Several legislations worldwide require assessing the health status of marine ecosystems. In Europe, the
Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD) demand the
evaluation of the status with the overall objective of achieving at least ‘‘Good Status’’, by 2015 and
2020, respectively. For this purpose, Member States are required to evaluate different biological, physico-
chemical and hydro-morphological quality elements (WFD), or qualitative descriptors (MSFD). The
assessments of both, the phytoplankton element and the eutrophication descriptor should include several
attributes, namely phytoplankton biomass, composition, abundance and blooms. However, few composition-
based indicators have been proposed. Principally, phytoplankton still relies on chlorophyll-a
concentration measurements, mainly due to the high cost of collecting information on this element
and the difficulties establishing significant pressure–impact relationships. This investigation describes
the difficulties in the development of a phytoplankton community composition indicator, the state of
the art and the main challenges for the future.