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Conceptual models are derived to indicate the signs and symptoms inherent in nutrient changes to brackish, estuarine and coastal areas of restricted circulation. These give a structured approach to detecting adverse symptoms of hypernutrification and eutrophication at all levels of biological organisation, from effects at cellular levels to the ecosystem approach. The conceptual models illustrate the bottom-up approaches to the detection and control of potential problems and the importance of top-down responses. The bottom-up approaches incorporate mechanisms with regard to inputs, retention of nutrients, biogeochemical cycling and the primary production response. The top-down approaches include the detection of responses in high-profile components of the marine system, such as fisheries, sea mammals and wading birds and seabirds, which are often of paramount socio-economic or conservation importance. The management of the above causes and consequences, and following from the adoption by signatories to proposals given by the Paris Commission (PARCOM), can be accomplished by the derivation of Ecological Quality Objectives (EcoQO) and Ecological Quality Standards (EcoQS). These are given here as a development from the Environmental Quality Objectives and Standards (EQO/EQS) approach. Such EcoQO and EcoQS are regarded as as an aid to monitoring and management of estuaries and coastal waters. That management includes recent proposals within European legislation aimed at monitoring and managing the health and integrity of coasts and estuaries, for example the implementation of the Nitrates, Species & Habitats, and Water Framework Directives. The paper, therefore, discusses both the quality and quantity of data involved in the science required by managers and the way ahead for assessing and managing the fate and effects of nutrients. Using European and U. S. examples, the paper introduces the major challenge of how the concerns highlighted can be addressed by policy action.

In a classical paper, Hutchinson (1961) argued that the large number of species in most plankton communities is remarkable in view of the competitive exclusion principle, which suggests that in homogeneous, well-mixed environments species that compete for the same resources cannot coexist. Few ideas in aquatic ecology have evoked more research than this 'paradox of the plankton'. This review is an effort to put the main solutions to the paradox that have been proposed over the years into perspective. Hutchinson himself already suggested that the explanation could be that plankton communities are not in equilibrium at all due to weather-driven fluctuations. Subsequent research confirmed that such externally imposed variability can allow many species to coexist. Another important point is that in practice the homogeneous well-mixed conditions assumed in the competitive exclusion principle hardly exist. Even the open ocean, for instance, has a spatial complexity resulting from meso-scale vortices and fronts that can facilitate coexistence of species. Perhaps most excitingly, theoretical work on species interactions has given a counter-intuitive new dimension to the understanding of diversity. Various competition and predation models suggest that even in homogeneous and constant environments plankton will never settle to equilibrium. Instead, interactions between multiple species may give rise to oscillations and chaos, with a continuous wax and wane of species within the community. Long-term laboratory experiments support this view. This chaotic behavior implies among other things that plankton dynamics are intrinsically unpredictable in the long run when viewed in detail. Nonetheless, on a higher aggregation level, indicators such as total algal biomass may show quite regular patterns.

The Netherlands are a small, low-lying delta in W. Europe (42 000 km 2;50degrees-54degrees N; 3degrees-8degrees E), mainly consisting of alluvial deposits from the North Sea and from the large rivers Rhine and Meuse. The country was 'created by man'. The conversion of natural aquatic and terrestrial ecosystems into drained agricultural land was a major cultural operation over the past 1000 years. Roughly 55% of the country's surface area is still agricultural land. Some decades ago, The Netherlands' landscape was characterised by an armoured coastline and bridled estuaries, a drastically reduced area of saline and freshwater marshes, fully regulated rivers and streams, and numerous artificial lakes. The aquatic ecosystems beyond the influence of the large rivers, the Pleistocene raised bogs and moor lands, have almost been completely annihilated in the past. Acidification and eutrophication led to the deterioration of the remaining softwater lake vegetation. Last but not least, an artificial drainage system was constructed, leading to an unnatural water table all over the country, high in summer, low in winter. Only very recently, some 25 years ago, the tide has been turned and ecological rehabilitation and restoration of disturbed ecosystems are in full swing now, enhanced by the European Union policy to set aside agricultural land in the Netherlands in favour of the development of 'nature'. The state of the art of aquatic and semi-aquatic ecological restoration projects in the Netherlands is given. Starting from the conceptual basis of restoration ecology, the successes and failures of hundreds of restoration projects are given. Numerous successful projects are mentioned. In general, ecological restoration endeavours are greatly benefiting from progressive experience in the course of the years. Failures mainly occur by insufficient application of physical, chemical or ecological principles. The spontaneous colonisation by plants and animals, following habitat reconstruction, is preferred. But sometimes the re-introduction of keystone species (e.g. eelgrass; salmon; beaver) is necessary in case the potential habitats are isolated or fragmented, or when a seed bank is lacking, thus not allowing viable populations to develop. Re-introduction of traditional management techniques (e. g. mowing without fertilisation; low intensity grazing) is important to rehabilitate the semi-natural and cultural landscapes, so characteristic for the Netherlands. For aquatic ecosystems proper (estuaries, rivers, streams, larger lakes) the rule of thumb is that re-establishment of the abiotic habitat conditions is a pre-requisite for the return of the target species. This implies rehabilitation of former hydrological end geomorphological conditions, and an increase in spatial heterogeneity. The 'bottom-up' technique of lake restoration, viz. reduction in nutrient loadings, and removal of nutrient-rich organic sediment, is the preferred strategy. The 'top-down' approach of curing eutrophicated ecosystems, that is drastic reduction of fish stock, mainly bream, and introduction of carnivorous fish, may be considered as complementary. For semi-aquatic ecosystems (river-fed and rain-fed peat moors, brook valleys, coastal dune slacks) it also counts that the abiotic constraints should be lifted, but here the species-oriented conservation strategy, the enhancement of the recovery of characteristic plant and animal species, is mainly followed. An important pre-requisite for the rehabilitation of the original natural or semi-natural vegetation is the presence of viable seed bank.Restoration of salt-marsh vegetation has to deal with a short-lived persistent seed bank, which means that transport of seeds by water currents is important. Isolated softwater ecosystems may rely on the long-lived seeds of the aquatic macrophytes. The paper ends with some notes on the predictability of the outcome of ecological restoration measures and the societal position of restoration ecology as a science. Scientists hold different views on the predictability of restoration measures. A fact is that the predictability of ecosystem development increases, with increasing knowledge of the underlying environmental processes.