Power law relationships among hierarchical taxonomic categories in algae reveal a new paradox of the plankton

Aim In this continental-scale study, the biodiversity of benthic and planktonic algal communities was explored. A recent analysis of extinct and extant tree communities by Enquist et al. (2002) showed that richness of higher taxa was a power function of species richness, invariant across temporal and spatial scales. Here we examined whether the relationships between algal richness at hierarchical taxonomic levels conform to power laws as seen for trees, and if these relationships differ between benthic and planktonic habitats. Location Streams from more than 50 major watersheds in the United States. Method A total of 3698 samples were collected from 1277 locations by the National Water-Quality Assessment Program. Three types of stream habitat were sampled: richest targeted habitats, depositional targeted habitats, and phytoplankton. The relationships between taxonomic richness at the species level vs. all higher categories from genus to phylum across the three habitats were examined by ordinary least squares (OLS) regressions after ln-transformation of all variables. The slopes, b, of these regressions represent the exponents of the power functions that scaled the richness of higher taxonomic levels (T) to species richness (S) in the form: T proportional to S-b. Results Algal richness at hierarchical taxonomic categories (genus to phylum) is a power function of species richness. The scaling exponent of this function, which captures the diversification of higher taxa, i.e. the rate of increase of their richness with the increase of species richness, is significantly different across environments. Main conclusions The differential algal diversification in the three studied habitats emphasizes the fundamental role of the environment in structuring the communities of simple organisms such as algae. The finding that the diversification of higher taxa is greater in the seemingly homogeneous planktonic environment, when compared to benthic habitats, encompassing an array of ecological niches, poses a new paradox of the plankton.