The color of the ocean is determined by the way light interacts with the water and contents within it. The ocean appears blue because water absorbs colors in the red end of the visible light spectrum. The bluer components of sunlight pass right through, and predominantly get reflected back out. This gives the ocean its characteristic blue color. However, under certain conditions, the ocean can also turn green or other colors.
Phytoplankton, tiny microscopic plants, contain chlorophyll which gives them a greenish color. When phytoplankton populations explode into massive blooms, the ocean surface takes on a greenish hue. This is why coastal regions will sometimes appear greenish, especially in spring and summer when phytoplankton are most abundant. Runoff from land can also add minerals, sediments, and organic matter that tints the ocean shades of green or brown near the coast.
Farther out in the open ocean, phytoplankton populations are sparser, and the water retains its characteristic deep blue color. But rising ocean temperatures, climate change, pollution, and other factors are changing phytoplankton communities globally. As populations fluctuate, ocean color changes as well. Understanding the factors that drive phytoplankton blooms and impact ocean color provides insight into ocean health.
What makes the ocean blue?
Water molecules naturally absorb light in the red end of the visible spectrum. Longer wavelengths corresponding to reds and oranges are absorbed, while shorter wavelengths in the blue/violet range pass through. When sunlight enters the ocean, the red components are selectively removed, leaving behind mostly blue light which gets reflected back out. This gives the ocean its deep blue appearance.
The exact shade depends on factors like:
- Water depth – Shallow coastal waters appear lighter blue compared to deeper shades offshore.
- Viewing angle – The ocean surface viewed straight down appears darker blue.
- Particles/sediments – More particles scatter light, making the water appear greener or murkier.
- Time of day – Overhead midday sun accentuates blue tones compared to morning/evening light.
But while water molecules account for the ocean’s inherent blueness, the exact color we see depends greatly on the water’s contents. Light interacts with any particles or organisms suspended in the water, selectively filtering wavelengths. Different ocean regions and marine communities create variation in ocean color.
What makes the ocean green?
The ocean surface takes on green hues primarily due to dense populations of phytoplankton. These microscopic photosynthetic organisms contain chlorophyll which strongly absorbs red and blue light, reflecting back green wavelengths. When massive phytoplankton blooms occur, the ocean surface appears green or even brownish from the high concentration of organisms and pigments.
Some major factors causing green ocean waters:
Upwelling
Upwelling brings cold, nutrient-rich waters from the ocean depths up to the sunlit surface layers. This fuels explosions of phytoplankton growth, creating greenish waters. Major upwelling regions include:
- Eastern boundary currents – the coasts of Peru, Namibia, Morocco
- Equatorial divergences
- Around islands
River plumes
Sediment-laden, nutrient-rich freshwater flowing into the ocean provides favorable conditions for phytoplankton blooms, often visible as green plumes extending from river mouths. Major river plumes include:
- Amazon River
- Yangtze River
- Mississippi River Delta
Shallow coastal areas
Sunlight and nutrient inputs fuel phytoplankton growth in shallow coastal areas and continental shelves worldwide. The Baltic Sea, North Sea, and East China Sea are examples of greenish coastal seas.
Melting sea ice
As sea ice melts in spring and summer, it releases nutrients into surface waters. This triggers phytoplankton blooms, evident as extensive green coloring across the Arctic Ocean and Southern Ocean.
Iron fertilization
Iron is an essential micronutrient for phytoplankton growth. Adding iron to iron-poor but nutrient-rich regions like the Southern Ocean induces major phytoplankton blooms and greens the water.
How do phytoplankton blooms impact ocean color?
Phytoplankton are diverse photosynthetic microalgae that form the base of the marine food web. Different phytoplankton groups impact ocean color in unique ways:
Diatoms
These single-celled algae are encased in ornate silica shells. Diatom blooms appear yellowish-brown and turbid.
Dinoflagellates
Flagellated single-celled organisms, some of which cause red tides. Dinoflagellate blooms range from greenish to brownish-red.
