Phytoplankton–zooplankton dynamics vary with nutrients: a microcosm study with the cyanobacterium Coleofasciculus chthonoplastes and cladoceran Moina micrura

Abstract

Anti-grazer responses of phytoplankton have been widely documented to vary with external abiotic factors (e.g. nutrient status), suggesting an environment-dependence of phytoplankton–zooplankton dynamics. Here, we examined the trophic interaction between the cyanobacteria Coleofasciculus chthonoplastes and the cladoceran Moina micrura under three nutrient levels from oligotrophic to hypereutrophic conditions (0.3, 3.0 and 100.0 μM PO43−; molar ratio NO3−:PO43−=16:1⁠) to determine the effect of nutrients on the stability of a phytoplankton–zooplankton system. In the absence of M. micrura, cell density and filament (single-filament) length of C. chthonoplastes were lower at the low nutrient level compared with those at the medium and high nutrient levels. When M. micrura was present, cell density, filament length, colony proportion and size (the mean number of cells per colony) of C. chthonoplastes were highest under the medium nutrient level, corresponding to the lowest amplitude of density fluctuations in M. micrura. Regression analyses revealed that low cell densities of C. chthonoplastes (due to its limited ability to form colonies and long filaments and hence a high consumption rate of M. micrura) at low and high nutrient levels limited the growth of M. micrura adult females, leading to a sharp decline in embryo length, clutch size and population density. In general, our results suggest that the stability of phytoplankton–zooplankton systems varies with nutrients because of the nutrient-dependent morphology in phytoplankton, and imply that extremely low or high nutrient load may destabilize phytoplankton–zooplankton systems.