Last year’s harmful algal bloom event in South
Australia illustrates how large-scale environmental impacts can arise from
natural processes, yet still generate considerable public concern and
speculation about human activities.
What are Harmful Algal Blooms?
Harmful algal blooms occur when algae grow rapidly and accumulate in large quantities, causing harm to people, animals, ecosystems and economies. These microscopic plant-like organisms are naturally present in aquatic environments such as lakes, rivers and oceans. However, under certain conditions they can multiply rapidly, forming a dense “bloom”.
A bloom becomes harmful when it produces toxins or depletes oxygen in the water. This can lead to severe ecological damage including fish kills, ecosystem disruption and water quality deterioration.
Beyond Dredging: What Really Drives Algal Blooms
When algal blooms occur near ports, coastal infrastructure or dredging operations, these activities are often quickly suspected as the cause. However, the South Australian event demonstrates that such phenomena are frequently driven by natural processes rather than sediment disturbance. Furthermore, during this event, the algae were found to produce toxins.
In most cases, the real drivers are nutrients and environmental conditions. Elevated concentrations of nutrients in the water column, particularly nitrogen and phosphorus, play a central role in algal bloom development. When combined with favourable environmental conditions such as:
- Warm temperatures;
- Stable water columns; and
- Adequate sunlight
these factors can trigger rapid phytoplankton growth, ultimately resulting in large-scale algal blooms.
Expanding Environmental Monitoring Beyond Turbidity
Dredging projects’ environmental monitoring programs typically focus on turbidity and suspended sediments, as these are directly linked to dredging operations and sediment plumes.
Continuous turbidity monitoring is standard practice for compliance with environmental permit conditions and for tracking potential impacts. However, key biological indicators, such as nutrients and chlorophyll concentrations, which increase as algae proliferate, are often excluded from monitoring programs, despite their importance in identifying bloom potential.
Chlorophyll-a, the primary photosynthetic pigment in almost all phytoplankton, is a strong indicator of algal biomass. Chlorophyll-a is also relatively easy to measure using standard water quality probes, however, it has limitations:
- It does not necessarily identify harmful species
- It does not necessarily correlate with toxicity
To provide a more comprehensive understanding of coastal water dynamics, it may be beneficial for monitoring programs to also include:
- Nutrients – such as nitrogen and phosphorous
- Dissolved oxygen
- Temperature
- Turbidity
- Direct toxin measurements – to confirm whether a bloom is harmful
Together, these parameters enhance understanding of ecosystem processes. They also improve the ability to detect and interpret algal bloom development, which often results from a combination of different factors.
Baseline Monitoring: The Foundation for Better Environmental Decisions
The harmful South Australia event started in March 2025 and persisted for a year. It reinforces the importance of robust baseline environmental monitoring before and during dredging and marine construction activities.
Environmental baseline studies:
- Establish natural environmental conditions
- Capture seasonal variability
- Enable differentiation between natural ecosystem events and project-related impacts.
Without this context, natural environmental changes can be incorrectly attributed to nearby dredging operations. This can lead to reputational damage, regulatory investigations and unnecessary project delays.
Comprehensive monitoring programs that include parameters such as nutrients and chlorophyll provide a stronger scientific foundation for environmental management. They also support clearer communication with stakeholders by ensuring natural environmental events are accurately understood and appropriately contextualised.
Key Lessons
The 2025 South Australian algal bloom highlights several important insights:
- Large algal blooms can occur naturally and independently of dredging activities.
- Nutrient enrichment, rather than sediment disturbance, is typically the primary driver.
- While dredging can resuspend sediments and release nutrients into the water column, most nutrients originate from land-based runoff and discharges.
- Dredging may contribute to algal blooms but is generally not the primary cause.
- Standard dredging monitoring focuses on turbidity and often excludes key environmental and biological indicators like nutrients and chlorophyll.
- Baseline environmental monitoring is essential for distinguishing natural events from project‑related impacts.
- Robust monitoring programs help protect both marine ecosystems and the reputation of coastal projects.