LAUSR

Abstract This review provides a broad conceptual overview of different approaches to measuring marine ecosystem condition and guidance for framework development (particularly relevant for developing countries). The objectives were to review definitions of ecosystem condition and identify the core approaches and key challenges within the research area. Furthermore, we reviewed the advantages and disadvantages of different indicators that can be used for condition assessments. A semi-systematic approach drawing from more than 300 studies covering a broad range of methods was followed. Generally, definitions of ecosystem condition are based on known changes to the ecosystem, but ultimately, they should align with the definition of an ecosystem and consider ecosystem structure and functioning. As far as possible, the definition should relate an observed state to a reference condition or ecological target. Importantly, definitions of condition should be practical, scalable and always provided when conducting condition assessments. Five indicator categories were identified including pressures, physical parameters, biological structure, functional structure and ecological models. Indicators mostly focus on drivers of change (i.e. ex situ pressures) or the structure of the ecosystem, as these data are easier to obtain. While valuable, these classes of indicators can only provide an indirect appraisal of ecosystem functioning in instances where the relationship between pressure, structure and functioning are known. Indicators that measure structure and functioning of an ecosystem can be used to directly report on condition; however, the data (and monitoring) requirements make them impractical in most instances. Ideally, integrated assessments, that measure indicators from all indicator categories, should be prioritised to provide a comprehensive overview of condition. However, this often requires transdisciplinary collaboration, substantial research capacity (and associated funding) and time to collect the necessary data. An alternative is to select a few strategic indicators that can provide data to address the definition of ecosystem condition and at the same time provide a feasible approach to address the project specific research objectives. Where possible, a data-derived approach should be followed as in situ measurements have potential to improve the understanding of impacts of anthropogenic pressures on ecosystem functioning, and in turn facilitate better management of human activities and enhance ecosystem functioning.