--- title: "What Color Is Plankton? Why the Sea Turns Green, Red or Milky" url: https://thesea.org/what-color-is-plankton/ date: 2026-11-13 modified: 2026-11-13 lang: en author: "Blane Perun" description: "TheSea.Org▸Bioluminescence & Plankton▸Plankton▸What Color Is Plankton? What Color Is Plankton? — Why the Sea Turns Green, Red or Milky Bioluminescence & Plankton By Blane Perun November 13, 2026 9 min..." categories: - "Bioluminescence & Plankton" word_count: 1981 --- # What Color Is Plankton? Why the Sea Turns Green, Red or Milky [TheSea.Org](/)▸[Bioluminescence & Plankton](/bioluminescence/)▸[Plankton](/plankton/)▸What Color Is Plankton? # What Color Is Plankton? — Why the Sea Turns Green, Red or Milky Bioluminescence & Plankton By [Blane Perun](/about-blane-perun/) November 13, 2026 9 min read What color is plankton? Most **phytoplankton are green or golden-brown**, coloured by chlorophyll plus accessory pigments; diatoms tend to be golden-brown, dinoflagellates reddish-brown and cyanobacteria blue-green or red. Most **zooplankton are nearly transparent**, though some copepods and krill are red or blue. In dense blooms, plankton can turn the sea **green, brown, red, rusty or milky turquoise**. Quick facts Main pigmentChlorophyll a (green) DiatomsGolden-brown (fucoxanthin) DinoflagellatesRed-brown (peridinin) CyanobacteriaBlue-green or red (phycobilins) Coccolithophore bloomsMilky turquoise Most zooplanktonTransparent Krill and many copepodsRed-orange (astaxanthin) Glow colour at nightMostly blue to blue-green A single plankton cell is too small to have a colour you can see. Put billions of them together, though, and they can tint a bay green, stain a coastline red, or turn hundreds of square kilometres of open ocean a pale, milky blue that shows up clearly from space. The colour comes from pigments, the same kind of chemistry that makes leaves green and carrots orange, and it tells scientists a great deal about what is living in the water. This page explains what colour plankton really is, group by group, and why the sea sometimes changes colour dramatically. For a primer on the two main plankton groups, see our [plankton](/plankton/) guide. Phytoplankton colour: chlorophyll and its helpers Every phytoplankton cell contains chlorophyll a, the green pigment that captures sunlight for photosynthesis. What makes different groups look different is the mix of accessory pigments added on top. Chlorophyll absorbs mainly blue and red light and reflects green, which is why green algae and many tiny flagellates look grass-green under the microscope. But sunlight changes as it passes through water: red light is absorbed within the first few metres, and below that the light is mostly blue and blue-green. Many phytoplankton have evolved extra pigments that catch those wavelengths and pass the energy on to chlorophyll. Those helpers, carotenoids and phycobilins, change the colour of the cell. The result is that “phytoplankton green” is really a spectrum. A sample from a spring bloom in temperate water may look olive or tea-brown. A sample from a warm lagoon may look blue-green. A sample from a red tide can look like tomato soup. Under the microscope, you can often tell the major groups apart by colour alone before you even look at their shapes; see [plankton under a microscope](/plankton-under-microscope/). | Plankton group | Typical colour | Key pigment | What a bloom looks like | | -------------- | -------------- | ----------- | ----------------------- | | Green algae | Bright green | Chlorophyll a and b | Pea-green water | | Diatoms | Golden-brown | Fucoxanthin | Brown, olive or tea-coloured water | | Dinoflagellates | Red-brown to golden | Peridinin | Red, rust or mahogany “red tide” | | Cyanobacteria | Blue-green, or red | Phycocyanin, phycoerythrin | Blue-green scum or rusty streaks | | Coccolithophores | Pale, chalky | Fucoxanthin + chalk plates | Milky turquoise water | | Zooplankton | Clear, some red or blue | Astaxanthin, blue carotenoproteins | Red patches from krill or copepods | ![Golden-brown diatoms and red-brown dinoflagellates under a microscope; what color is plankton](https://thesea.org/wp-content/uploads/what-color-is-plankton-1-thesea-org-by-blane-perun.webp)Golden-brown diatoms and red-brown dinoflagellates under a microscope; what color is plankton Golden-brown diatoms and red-brown dinoflagellates Diatoms carry a carotenoid called fucoxanthin, the same pigment that makes kelp and other brown seaweeds brown. It masks much of the chlorophyll green, so diatoms look golden-brown. When diatoms bloom in spring in coastal and temperate waters, the sea often turns olive, brown or murky green rather than clear blue. Reef keepers see the same golden-brown colour when diatoms coat the sand and glass of