Monitoring Cyanobacterial Fluctuations in Coastal Mangrove Waters

Mangrove forests, situated in the intertidal zones of tropical and subtropical coastlines, represent one of the most productive and ecologically significant ecosystems on Earth. They provide critical habitats for marine species, stabilize shorelines, sequester carbon, and act as natural water purification systems. 

However, these ecosystems are increasingly threatened by anthropogenic pressures, among which the proliferation of cyanobacteria—commonly known as blue-green algae—has emerged as a major concern.

Cyanobacteria as Sentinels of Eutrophication

Coastal mangrove waters receive continuous inputs of nutrients from tidal flushing, surface runoff, and industrial and domestic wastewater discharges. As nitrogen and phosphorus accumulate, they fuel the excessive growth of phytoplankton. Cyanobacteria, with their ability to thrive in low-light, high-turbidity conditions, often outcompete other algal groups and shift the phytoplankton community toward a cyanobacteria-dominated state. 

This shift is not merely a compositional change—it is a diagnostic signal of ecosystem degradation, marking the transition from a clear, macrophyte-dominated state to a turbid, algae-dominated one. Thus, systematic monitoring of cyanobacterial biomass and community structure provides an early and reliable indicator of nutrient enrichment and eutrophication trends in mangrove waters.

Ecological Threats Posed by Cyanobacterial Blooms

Excessive cyanobacterial proliferation—harmful algal blooms (HABs)—poses systemic risks to mangrove ecosystem structure and function. In Shenzhen Bay alone, over 30 HAB events have been recorded in coastal waters since the 1980s. 

During bloom events, massive algal die-off and decomposition rapidly deplete dissolved oxygen, creating extensive hypoxic zones that cause mass mortality of fish and benthic organisms. Dense algal mats also shade submerged vegetation, suppressing photosynthesis in mangrove seedlings and associated macrophytes. Furthermore, toxin-producing cyanobacteria—such as Microcystis and Anabaena—release microcystins and neurotoxins that directly poison aquatic fauna and, through food chain transfer, pose risks to human health. 

As climate change and human activities continue to intensify, the frequency and spatial extent of coastal HABs are expected to rise. Routine cyanobacterial monitoring is therefore an essential prerequisite for early warning and proactive management of bloom risks.