Differential cell size optimization strategies produce distinct diatom richness–body size relationships in stream benthos and plankton

Summary1 The relationship between species richness and body size is one of the most thoroughly studied subjects in animal ecology; however, this relationship and its underlying mechanisms are largely unknown in photosynthetic organisms, especially protists. 2 In this continental study, I first examined the number of diatom species across the cell size spectrum in benthic and planktonic stream habitats. The relationship was right‐skewed unimodal and was significantly different between the benthos and the plankton; larger sizes were more speciose in the benthos, and smaller sizes in the plankton. The species richness peaks were explained with allometric trade‐offs between maximum nutrient uptake rate and dispersal in the benthos but maximum nutrient uptake rate and sinking resistance in the phytoplankton. 3 I also explored the cell size similarity among species and across environments. Small diatoms were significantly more similar in size than large diatoms, and benthic diatoms were significantly more similar than planktonic diatoms. 4 This is the first continental study on the richness–body size relationship in algae, which suggests that the environmental differences between benthic and planktonic habitats generated allometric trade‐offs that have driven the cell size optimization towards larger species in the benthos but smaller in the plankton. The patterns of cell size similarity revealed a higher niche overlap in the benthos than in the phytoplankton and among small species than among large species. These findings indicate that interspecific competition in stream diatoms, which is a function of niche differentiation, is habitat‐specific and inversely related to cell size.