a new tank; see [diatoms in reef tanks](/diatoms-reef-tank/). Dinoflagellates carry another carotenoid, peridinin, which gives many species a red-brown or golden colour. In dense blooms they produce the classic red tide: water that looks red, rusty, brown or even purple, depending on the species and the light. Not every red tide is red, and not every discoloured patch is harmful, but many dinoflagellate blooms produce toxins. On Florida’s Gulf coast, the dinoflagellate *Karenia brevis* causes recurring red tides that kill fish and cause coughing and eye irritation on the beach. Our [red tide](/red-tide/) page explains how those blooms work. Blue-green and red cyanobacteria Cyanobacteria, once called blue-green algae, use phycobilin pigments. Phycocyanin is blue, and phycoerythrin is red or pink, and the balance between them sets the colour. Freshwater cyanobacteria blooms are often a vivid blue-green scum. In the ocean, the filamentous cyanobacterium *Trichodesmium* forms floating rafts that sailors called “sea sawdust”. Its blooms are often yellowish-brown or rusty, and some species can turn reddish as they age. One popular explanation for the name of the Red Sea, which I have dived, is *Trichodesmium erythraeum* blooms that tint the water there; it is a plausible theory, though historians also offer other explanations. See our [Red Sea dive](/red-sea-dive/) page for the reefs. The tiniest cyanobacteria, *Prochlorococcus* and *Synechococcus*, are so dilute in open ocean water that they do not visibly colour it at all. Their combined effect is still enormous, because they do much of the photosynthesis in the nutrient-poor tropics. 1Pigment all phytoplankton share: chlorophyll a BlueColour of light that penetrates deepest in clear seawater 2024NASA’s PACE ocean-colour satellite launched Milky turquoise: coccolithophore blooms Some of the most striking plankton colours do not come from pigment at all. Coccolithophores are tiny algae that cover themselves in plates of calcium carbonate, called coccoliths. When a species such as *Emiliania huxleyi* blooms, the plates shed into the water by the trillion and scatter sunlight like chalk dust in a glass. The sea turns a pale, milky turquoise. These blooms can cover enormous areas of the North Atlantic, the Bering Sea, the Black Sea and the waters off Patagonia and are easy to pick out in satellite images. Fishermen and sailors sometimes notice the water looks bright and cloudy rather than dark blue. What colour is zooplankton? Most zooplankton are close to invisible. Living in open water with nowhere to hide, many animals have evolved transparent bodies so that predators looking for them see straight through. Jellyfish, comb jellies, salps, arrow worms and many larvae are glass-clear, sometimes with only their eyes and guts visible. There are colourful exceptions. Krill and many copepods are red or orange because they store astaxanthin, a carotenoid they get from the algae they eat. Red is a good colour to be at depth, because red light does not reach far into the sea and a red animal looks black and hard to see. When krill or red copepods swarm near the surface, they can turn patches of water pink or red, and the red colour of salmon flesh and flamingo feathers ultimately traces back to the same pigment moving up the food chain. Some surface-dwelling copepods, especially the pontellids, are bright blue, a colour thought to protect them from strong sunlight. The bright, clear water of the open ocean is partly blue because there is so little plankton in it; see [zooplankton](/zooplankton/) for more on these animals. ![Waves glowing electric blue at night, what color plankton turns when it lights up](https://thesea.org/wp-content/uploads/what-color-is-plankton-2-thesea-org-by-blane-perun.webp)Waves glowing electric blue at night, what color plankton turns when it lights up Plankton colour at night: glowing blue After dark, some plankton make their own colour. Bioluminescent dinoflagellates such as *Noctiluca scintillans* and *Lingulodinium polyedra* flash blue when they are disturbed, which is why breaking waves, boat wakes and swimmers can glow electric blue on certain nights. Interestingly, the same species can be a completely different colour by day: *Lingulodinium* blooms often look red-brown in daylight, and *Noctiluca* comes in a red form and a green form (the green form carries tiny symbiotic algae). Most marine light is blue or blue-green because those wavelengths travel furthest through seawater. Our [bioluminescent plankton](/bioluminescent-plankton/) and [milky seas](/milky-seas/) pages cover the glow in detail. Brown, green or red water: is it dangerous? Colour alone cannot tell you whether a bloom is harmful. Plenty of harmless diatom blooms turn water