Recovery of freshwater algal assemblage from copper and zinc toxicity in a semi-continuous culture system

Abstract

Metal contamination in freshwater ecosystems can substantially alter algal community structure and function. Although many studies have documented the toxic effects of heavy metals on algal growth, the mechanisms and dynamics of community recovery following the cessation of metal stress remain poorly understood. In this study, we examined the recovery of a freshwater algal assemblage comprising Chlorophyta, Cyanophyta, and Bacillariophyta after exposure to copper (Cu) and zinc (Zn) in a semi-continuous culture system. During the initial toxicity phase, metal exposure caused a 65–80% reduction in total algal biovolume, with pronounced suppression of cyanophytes, followed by chlorophytes and diatoms. After transferring the metal-treated algal assemblage to fresh culture medium without excess heavy metals, the cultures showed progressive recovery over a 25-day period, characterized by the reappearance and succession of previously suppressed taxa. Recovery initiated within 5–7 days; by day 25, Zn-exposed cultures reached 78% of the control biovolume, whereas Cu-treated cultures recovered only to 52%. The stronger recovery in Zn-treated cultures suggests differential toxicity and species-specific resilience. The semi-continuous culture approach enabled simultaneous observation of the inhibition and recovery phases to assess community-level responses to metal stress. These findings illustrate the contrasting impacts of Cu and Zn on algal recovery and may contribute to future research aimed at developing recovery-based indicators for aquatic ecosystem assessment and restoration.