Coccolithophores
Algae with calcite plates that reflect light, giving blooms a striking turquoise color.
Trichodesmium
Cyanobacteria that form extensive blooms, turning waters a vibrant turquoise.
Picocyanobacteria
Tiny photosynthetic bacteria that multiply rapidly, altering water color to greenish-blue.
As different phytoplankton rise and fall with changing environmental conditions, they create a patchwork of color across the ocean surface that shifts seasonally. Monitoring phytoplankton types using ocean color data provides insight into marine ecology and biogeochemistry.
How do climate factors drive ocean color changes?
Climate variability and change exert profound influences on ocean processes, altering conditions for phytoplankton growth and impacting ocean color:
Increasing sea surface temperatures
Warmer oceans experience intensified stratification, reducing mixing and upwelling. This limits nutrient supply for phytoplankton in some regions, reducing blooms and leaving oceans bluer. However warmer waters also increase metabolism, potentially fueling more growth.
Shifting wind patterns
Changing wind strength and direction affects upwelling and ocean circulation patterns. This can inhibit or enhance nutrient supply to surface waters, causing phytoplankton concentrations to fall or rise.
Sea ice changes
Declining sea ice extent in the Arctic exposes more open water, fueling massive phytoplankton blooms that turn waters green. Increased meltwater also stabilizes the water column, favoring phytoplankton growth.
Increasing stratification
Stronger density differences between warm surface and cold deep waters reduces overturning and mixing. This limits nutrient exchange to the surface, potentially suppressing phytoplankton growth.
Ocean acidification
Increasing ocean acidity impacts phytoplankton communities, making them less productive in some regions while favoring growth in others. Community shifts alter regional ocean colors.
How do other human activities impact ocean color?
In addition to climate change, direct human pressures like overfishing, pollution, and coastal development are impacting ocean biology and altering phytoplankton communities and ocean color:
Nutrient pollution
Agricultural runoff and sewage effluent enrich coastal waters with nitrogen and phosphorus, fueling phytoplankton blooms and greening/browning nearshore waters.
Sediment runoff
Soil erosion from deforestation, mining, and development mobilizes sediments into the ocean, increasing turbidity and discoloration.
Overfishing
Removing too many fish depletes predators in the food web, allowing phytoplankton growth to spike unchecked, changing ocean color.
Invasive species
Introduced species like crabs and jellyfish can radically alter food webs, causing shifts in phytoplankton abundance and water color.
Oil spills
Oil slicks and dispersants used to clean them up are toxic to many phytoplankton. This suppresses blooms, leaving the water clearer and bluer locally.
What tools do scientists use to study ocean color?
Satellite ocean color sensors are critical for analyzing ocean color trends over different time scales:
| Satellite | Agency | Lifespan |
|---|---|---|
| SeaWiFS | NASA | 1997-2010 |
| MODIS | NASA | 1999-present |
| MERIS | ESA | 2002-2012 |
| VIIRS | NASA/NOAA | 2011-present |
These sensors measure light at multiple wavelengths to derive chlorophyll and other water quality parameters used to study phytoplankton and infer ocean color. Scientists also use shipboard measurements, moorings, and autonomous vehicles to collect detailed water sample data on phytoplankton and other particulate matter affecting ocean color. Understanding connections between in-water constituents and satellite ocean color enables global analysis of how marine ecosystems are changing.
Conclusion
The ocean’s blue color results from selective absorption of red/orange wavelengths by water molecules. But variations in phytoplankton populations and other particulates alter this to generate green, brown, and other colors. Climate change, pollution, and other anthropogenic factors are shifting phytoplankton communities, driving widespread changes in ocean color patterns detected by satellites. Tracking ocean color provides insight into the health of marine ecosystems and how they are responding to global pressures. Sustained satellite monitoring combined with in-water sampling is essential for diagnosing ocean changes and informing management strategies.
