Application of a new multi-metric phytoplankton index to the assessment of ecological status in marine and transitional waters

► Plankton size structure as a tool for the study of aquatic ecosystems. ► Test of the ISS-Phyto approach to ecological status assessment in transitional waters. ► Integration of size structure metrics with biomass, taxonomic richness and size class sensitivity. ► ISS-Phyto consistently discriminate between anthropogenic and natural disturbance conditions. ► ISS-Phyto explains the relationships between anthropogenic impact and ecosystem response. Patterns of phytoplankton size spectra variation with gradients of environmental stress have been observed in freshwater, transitional waters and marine ecosystems, driving the development of size spectra based assessment tools. In this study, we have tested on transitional and coastal waters a new Index of Size spectra Sensitivity of Phytoplankton (ISS-Phyto), which integrates simple size spectra metrics, size class sensitivity to anthropogenic disturbance, phytoplankton biomass (chlorophyll a) and taxonomic richness thresholds. ISS-Phyto has been tested both among and within ecosystems along pressure gradients based on expert view assessment; the adequacy of symmetric and both left and right asymmetric models of phytoplankton size class sensitivity have been compared. The results showed that ISS-Phyto consistently discriminated between anthropogenic and natural disturbance conditions. Left asymmetric models of size spectra sensitivity, assuming greater disturbance tolerance with respect to eutrophication and organic enrichment of increasingly large size classes, showed the best fit comparing all ecosystems; in three of the four considered ecosystems (Varna, Helsinki, Mompás-Pasaia), they seemed to discriminate best between different levels of disturbance also within ecosystems. Moreover, they demonstrated significant and inverse patterns of variation along the overall pressure gradient as well as along the inorganic phosphorus (DIP), chlorophyll a and trophic index (TRIX) gradients. Therefore, ISS-Phyto, originally developed for transitional waters, seems to be an adequate assessment tool of ecological status also in coastal marine waters; moreover, it seems adequate to describe within ecosystem disturbance gradients. Hence, ISS-Phyto helps to understand the relationships between anthropogenic impact and ecosystem response from the individual point of view, with reference to the simple parameter of body size.