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Marine and coastal ecosystems are subject to diverse and increasingly intensive anthropogenic activities, making understanding cumulative effects critically important. However, accurately accounting for the cumulative effects of human impacts can be difficult, with the possibility of multiple stressors interacting and having greater impacts than expected, compounding direct and indirect effects on individuals, populations, communities and ecosystems. Assessment of multiple stressors therefore requires extensive scientific research that directly tests how single or multiple ecological components are affected by stressors, both singly and when combined, and as a consequence, cumulative effects assessments are now increasingly included in environmental assessments. Currently, there is a need to assess these at larger spatial scales, with additional research also urgently needed on the responses of ecological components, processes and functions to single and cumulative stressors. As cumulative environmental impacts could be better addressed by regional stressor effects assessments that combine methods for predicting multiple pressures on ecosystem recovery alongside degradation, this study used several separate approaches that can be used in parallel to give support for local management measures. I tested four completely different methods - a range of multi-metric indices, a food web model (Ecopath), a predictive model (Ecosim) and a Bayesian Belief Network model. Each approach was tested and compared in two shallow water estuarine systems, in Scotland and England, initially concerning the impact of nutrient enrichment and subsequent recovery and was followed by an investigation of how the addition of multiple stressors (nutrient levels, temperature and river-flow rates) would impact the future state of each system. The response to stressors was highly context dependent, varying between and within geographic locations. Overall, each of the four different approaches complemented each other and gave strong support for the need to make big reductions in the pressures and to consider trade-offs between impacting pressures. The models and tools also indicate that in order to reach an improved overall environmental state of each ecosystem, a focus on nutrient reductions are likely to be the most effective of the controls on stressors explored and that cumulative effects of the management of nutrient inputs and increased water temperatures and river-flow are likely to exist.

The ICES International Bottom Trawl Survey (IBTS) quarter 3 (Q3) data, Dutch Beam Trawl Survey (BTS) Q3 data, and ICES stock assessment data were analysed to model the catchability of each 1-cm size class of demersal fish caught in the Q3 IBTS. Estimates of catchability were generally low, suggesting that raw trawl survey density data seriously underestimated actual densities.

As oceans warm due to anthropogenic climate change, the global ocean oxygen inventory decreases in a process known as ocean deoxygenation. Since oxygen is fundamental for aerobic life, ocean deoxygenation can impact marine organisms across multiple levels of biological organization. This research used the steep gradients in oxygen characteristic of upwelling margins with oxygen minimum zones (OMZs), as a natural experiment to study how hypoxia affects the composition, diversity, trophic structure and dynamics of demersal fish communities. Despite the severely hypoxic conditions ([O2] < 22 μmol kg-1), demersal fish are shown to be present in OMZ systems around the world; several species exhibit remarkable hypoxia tolerance in the Gulf of California ([O2] < 2 μmol kg-1). Demersal fish diversity is sensitive to hypoxia, and diversity shows a non-linear response to low oxygen conditions, decreasing rapidly below a region-specific oxygen threshold. Density patterns are less predictable in relation to oxygen levels. Paired video and environmental samples, taken using an autonomous nanolander, show that seafloor communities on the upper margin (100-400 m) in the Southern California Bight (SCB) experience substantial natural oxygen variability at daily and weekly timescales, but communities do not respond to low-oxygen events at these timescales. Feeding relationships are also altered by hypoxia. At the upper OMZ boundary in the SCB a transition occurs from primarily pelagic-feeding to benthic-feeding fishes, which contrasts with the pattern on non-upwelling margins. The expansion of low-oxygen conditions in the E. Pacific may lead to a shift in community composition as intolerant species are excluded and hypoxia-tolerant species become competitive, an overall decrease in diversity, a reduction in bentho-pelagic coupling, and a lengthening of the food chain as demersal fish shift to utilizing benthic resources. Considering the natural variation of oxygen conditions across different timescales is important as existing modes of variability determine exposure histories of animals and likely influence sensitivity of communities to deoxygenation. Despite major ecological consequences of ocean deoxygenation, an analysis of Nationally Determined Contributions submitted under the Paris Agreement indicate that oxygen loss remains an under acknowledged topic by policy-makers in the international climate community.