Morphological and growth responses of two green algal strains to toxic Microcystis are dependent on the cultivation growth phase of filtrate and target strain

Allelopathic interactions amongst phytoplankton are considered an important factor contributing to species competition and succession in aquatic ecosystems, but their mechanisms in plankton dynamics are poorly described. In this study, whether toxic Microcystis aeruginosa could affect the growth of Chlorella vulgaris and Kirchneriella sp. was examined according to filtrate experiments at different cultivation phases. Results indicated that M. aeruginosa filtrate significantly influenced the growth and morphological characteristics of the two target green algae, which were dependent on the cultivation growth phase of filtrate and target strain. At the beginning of the experiment, the formation of a large C. vulgaris colony was induced by M. aeruginosa filtrate. The effects of filtrate in the stationary phase (SP) was more significant than that of the exponential phase (EP). Subsequently, the colonies gradually broke into small colonies or single cells. The growth rate of C. vulgaris was finally promoted in the filtrate treatment. For Kirchneriella sp., the colonies formed and remained in M. aeruginosa filtrate under EP until the end of the experiment. Smaller colonies were observed in Kirchneriella sp. by M. aeruginosa filtrate under SP than those in the control, and larger colonies were not detected. The growth rate of Kirchneriella sp. was inhibited in the filtrate of EP but was promoted in SP. This study provided new insights into the interaction between the morphological responses and growth effects of algae and proposed a new theoretical basis for algal succession in aquatic ecosystems.

Benthic algal biomass and assemblage changes following environmental flow releases and unregulated tributary flows downstream of a major storage

A large dam reducing the magnitude of flows regulates the Severn River, Australia. Environmental flows (EFs) are designed to increase the magnitude of flow and improve ecological outcomes such as reducing filamentous algal biomass and re-setting algal succession. The effectiveness of EF releases to alter benthic algal assemblages is poorly understood. We examined benthic algal biomass and assemblage structure at two cobble-dominated riffle sites downstream of Pindari Dam, before and after two EFs. Both EFs had discharges of ~11.6 m3 s–1 (velocity of ~0.9 m s–1). Neither EF reduced benthic algal biomass, and sometimes led to increases, with density of some filamentous algae increasing (Stigeoclonium and Leptolyngbya). An unregulated flow from a tributary between the two sites increased discharge to 25.2 m3 s–1 (velocity of ~1.2 m s–1), decreasing biomass and density of filamentous algae. The similarity in flow velocities between scouring and non-scouring events suggests that thresholds may exist and/or suspended sediments carried from unregulated tributaries may contribute to reduce algal biomass. Identifying velocities needed to reduce algal biomass are useful. Accordingly, EFs with flow velocities ~1.2 m s–1 may achieve this in river cobble-dominated riffle sections dominated by filamentous algae. Lower flow velocities of <0.9 m s–1 may result in no change or an increase in filamentous algae.