Chlorophyll-a is the primary green photosynthetic pigment found in phytoplankton—microscopic marine algae that drift in the ocean. Just like terrestrial plants, these single-celled organisms use Chlorophyll-a to capture sunlight and convert it into energy.

When phytoplankton populations explode into an “algal bloom,” the sheer volume of Chlorophyll-a in the water column drastically alters the optical properties of the ocean. The pigment absorbs red and blue light while reflecting green, turning the water a murky, emerald colour.

What Causes the Blooms?

Phytoplankton require two primary ingredients to multiply: sunlight and nutrients (like nitrogen and phosphorus). Along the New South Wales coast, these explosive blooms are typically triggered by a combination of factors:

• Coastal Upwelling: As the East Australian Current (EAC) flows southward and interacts with the continental shelf, it often drives cold, nutrient-rich deep water to the surface.
• Terrestrial Runoff: Heavy coastal rains flush organic matter and fertilizers from land catchments directly into the nearshore environment, providing a sudden nutrient feast for algae.
• Optimal Sunlight: As the water warms and daylight hours increase, the photosynthetic engine of the phytoplankton accelerates.

Seasonality and Duration

• When it happens: While Chlorophyll-a is always present in the water, the “Temperate East” marine bioregion (which includes the Sydney coastline) experiences its most massive, classic phytoplankton blooms during Spring (September through November). During these months, the combination of increasing solar radiation and nutrient availability creates the perfect storm for algal growth. Smaller, secondary spikes can also occur in summer driven by intense EAC upwelling events.
• How long it lasts: The biological life cycle of a bloom takes time to spin up and eventually dissipate. While the physical upwelling of cold water might occur rapidly over 5 to 10 days, the resulting phytoplankton blooms typically develop and persist over 4 to 8 weeks. However, the actual duration of poor visibility at a specific coastal dive site may only last a few days to a couple of weeks, depending on how quickly ocean currents flush the bay and how fast local nutrients are depleted.

Contains modified Copernicus Sentinel data 2026, processed for this card — with VIIRS NOAA-21, PACE OCI and Sentinel-3 OLCI via NASA GIBS behind it. Whichever instrument produced the scene on screen is named above it.