13/02/2026
There’s a lot of this in the marina’s at the moment 😟🥴
Algal blooms spotted in Marlborough's waters in recent weeks may appear concerning, but the public can rest assured that they are generally harmless.
Senior Coastal Scientist Katie Littlewood said marine algal blooms are common at this time of year and occur when microscopic algae and plankton grow rapidly in the water.
They are a natural part of coastal ecosystems and become more common when warm temperatures and nutrient levels increase.
“We’ve had a particularly warm and wet summer which has provided perfect conditions for algal blooms, especially in the Marlborough Sounds. Warmer water enables algae to grow faster, while heavy rainfall washes nutrients from the land into rivers and coastal areas. Calm conditions in sheltered bays and inlets allows algae to build up at the surface, which makes blooms more visible and longer lasting.
“While many are harmless, large blooms can affect water quality and place stress on marine life as they break down. Any bloom carries a risk for human health; however Council’s water quality monitoring shows this is a species called Noctoluca scintillans which is generally harmless. However we recommend not swimming directly in it, eating shellfish from that area, and keep your dog out of the water.”
Noctoluca is commonly bioluminescent and produces a shimmering blue light when disturbed - an exciting sight to see at night.
Algal blooms are monitored by the Ministry for Primary Industries and people should check their website for up-to-date warnings and further information. You can also sign up for email updates about shellfish biotoxin warnings - see the link in the comments.
Conversely, with the recent rainfall there is currently very little toxic algae in Marlborough’s rivers. This could change if the weather is consistently warm and sunny. Here, the most common freshwater toxic algae are Microcoleus*, which form dark brown mats growing on rocks in the river bed, usually in fast-flowing water. When the algae mats become very thick they detach and float in the water, accumulating along the edges of the river. This is when they present the greatest risk to recreational users and dogs.
*Previously known as Phormidium.
📸: The Coastal Science team collects water samples at Torea Bay in the Marlborough Sounds.
