Diatoms often break the smaller-when-warmer rule
Across a 20°C Icelandic stream gradient, diatom size did not shrink with warmth; composition, not plastic shrinkage, drove size differences.
Source
Diatoms can be an important exception to temperature-size rules at species and community levels of organization
Inspect source passages
Diatoms often break the smaller-when-warmer rule
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
31 diatom species present in two or more streams were used for the intraspecific analysis
There were 31 diatom species present in two or more streams, and these were used in the intraspecific analysis.
Of 91 pairwise comparisons, 51 size differences were solely due to compositional shifts
Of 91 pairwise comparisons between average community body size in the fourteen streams (), 51 showed that the observed difference in average community body size was solely due to compositional shifts
What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Temperature–size rule / James’ rule
- The claim that individuals of a species are smaller in warmer waters.
- Bergmann’s rule (community)
- Here, the claim that whole assemblages shift toward smaller species in warmer sites.
- Compositional shift
- Community mean size changes because species identities or abundances change, not because each species shrinks.
- Species turnover
- The extreme compositional route: local loss or gain of species along the gradient.
- Geothermal stream gradient
- Fourteen Icelandic streams ~2 km apart spanning ~20°C, a natural warming experiment with limited biogeographic confounding.
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The Icelandic streams spanned about:
Common questions
Did most diatom species get smaller in warmer streams?
No. Slopes were split roughly 16 negative vs 15 positive, with no average shrinking.
Did the whole community get smaller?
No significant linear or quadratic relationship with temperature.
What actually caused size differences among streams?
Mostly which species were common, not plastic shrinkage of each species.
Why do diatoms matter for climate?
They are ~23% of global primary production and size structures food webs and carbon cycling.
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