brown, and some toxic blooms are barely visible, because a few species can cause problems at fairly low densities. A bright red or rusty patch is a reason for caution, not proof of danger, and a clear-looking beach can still be under a shellfish harvesting closure. The safest approach is to check local advisories. In the United States, state agencies and NOAA publish harmful algal bloom reports, and in Florida the Fish and Wildlife Conservation Commission posts regular red tide sampling maps. If water looks discoloured, smells unusual, or there are dead fish on the beach, it makes sense to stay out of it, keep pets away and avoid eating shellfish gathered locally until officials say it is safe. Most of the time, though, coloured water simply means the ocean is busy growing its own food. Reading the ocean’s colour from space Because phytoplankton change the colour of the water, satellites can map them. Instruments such as NASA’s SeaWiFS, MODIS and the newer PACE mission measure how much blue and green light comes back from the sea surface. Clear, plankton-poor water looks deep blue; water rich in chlorophyll reflects more green. From those measurements scientists estimate chlorophyll concentrations across entire oceans, track blooms, follow seasonal changes and watch for long-term shifts linked to warming. PACE, launched in 2024, measures many more wavelengths than earlier satellites, which is meant to help identify which types of phytoplankton are present, not just how much. The same colour shifts are useful closer to home. Water quality agencies use ocean colour to watch for harmful algal blooms, and some red tide forecasts in Florida draw on satellite imagery alongside water samples. My take: colour is the first clue From years of reef keeping since 1996 and many dives across the Caribbean and the Pacific, I have come to treat water colour as the first clue to what is happening in it. Deep, clear blue usually means low nutrients and little plankton, which is exactly what most coral reefs want. Green or brown tints usually mean plenty of phytoplankton, which is good for fish and filter feeders but means poorer visibility. In a reef tank, the same logic applies on a smaller scale: a brown film is usually diatoms, a red slime is often cyanobacteria, and a stringy brown film with bubbles may be dinoflagellates. If I see a colour change, my first step is always to figure out which organism is causing it before deciding whether it is a problem. So, what colour is plankton? Mostly green, gold, brown or clear, with flashes of red, blue and milky white when conditions are right. The colour of the sea is, in large part, the colour of what is living in it. ![Bright green phytoplankton bloom seen from the air, the usual answer to what color is plankton](https://thesea.org/wp-content/uploads/what-color-is-plankton-3-thesea-org-by-blane-perun.webp)Bright green phytoplankton bloom seen from the air, the usual answer to what color is plankton Frequently asked questions What color is phytoplankton? Usually green to golden-brown, depending on its pigments. Diatoms are golden-brown and many dinoflagellates are red-brown. What color is zooplankton? Mostly transparent. Krill and many copepods are red or orange, and some surface copepods are blue. Why does plankton make the water green? Chlorophyll in phytoplankton reflects green light, so dense phytoplankton turns the water green. What plankton turns the ocean red? Usually dinoflagellates during red tides, sometimes cyanobacteria or swarms of red zooplankton. What makes the sea milky blue? Blooms of coccolithophores, whose chalk plates scatter light and turn the water milky turquoise. What color is bioluminescent plankton? The glow is mostly blue or blue-green, even if the plankton looks red-brown by day. Sources & further reading - NASA Earth Observatory: What are phytoplankton? - NASA PACE mission: Plankton, Aerosol, Cloud, ocean Ecosystem. - NOAA National Ocean Service: What is a harmful algal bloom? - Florida Fish and Wildlife Conservation Commission: Red tide (*Karenia brevis*) status. - Falkowski, P.G. & Raven, J.A. (2007). *Aquatic Photosynthesis*. Princeton University Press. Continue reading More about plankton [PlanktonRed TideWhen dinoflagellates turn the water red. ](/red-tide/) [PlanktonPhytoplankton BloomsWhat causes them and why they matter. ](/phytoplankton-bloom/) [BioluminescenceMilky SeasThe glowing ocean seen from space. ](/milky-seas/) BP Last updated November 13, 2026 Written by Blane Perun Blane Perun has been exploring Earth’s oceans and marine life for over 25 years. Founder of TheSea.Org in 1999, underwater photographer, coral aquaculture pioneer, and explorer of 80+ countries. [About Blane Perun →](/about-blane-perun/) [More on plankton →](/plankton/)