Discover real natural events happening around our amazing planet right now!
Real-time algal bloom and water quality data from NASA EONET ocean colour satellites
View on Globe →Sometimes the water in a lake, river, or part of the ocean suddenly changes colour — turning bright green, red, orange, or brown. This can happen when tiny plants called algae grow so fast they cover the surface, or when pollution flows in. Some algae blooms can be toxic to fish and other wildlife.
NASA's ocean colour satellites can detect these changes from orbit — water that is packed with algae reflects light differently than clean water, creating distinctive colour patterns that are clearly visible from space. This allows scientists to map the extent of algal blooms covering hundreds of square kilometres within hours of detection.
NASA's MODIS Aqua satellite carries a specialised Ocean Color instrument that measures how different wavelengths of light are absorbed and reflected by the ocean surface. Clean, deep ocean water appears intensely blue because it absorbs red and green light. Water filled with algae (chlorophyll) appears green, while sediment-laden water appears brown. These spectral signatures allow NASA to identify and map algal blooms anywhere on Earth within a day of satellite acquisition.
The ESA Sentinel-3 satellite — operated in partnership with data shared with NASA — maps ocean and lake colour at 300-metre resolution, providing finer detail than MODIS in coastal waters and lakes where blooms most often form near populated areas. Its daily global coverage enables scientists to watch blooms form, spread, and dissipate over days to weeks.
NASA's Ocean Color Web makes this data freely available to researchers, fisheries managers, water utility operators, and environmental agencies worldwide, helping them protect public health and manage ecosystems in near real-time.
Earth Explorer displays water colour events tracked by NASA's EONET monitoring network. Significant algal blooms, unusual water discoloration, and notable water quality events in lakes, rivers, and coastal oceans worldwide appear as glowing dots on the interactive globe.
View Live Water Color Events →Water colour depends on what is dissolved or suspended in it. Clear ocean water appears blue because it absorbs all colours of light except blue. Algae make water green because they contain green chlorophyll. Sediment, tannins from leaves, and organic matter make water brown or yellow. Some specific algae, bacteria, or minerals produce red, orange, or pink water. Satellites detect these changes by measuring how water reflects different wavelengths of sunlight.
An algal bloom is an explosive growth of algae when conditions are ideal: warm water, bright sunlight, and plenty of nutrients. Algae can double their population every day during a bloom, covering hundreds of square kilometres of water surface in vivid green or other colours. These blooms are clearly visible from NASA satellites and are tracked as significant environmental events when they threaten ecosystems or public health.
Many algal blooms are harmless — they look alarming but cause no health risk. However, Harmful Algal Blooms (HABs) produce toxins that can kill fish in large numbers, make shellfish unsafe to eat, and cause illness in people who swim in or drink affected water. Some toxins can even become airborne near the shore, causing respiratory irritation. People, pets, and livestock have died from exposure to HAB toxins.
HABs are driven by excess nutrients — primarily nitrogen and phosphorus — from agricultural fertiliser runoff, sewage overflows, and urban stormwater. Warm temperatures, calm water, and sunshine allow algae to explode in population once nutrients are abundant. Climate change is making HABs more frequent by warming water temperatures and intensifying the heavy rainfall events that wash nutrients from land into water bodies.
Do not swim in water that appears unusually green, red, brown, or foamy, or has a strange smell. Keep pets and children away from discoloured water. Do not eat fish or shellfish from areas with active algal blooms. Report suspicious water colour to your local environmental agency — they can test the water and issue public health advisories. Rinse off thoroughly if you accidentally come into contact with affected water.
MODIS Aqua's Ocean Color instrument measures how water reflects light at multiple wavelengths — a spectral fingerprint that reveals what is in the water. Green chlorophyll from algae, brown sediment, and yellow dissolved organic matter each produce unique signatures. Sentinel-3 maps coastal and lake colour at 300-metre resolution. NASA makes this data freely available through the Ocean Color Web, enabling real-time monitoring worldwide.