OSPAR CEMP Guideline. Common indicator PH3 “Changes in Plankton Diversity"

Abstract

Species composition and abundance of plankton assemblages are influenced by environmental conditions and their variability, such as available light, nutrients, prey, currents and climate. As a result, plankton communities fluctuate in space and time. Environmental perturbations such as pollution and/or eutrophication (i.e. excessive nutrients) can create unusual marked changes in community composition because only a small number of species can cope with/benefit rapidly from the new situation. In the Baltic Sea, for example, phytoplankton species composition has been observed to change with different nutrient levels and ratios (HELCOM, 2017 and references therein). Monitoring plankton diversity is important since long-term and drastic changes in biodiversity can alter marine ecosystems in terms of their functioning, such as food webs and the uptake and transfer of carbon in the oceans, and the services they provide. In order to quantify changes in biodiversity, indices based on the number of species and/or their relative abundances in the community can be calculated for water quality assessment. A plethora of indices exist in the scientific literature but their use depends on (1) the objective of the study, (2) their ecological relevance and (3) mathematical properties. Currently, taxonomic diversity indicators for plankton are being revised within the Marine Strategy Framework Directive for French waters (Duflos et al., 2018). In a wider management context (MSFD), only a few community composition indicators are currently applied and this probably reflects the difficulty in setting reference conditions and environmental objectives for these indicators (Garmendia et al., 2013). On the other hand, diversity indices are relatively easy to calculate and their interpretations are intuitive.