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• Diatoms phenology is controlled by the niche-environment interaction resulting from changes in morphological traits. • Cell shape is a key adaptation that confers to each species a unique niche and a place in the sequence of annual succession. • As morphology influences long-term changes in abundance, current biogeochemical models should consider cell shape. Annual phytoplankton succession is a key ecological phenomenon that drives marine species' life cycles and energy flows within marine ecosystems. Identifying processes that control annual succession is critical to anticipate climate-induced environmental perturbations of this phenomenon and the consequences upon ecosystem functioning. Here, we demonstrate that diatoms in the North Sea undergo strong morphological changes throughout the year and that species with similar phenology possess comparable morphological traits (e.g. cell elongation) and ecological niches. The spring and autumn periods appear to be dominated by oblates (flattened cells) whereas the summer period is dominated by prolates (elongated cells). Elongation of the cell shape enhances buoyancy and confers a selective advantage in stratified low-nutrient waters typical of summer without changing a diatom's surface area/volume ratio or its ability to absorb nutrients. Diatom shape thus appears to have evolved as a key adaptation to a specific environment and confers upon a species its specific niche and phenology, and therefore its place in the sequence of annual succession. As a result, shape influences temporal changes in the abundance of diatoms and their putative response to environmental forcing. We suggest that biogeochemical and earth-system models should include diatom cell shape as a parameter in order to improve model predictions and help our understanding of the consequences of climate change on marine ecosystems. [ABSTRACT FROM AUTHOR]

• High overwintering biomass in Norwegian Trench and north-west North Sea shelf. • Inflow accounts for 41% of North Sea biomass and drives interannual variability. • Norwegian Trench and East Shetland Atlantic Inflow are important inflow pathways. • C. finmarchicus in the North Sea is not self-sustained but dependent on the inflow. • Biomass carried by East Shetland Atlantic Inflow decreases over 2000–2016. Calanoid copepods are key taxa in the North Sea as they are the main food source for many fish stocks, such as herring, mackerel and cod. In this study we use an individual-based model for Calanus finmarchicus embedded in the NORWegian ECOlogical Model system (NORWECOM) to investigate important population parameters such as biomass and abundance, distribution and interannual variability of the overwintering population, as well as the inflow of C. finmarchicus into the North Sea from adjacent areas for the 2000–2016 period. The modelled spatial–temporal patterns of C. finmarchicus abundance is comparable with the Continuous Plankton Recorder (CPR) Survey data in the northern North Sea. The simulated annual mean biomass of C. finmarchicus amounts to 0.94 million-tonnes of carbon. High overwintering biomass appears in the Norwegian Trench as well as in the north-west shelf region of the North Sea. A decreasing trend in the overwintering biomass has been detected on the path of the East Shetland Atlantic Inflow (ESAI) over the simulated period. The inflow of C. finmarchicus biomass into the North Sea from the north constitutes on average 41% of the annual mean biomass in the North Sea during the simulated 17 years, and thus determines the interannual variability of the biomass. We conclude that the C. finmarchicus population in the North Sea is not self-sustained and is highly dependent on the inflow of C. finmarchicus from the Faroe-Shetland Channel and south of the Norwegian Sea. C. finmarchicus enter the North Sea via three branches of the North Atlantic current with variable depths depending on seasons and topography. Beside the western flank of the Norwegian Trench (carrying 57% of the inflow biomass), we suggest that the ESAI is also an important agent carrying 37% of the total C. finmarchicus inflow biomass through the shelf area into the north-west of the North Sea. The annual mean outflow biomass is larger than the inflow biomass (0.52 versus 0.39 million-tonnes carbon per year), which indicates that the North Sea serves as a feeding ground and growth region for C. finmarchicus. This study is a first step towards a better understanding and quantification of the exchange of C. finmarchicus between the open seas, coastal waters and the fjords. [ABSTRACT FROM AUTHOR]