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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The photosynthetic rate of natural populations of benthic microalgae was determined in the laboratory over a r nge of irradiances (75 to 1500 μΕ m~2 s-1 ) and at 15 °C and 20 °C. Values for P B max, 5.3 and 11.7 μg O 2 (μg chl-a)" 1 h -1 and the initial slope of the photosynthesis versus irradiance curve 0.035 and 0.031 μg O 2 (μg chl-fl)-1 h" 1 (μΕ m~2 s" 1 )" 1 at 20 °C and 15°C respectively, feil within the ranges quoted by a number of other workers. However, the overall response of the microalgae to light suggested that they were from a 'sun adapted' algal Community. No photoinhibition was observed, even after 2 hours exposure to the highest irradiance. The low value calculated for Quantum Efficiency (0.025) suggest that these organisms were nutrient limited.