Climate impacts
Diatoms often break the smaller-when-warmer rule
Open access · cc by · source: Europe PMC
Across a 20°C Icelandic stream gradient, diatom size did not shrink with warmth; composition, not plastic shrinkage, drove size differences.
Key findings
Among 31 species, 16 had negative size–temperature slopes and 15 positive; the combined test for shrinking was nonsignificant (P=0.186). Community mean size had no clear temperature relationship. Of 91 pairwise comparisons, 51 were due solely to compositional shifts; mixed cases averaged only 21.8% from intraspecific change. Species turnover averaged 14.8% of compositional differences—relative abundance dominated.
Methodology
Authors sampled diatom assemblages in 14 geothermally heated Icelandic streams spanning about 20°C within 2 km. They measured body size for 31 species present in two or more streams, tested James’ and Bergmann’s rules, and partitioned community size differences into intraspecific change, relative-abundance shifts, and species turnover.
Limitations
Geothermal warming is a spatial proxy, not a multi-decadal climate time series. Other variables (pH, silicate) also related to size. Results are for benthic stream diatoms, not ocean phytoplankton. Mechanisms (metabolism vs competition vs grazing) were not experimentally isolated.
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