Ocean observing systems like SOOS are expanding worldwide and need assistance to identify the biological and ecological variables that they should measure. Essential variables of the physical marine system are relatively well developed, and can be readily interpreted and used in the development and application of models of the physical systems. In comparison, Ecosystem Essential Ocean Variables (eEOVs) that indicate not only the status and change in marine habitats and the biotic components of the ecosystem, but also the function and dynamics of the ecosystem, need to be developed in order to establish research and monitoring programs to parallel monitoring of physical parameters (e.g., temperature, salinity). The readiness of eEOV observations will need to be assessed in order to design research efforts aimed at creating and using new eEOVs. Tools are now available to evaluate the efficacy of indicators and different field sampling designs (spatial and temporal sampling programs) and it is timely to begin to apply such methods to evaluate case studies of different marine ecosystems.
eEOVs need to be measured relatively easily and cost-effectively in order to achieve coverage in space and time to signal how ecosystems are changing. They need to encompass key biological attributes of marine ecosystems that underpin structure, function, and dynamics. Their identification requires the involvement of experts ranging from observations of key types of marine organisms to those with a theoretical understanding of the dynamics of ecosystems and the key drivers of their structure and function.