes of Lake Vechten. Species composition, spatial distribution and production.- Structural and functional characteristics of epiphyton and epipelon in relation to their distribution in Lake Vechten.- Macro-invertebrate communities associated with the macrophytes of Lake Vechten: structural and functional relationships.- The zooplankton: its community structure, food and feeding, and role in the ecosystem of Lake Vechten.- Sedimentation and breakdown kinetics of organic matter in the anaerobic zone of Lake Vechten.- Aspects of aerobic mineralization during spring in Lake Vechten with special reference to the 14C-labelling technique.- Nitrogen cycle of Lake Vechten: concentration patterns and internal mass-balance.- Two: Tjeukemeer.- Hydrology of Tjeukemeer.- Physico-chemical environment in Tjeukemeer with special reference to speciation of algal nutrients.- The algal periodicity in Tjeukemeer during 19681978.- Population dynamics and production of Daphnia hyalina Leydig and Daphnia cucullata Sars in Tjeukemeer.- Population dynamics and production of Acanthocyclops robustus (Sars) and Mesocyclops leuckarti (Claus) in Tjeukemeer.- INSTAR, a discrete event model for simulating zooplankton population dynamics.- Distribution and production of the larval chironomid populations in Tjeukemeer.- A note onthe significance of larger bivalve molluscs (Anodonta spp. and Dreissena sp.) in the food of the eel (Anguilla anguilla) in Tjeukemeer.- Growth, condition and gonad development of bream (Abramis brama L.) in relation to its feeding conditions in Tjeukemeer.- The relations between 0+ fish density, zooplankton size and the vulnerability of pikeperch, Stizostedion lucioperca, to angling in the Frisian lakes.- Three: Miscellaneous.- Effects of thermal effluents from the Bergum Power Station on the zooplankton in the Bergumermeer.- Effects of entrapment and cooling water discharge by the Bergum Power Station on 0+ fish in the Bergumermeer.- Temporal and spatial variations of dissolved oxygen concentrations in some Dutch polder ditches.