The relationship of diversity and biomass in phytoplankton communities weakens when accounting for species proportions

The relationship between productivity (or biomass) and species diversity in ecological communities remains a hotly debated topic. While much is already known about vascular plants, little is known in other types of organisms. We used a broad and standardized database of phytoplankton samples from the Czech Republic, containing 413 samples of various types of stagnant waters to evaluate this relationship. Biomass was characterized by the total biovolume/ml, the total number of individuals/ml and cells/ml all giving similar results. All these indicators spanned over five orders of magnitude while the number of species ranged between 1 and 57. Diversity was characterized by indices of Hill's unified notation series progressively accounting for species proportion effects. The number of species showed an asymmetric unimodal relationship with biomass. The relationship weakened when considering diversity indices including species proportions. At very low productivity values (characterized by low biomass), diversity was probably restricted by the ability of algae and cyanobacteria to survive a lack of nutrients, in high productivities, by the competition for light. Medium productivities, where maximum diversity was found, exhibited large variability of diversity values (including very low ones), suggesting that low diversity of phytoplankton samples can be caused by multitude of factors.