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29 30Functional classification based on species traits brought a revolution in community ecology, 31and also boosted phytoplankton and in phytobenton (diatom) research. Several studies 32 stressed the usefulness of phytoplankton functional groups in ecological status assessment, 33and there is also a strong emphasis to use combined traits in ecological assessments of diatom 34 assemblages. The Combined Eco-Morphological Functional Groups (CEMFGs) help to reveal 35 species-environmental correlations, which can be hidden, controlling traits separately. between the correlations of CEMFGs-P or CEMFGs-RB and abiotic factors were 50 hypothesised in the case of those functional groups which contained reassigned taxa with high 51 frequency and/or abundance. Our results confirmed this hypothesis. Furthermore, the present 52 study also highlighted the relevance of well-defined trait classification. Abundant and/or 53 frequent taxa, which are able to change their life forms, can modify significantly the 54 relationship between the functional group containing them and environmental factors. In the 55 future, both field and laboratory studies should focus on revealing the circumstances, which 56 cause the mentioned changes in traits of diatoms. 57 58

Summary Modelling the relationship between biomass and diversity in phytoplankton assemblages provides new insights into the mechanisms responsible for the coexistence of species, even in terrestrial ecosystems. We tested the biomass–diversity relationship in lake phytoplankton along a wide biomass gradient using functional species groups. We hypothesized that changes in the taxonomic diversity of the phytoplankton along a biomass gradient are associated with altered functional diversity. For the analyses, in total 768 samples were collected from 30 oxbows, reservoirs and lakes in the Hungarian Lowland Region and analysed between 1992 and 2002. We found that the diversity and also the number of functional species groups showed a humped‐back curve similar to the species richness. The changes in functional group composition act as a good proxy for phytoplankton species responses. We found that the peak of the number of strategy groups and their Shannon diversity was at a much lower biomass than that of species richness. We revealed the fine‐scale effects of increasing the dominance of respective species or species groups with increasing biomass. This increase was well reflected by the changes in the functional characteristics: first, the species evenness; then, the Shannon diversity; and finally, the species richness started to decrease with increasing biomass. Cyanoprokaryota were positively correlated with increasing biomass and negatively with the increase in species richness; thus, the high increase both in their abundance and biomass can be responsible for the abruptly decreasing part of the humped‐back curve. We detected a humped‐back curve between biomass and diversity, where the peak compared to terrestrial plant communities tended to be towards high biomass scores, that is, >60% instead of the 20–60% of the biomass range typical for terrestrial plant communities. Marked differences in the structural and dynamic features of phytoplankton assemblages and terrestrial plant communities are likely responsible for this difference.

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