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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

The modelling community has identified challenges for the integration and assessment of lake models due to the diversity of modelling approaches and lakes. In this study, we develop and assess a one-dimensional lake model and apply it to 32 lakes from a global observatory network. The data set included lakes over broad ranges in latitude, climatic zones, size, residence time, mixing regime and trophic level. Model performance was evaluated using several error assessment metrics, and a sensitivity analysis was conducted for nine parameters that governed the surface heat exchange and mixing efficiency. There was low correlation between input data uncertainty and model performance and predictions of temperature were less sensitive to model parameters than prediction of thermocline depth and Schmidt stability. The study provides guidance to where the general model approach and associated assumptions work, and cases where adjustments to model parameterisations and/or structure are required. (c) 2017 Published by Elsevier Ltd. ; Australian Research Council (ARC)Australian Research Council [DP130104078, LP130100756] ; GLM development and funding support for LCB, BDB, CB and MRH was provided by the Australian Research Council (ARC) (grants DP130104078 & LP130100756). Additional contributions from individuals and organisations as well as sources of data, provided from a variety of organisations are summarised in Appendix D. This study was made possible through the sharing of ideas, data and models across the AEMON and GLEON networks as well as discussions and working groups held during AEMON workshops and GLEON meetings. ; Public domain authored by a U.S. government employee