How many indicators make a good assessment?

Marine ecosystem assessments use indicators and various integration methods to conclude on ecosystem state. The integration methods vary in their structures, which will influence the number of indicators required. Thus, the choice of an assessment method will have cost consequences, but if the indicator pool is fixed, it is important to select a suitable method to ensure scientific robustness of the results. While science has been hesitant to provide methods for simplifying inherently dynamic ecosystems, the integration of ecosystem indicators is a necessity for answering the demands from policy, societies and wider audience. Some integration methods emphasize the precautionary approach or finding average conditions, some want to include detailed indicator data and some paint with broader brush of ecosystem conditions. In this study, we tested how data-hungry three common integration methods are and how indicator availability impact the robustness of the integrated result. We used real indicators from three European sea regions in five test sites. The results showed that uncertainty in an assessment result increases in more structured integration methods if indicator availability is limited. This should raise warning flags for managers with limited monitoring resources to prioritize indicators or change the integration method.