Reduced body size is among the universal ecological responses to global warming. Our knowledge on how altered body size affects ecosystem functioning in ectothermic aquatic organisms is still limited. We analysed trends in the cell size structure of phytoplankton in the middle Danube River over a 34-year period at multiple levels: (1) average cell size of assemblages (ACS), (2) within the centric diatom community and (3) in the dominant centric diatom taxon: Stephanodiscus. We asked whether global warming and human impacts affected the average cell size of phytoplankton. Also, whether the altered size structure affected how chlorophyll-a, as an ecosystem functioning measure, relates to the ACS of phytoplankton. The cell size of phytoplankton decreased significantly at all organisation levels, and the assemblages became more dispersed in cell size over time. Environmental variables related to global warming and human impacts affected the ACS of phytoplankton significantly. The relationship between chlorophyll-a and the ACS of phytoplankton shifted from negative linear to broad and then narrow hump shape over time. Longer water residence time, warming and decline in nutrients and suspended solids decrease the ACS of phytoplankton in the middle Danube and expectedly in other large rivers. Our results suggest that cell size decrease in phytoplankton, especially of centric diatoms, constrains planktic algal biomass production in large rivers, independently of algal density. Such cell size decrease may also affect higher trophic levels and enhance the more frequent occurrence of “clear-water” plankton in large, human-impacted rivers under global change.

(1) Urbanization significantly influences the ecosystems of rivers in various ways, including the so-called loading effect of wastewater production. Benthic diatoms are used in ecological status assessments of waters. Beside species composition, traits can be used as indicators. We aimed to evaluate how the loading of the large city of Budapest manifests in the physico-chemical variables of the River Danube and what species composition and trait response this loading results in for the benthic diatom communities. (2) Weekly samplings were performed at points upstream and downstream of Budapest on both riverbanks. Samples were compared, based on general physical-chemical variables and the concentration of thirty-four elements, as well as species composition and seven traits of species of diatom communities. Ecological status was assessed using the Specific Pollution Sensitivity Index (IPS). (3) Only a few measured environmental variables showed differences between the sampling points, suggesting that the nutrient loading has significantly decreased due to the installation of several efficiently working wastewater treatment plants since the introduction of the European Union Water Framework Directive. In contrast, the species composition and traits of species showed the effect of land use. Benthic diatoms indicate the environmental changes caused by land use in the longer-term, while chemical measurements reflect instantaneous status.

Stephanodiscus triporus was described in 1978, based on a study from the phytoplankton of Volgograd Reservoir that used transmission electron microscopy. This species is small, diameter 3.7–10.6 µm, with 14–30 striae and 30–60 areolae in 10 µm, and differs from other Stephanodiscus species by the presence of three satellite pores in the single central fultoportula. Later a new species similar in morphology to S. triporus, was described from Iowa, USA, namely S. vestibulis. A large population of S. vestibulis was found in Lake Balaton and the species also occurs in different Hungarian and French waters. Detailed comparison of S. triporus and S. vestibulis, based on our results and literature, showed they are very similar. Therefore we reinvestigated the type material of both and compared them with the Hungarian and French specimens. Conventional and geometric morphometric analyses were carried out, also including comparisons with the morphologically closest taxon, S. minutulus. There is a continuum of variation between S. triporus and S. vestibulis (in diameter, the number and morphology of the striae, the position of the valve face fultoportula with three satellite pores, the presence of a vestibulum having more or less the same shape). Hence we suggest that they are conspecific, with S. vestibulis a later synonym of S. triporus.