Body size dependency of natural variability in marine ecosystems: insights on defining monitoring plans for MSFD

Assessing ‘natural’ variability of structural and functional components of ecosystems is a main target of MSFD, responding to the need of disentangling biological responses to anthropogenic stresses from those on which human impact is minimal. The scale and the extent of ecosystem components variability are main determinants of ecological patterns and process. Moreover, variability within ecosystems depends on both internal ecosystem dynamics and larger scale drivers and processes. Here we focus specifically on natural variability of biotic ecosystem components and ecosystem processes, which are either descriptors or parameters of relevance in the MSFD monitoring scheme. Metabolic theories have highlighted how the individual body size can be considered as a main, intrinsic driver of variation in populations and communities functional properties. Indeed, individual energetic and behaviour show body size dependent variation according to an x/4 rule. Cascading deterministic effects of individual level size-dependencies have been demonstrated on both population and community levels as well as on functional properties. In the presentation: i. We develop a conceptual analysis of the implication of body size dependencies of individual energetic and behaviour on scale and extent of natural variability in the individual, population and community parameters considered in the MSFD; ii. We present an example of phytoplankton parameter assessments with changing temporal scales of assessment in order to account for the natural variability of cell densities in the phytoplankton guilds; and iii. We infer from implications of biological parameter size dependencies on MSFD monitoring plan accounting for natural variability and derive operational proposals for adaptive monitoring plans.