Although the terms 'health' and 'healthy' are often applied to marine ecosystems and communicate information about holistic condition (e.g. as required by the Ecosystem Approach), their meaning is unclear. Ecosystems have been understood in various ways, from non-interacting populations of species to complex integrated systems. Health has been seen as a metaphor, an indicator that aggregates over system components, or a non-localized emergent system property. After a review, we define good ecosystem health as: 'the condition of a system that is self-maintaining, vigorous, resilient to externally imposed pressures, and able to sustain services to humans. It contains healthy organisms and populations, and adequate functional diversity and functional response diversity. All expected trophic levels are present and well interconnected, and there is good spatial connectivity amongst subsystems.' We equate this condition with good ecological or environmental status, e.g. as referred to by recent EU Directives. Resilience is central to health, but difficult to measure directly. Ecosystems under anthropogenic pressure are at risk of losing resilience, and thus of suffering regime shifts and loss of services. For monitoring whole ecosystems, we propose an approach based on 'trajectories in ecosystem state space', illustrated with time-series from the northwestern North Sea. Change is visualized as Euclidian distance from an arbitrary reference state. Variability about a trend in distance is used as a proxy for inverse resilience. We identify the need for institutional support for long time-series to underpin this approach, and for research to establish state space co-ordinates for systems in good health.

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Marine legislation is becoming more complex and marine ecosystem-based management is specified in national and regional legislative frameworks. Shelf-seas community and ecosystem models (hereafter termed ecosystem models) are central to the delivery of ecosystem-based management, but there is limited uptake and use of model products by decision makers in Europe and the UK in comparison with other countries. In this study, the challenges to the uptake and use of ecosystem models in support of marine environmental management are assessed using the UK capability as an example. The UK has a broad capability in marine ecosystem modelling, with at least 14 different models that support management, but few examples exist of ecosystem modelling that underpin policy or management decisions. To improve understanding of policy and management issues that can be addressed using ecosystem models, a workshop was convened that brought together advisors, assessors, biologists, social scientists, economists, modellers, statisticians, policy makers, and funders. Some policy requirements were identified that can be addressed without further model development including: attribution of environmental change to underlying drivers, integration of models and observations to develop more efficient monitoring programmes, assessment of indicator performance for different management goals, and the costs and benefit of legislation. Multi-model ensembles are being developed in cases where many models exist, but model structures are very diverse making a standardised approach of combining outputs a significant challenge, and there is a need for new methodologies for describing, analysing, and visualising uncertainties. A stronger link to social and economic systems is needed to increase the range of policy-related questions that can be addressed. It is also important to improve communication between policy and modelling communities so that there is a shared understanding of the strengths and limitations of ecosystem models

Nitrogen fluxes through the lower estuary of the river Great Ouse, England: the role of the bottom sediments 'Department of Biological Sciences. University of Essex, Colchester C 0 4 3SQ, United Kingdom 'Centre for Environment Fisheries and Aquaculture Sciences, Pakefield Road, Lowestoft, Suffolk NR33 OHT, United Kingdom ABSTRACT: Sediment-water nutrient exchange, oxygen uptake, denitrification (acetylene blockage) and pore water nutnent concentration profiles were measured at intertidal sediment (predominantly silWclays) sites in the lower estuary of the nver Great Ouse, England. Sedments were consistent sinks for No3-(310 pm01 m-2 h-', mean sites 4 to 9) and O2 (2800 pm01 m-2 h-', mean sites 4 to 7 ) , sources of NH,' (270 p o l m-2 h-', mean sites 4 to 9) but neutral with respect to NOz-and urea Oxygen uptake was significantly correlated (p c 0.05) 14th seasonal temperature. Nitrate exchange became saturated at No3-concentrations > 400 pM, at a rate of about 400 pm01 NO3-m-' h-'. Denitrification accounted annually for 46% of the NO,-exchanged into the sediments and approached asymptotic rates during spnng and summer at NO3-concentrations > 400 yM. Of the total N flux through the sediments, NH,' efflux accounted for 51 %, whist 49% was converted to gases, compared to >90% in the upper estuary Freshwater flushlng tunes were calculated for a defined area of the estuary and ranged from 20.5 d in June to 3.25 d in November. Attenuation of the riverine total oxidlsed nitrogen (NO,-+ NO2-) load to the estuary ranged from 1 % in the middle of winter to 56'::) at the height of summer and annually the sehments denitrified 38.4 Mm01 N. Recycling of nitrogen in the sedirnents, via N O 3 arnmonification (calculated by difference) and organic ammonification, generated an annual NH,' efflux of 128 Mm01 N, equivalent to 22% of the primary production N requirement in the Wash.

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