The influence of temporal scale selection on pelagic habitat biodiversity indicators

The development of biodiversity indicators is an integral component of forming marine strategies under the European Marine Strategy Framework Directive (MSFD). A key stage in the development of biodiversity indicators is the selection of an appropriate temporal scale over which to assess change in indicator state. This presents a particular challenge for the development of plankton indicators for assessing the state of pelagic habitats, due to the inherent stochasticity of plankton dynamics and the sensitivity of indicators to both climatedriven change and directly manageable pressures. Using two plankton indicator metrics, we demonstrate that the outcome of indicator assessments is inherently influenced by the temporal scale of the time-period over which the indicators are assessed. For example, we show that the inclusion of data from before a regime shift that occurred in the 1980s often alters the assessment of change compared to only including data from after the regime shift. We highlight that ultimately the appropriate temporal scale selected depends on the policy questions being addressed. We also suggest that reporting indicators over multiple temporal scales within an assessment helps provide information relevant to contemporary management, whilst also retaining crucial multidecadal trends such as those caused by climate change.