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Cell size is a master trait in the functional ecology of phytoplankton correlating with numerous morphological, physiological, and life‐cycle characteristics of species that constrain their nutrient use, growth, and edibility. In contrast to well‐known spatial patterns in cell size at macroecological scales or temporal changes in experimental contexts, few data sets allow testing temporal changes in cell sizes within ecosystems. To analyze the temporal changes of intraspecific and community‐wide cell size, we use the phytoplankton data derived from the Lower Saxony Wadden Sea monitoring program, which comprises sample‐ and species‐specific measurements of cell volume from 1710 samples collected over 14 yr. We find significant reductions in both the cell volume of most species and the weighted mean cell size of communities. Mainly diatoms showed this decline, whereas the size of dinoflagellates seemed to be less responsive. The magnitude of the trend indicates that cell volumes are about 30% smaller now than a decade ago. This interannual trend is overlayed by seasonal cycles with smaller cells typically observed in summer. In the subset of samples including environmental conditions, small community cell size was strongly related to high temperatures and low total phosphorus concentration. We conclude that cell size captures ongoing changes in phytoplankton communities beyond the changes in species composition. In addition, based on the changes in species biovolumes revealed by our analysis, we warn that using standard cell size values in phytoplankton assessment will not only miss temporal changes in size, but also lead to systematic errors in biomass estimates over time. ; Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347 ; Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659 ; Interreg V A program Deutschland‐Nederland of the European Union ; Niedersächsisches Ministerium für Wissenschaft und Kultur http://dx.doi.org/10.13039/501100010570 ; https://doi.org/10.5281/zenodo.5799263

Ziel des Vorhabens war es, dass die teilnehmenden Projektpartner das Umweltbundesamt (UBA) bei den Aufgaben zur Umsetzung der Meeresstrategie-Rahmenrichtlinie (MSRL) und der dazu erforderlichen Kooperationen auf nationaler, regionaler und EU-Ebene fachlich unterstützen und begleiten. Dazu wurden nach Bedarf und auf Anforderung verschiedene Themenkomplexe zur Weiterentwicklung von Indikatoren und Bewertungssystemen, zur Entwicklung und Umsetzung von Maßnahmen sowie zur Information der Öffentlichkeit bearbeitet.

In 2008, the guideline for establishing a framework for Community action in the field of marine environment (Marine Strategy Framework Directive - MSFD, 2008/56/EG) was published. The overall objective of the guideline is to achieve and/or maintain a good status of the marine environment before the year 2020. The good environmental status has to be defined in accordance with qualitative Descriptors as listed in Annex I and specified through respective Criteria and Indicators given by the European commission. While recent projects have focussed mainly on the so-called 'state' Descriptors of the MSFD, the focus of the current project has been on 'pressure' and 'impact' Descriptors. For these, assessment systems were not yet available. Within the project, we have been identifed existing deficits and presented possible solutions, for example by developing respective assessment systems. For Descriptor 2 (non-indigenous, invasive species) an assessment system has been developed which considers the amount of foreign species within an ecosystem, the impact on native communities, and the trend indicator. The approach proposed for Descriptor 6 (seafloor integrity) is based on modelling the impact by combining pressure-specific sensitivity information for benthic habitats with data on the spatial and temporal extent of physical loss and damage. Due to large gaps in knowledge on pressures and impacts on hydrographical conditions and ecosystem components, it is currently not possible to present a detailed assessment concept for Descriptor 7 (hydrographical conditions). Instead, a first draft of an assessment concept is briefly outlined as a basic framework which should be open to changes and adaptable for future developments in research. One of the most important stressors in the marine environment is the chemical pollution which is covered by Descriptor 8 (contaminants). Initially, relevance of the contaminants, environmental quality targets, the biological effects in common, and the effects on marine mammals in particular have been examined. For Descriptor 10 (marine litter) an assessment system for litter on beaches has been developed. Concerning litter at the water surface, within the water column, and at the seafloor (particularly fisheries nets) existing data have been analysed to suggest environmental quality targets and effective monitoring strategies. In addition, the impact on marine birds has been examined when swallowing up litter. Moreover, possible approaches for an overall assessment concept for 'the good environmental status' according to the MSFD have been developed, with special regard to the results of recent MSFD projects.