OSPAR CEMP Guideline. Common indicator PH2 “Changes in Phytoplankton Biomass and Zooplankton Abundance”

Abstract

Plankton biomass and/or abundance in the ocean are hydro-climatic variables and as such have been demonstrated to reflect environmental changes, as illustrated by already numerous phytoplankton and zooplankton published studies. Being at the base of the food-web and representing a food of importance for numerous species of higher trophic levels, such as fish of commercial interest, the fluctuation of plankton biomass and/or abundance can have significant impacts on the whole trophic food web but also on carbon cycles and nutrient recycling. The intrinsic characteristics of these organisms at the base of the food web, such as small size, short life cycles and distribution over the whole globe, render them particularly interesting in the frame of monitoring programmes and they have a high potential to reflect environmental changes at short and long-term scales in the marine systems. In practice, the use of total biomass and/or abundance is often favoured over indicators using species, since indices of species-specific abundance are frequently subject to large inter-annual variation, often due to natural physical dynamics rather than anthropogenic stressors (de Jonge, 2007). Combining both phytoplankton biomass and zooplankton abundance can provide an indication of changes in the energy transfer from primary to secondary producers. The indicator is still under development. Further investigations are needed to precise the assessment method, and to make the indicator flexible enough to include data from innovative approaches and techniques (see further). Since different indices provide complementary information on the community structure, we propose a combination of diversity indices to assess GES for plankton communities. Moreover, each PH indicator considers the community at different resolutions, PH1 at the life-form level of the community, PH2 the total biomass/abundance of the community and PH3 at the species level. Hence, by combining the information from these three indicators, a more holistic assessment of plankton dynamics can be obtained than from each indicator individually.