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Why is the sea blue?

Updated: May 29, 2020


The ocean makes up 71% of Earth’s surface and is characterised by it’s colour without a second thought. But why is the sea actually blue? And, in saying that, why is it not covered in photosynthetic plants resulting in a green sea? What are the processes that create the beautiful, shimmering and clear colour of the body we know and love?

I’m a marine biologist. I swim, snorkel, SCUBA dive, surf, research and spend my summers lying beside Earth’s magnificent body of an ocean. But one of the most obvious questions remains unanswered: why is the sea blue?


Well, obviously this can be easily solved by light absorption: as sunlight passes through water, the colours of the spectrum are absorbed at different rates with the long wavelengths, such as red, absorbed first and shorter wavelengths, such as blue, penetrating deeper into the ocean depths (Fig. 1). Therefore, underwater available light is predominantly blue.






But, if that’s the answer, then essentially seawater is blue because it’s clear and we can see through it? So, we then have to follow up and ask how it is so clear? Why is it not covered by photosynthetic plants such as seaweeds and phytoplankton utilising all available light, resulting in a green sea? In asking why the sea is blue, we must also answer why the sea is not green.


Let’s take a look at the ocean’s structure to try understand why the sea is not green. The ocean’s surface is known as the euphotic zone, defined as the point extending to where only 1% of photosynthetically usable light is available (Fig. 2). The lower limit of the euphotic zone explains why most of the volume of the oceans is not green – it is simply too dark for photosynthesis. But, if these upper levels of the ocean provide all the energy needed for photosynthesis, why is the ocean surface not thick with plants, or at least photosynthetic organisms, therefore being green when we look at it?


Well, to be green we would either need an ocean of tiny photosynthetic organisms such as diatoms and phytoplankton, or, some density of large plants. When conditions are suitable, we can see a considerable biomass of seaweeds from the ocean surface and under the right conditions, marine ‘plants’ can grow remarkably quickly. But, since seaweeds are limited to shallow water due to their light dependency they also appear to be limited by other physical aspects of their environment such as high wave action and the ability to hold fast on substrata. Therefore, it is clear that neither sea grasses nor macroalgae attached to substrate can make the sea green because they can only survive in very shallow waters around coasts and so are absent from most of the oceans. But what about floating plants?


In freshwater systems, there are plants with leaves that float on the water surface without any connection to the sediment or substrate, such as hyacinths and duckweed. Why are the ocean’s surfaces not dominated by floating plants? Floating adaptations tend to be unsuccessful in the ocean due to wind associated with large bodies of water movements resulting in floating objects being washed up on the beach (as seen with floating plastics). So, with no roots or swimming ability in plants, a floating adaptation is unsuccessful in most plants, with few exceptions.


Sargasso weed is one of the exceptions for floating plants in the ocean (Fig. 3). It’s habitat within the Sargasso Sea gyre (an ocean region bounded by four currents) allows the weed to maintain it’s position in its centre so it can live in large patches on the water surface. But, even in the Sargasso Sea, floating plants fail to cover the whole surface, and are often arranged into lines by wind action leaving large expanses uncovered by plants. So, we can count out the ability of large plants and macroalgae to make the sea green. As a result, marine photosynthesis is dominated by microorganisms. But do these have the ability to turn the sea green?

Phytoplankton are the microscopic organisms responsible for approximately half of all global primary production. One litre of seawater can contain an estimated 20,000 species of microbes! Phytoplankton can be broken down into two groups: picoplankton (very tiny photosynthetic species such as cyanobacteria - which also happens to be the most common species on Earth!) and microplankton (less tiny but still small species such as diatoms). Despite this great diversity of phytoplankton, they rarely occur at densities high enough to make seawater green. This only happens during phytoplankton blooms, induced by high levels of nutrients via upwelling (Fig. 4). If it’s possible for phytoplankton blooms to cause a green ocean, then is a lack of nutrients responsible for our blue sea?

A vast range of chemicals are dissolved into the ocean (chloride, sodium, sulphate, magnesium, calcium), some of which are important for plankton (phosphate, nitrate, iron), but are all at low concentrations in seawater. These marine salts come from weathering of rocks on land, from wind-blown dust and from hydrothermal vents. A short supply of nutrients, such as iron, will limit phytoplankton production and thus food availability for zooplankton, and other animals, thereby not creating a green sea.


What if we increase iron? Well, iron is actually the fourth most abundant element in the Earth’s crust, but unfortunately it’s not easily oxidised in the ocean because it’s insoluble above a pH of 4, with the pH of ocean water being around 8. Since increased iron nutrients has increased phytoplankton blooms and therefore resulted in increased removal of carbon dioxide from the atmosphere through increased photosynthesis, the idea of artificially adding iron to the ocean to remove anthropogenic carbon dioxide has been toyed with for many years. Large-scale experiments have monitored effects when iron is added to patches of ocean. These experiments led to increased plankton primary production, mostly by diatoms. Natural experiments have also been carried out measuring iron inputs over a longer time period from upwelling resulting in a much better outcome than patch experiments due to the steady input of iron. But, before we rush in and claim this as the answer to reducing carbon emission, this solution is not the be all end all. In Indonesia, 1997, excessive amounts of iron washed into the ocean following terrestrial fires causing extensive mortality to corals and fish populations due to oxygen shortages. Therefore, iron fertilisation does not benefit all marine organisms and would be hazardous to attempt on a large scale before further studies have been conducted.


Plus, would an increase in iron necessarily lead to an increase in phytoplankton? No! There are also other limiting factors that interact, such as macronutrients like phosphorus and nitrogen that are required by phytoplankton in large amounts. While both of these nutrients can be supplied from land, and nitrogen can be fixed to a usable form by nitrogen-fixing microbes, these nutrients are often biologically unavailable. Furthermore, the ratio of nitrogen to phosphorus in plankton is 16:1 – the same as the ratio of nitrate to phosphate in seawater. If phosphate increased in seawater, then nitrogen fixation would allow plankton numbers to increase to a point where they had used up all additional phosphate. Therefore, water chemistry is biologically controlled to the extent where nitrogen taken out of water only occurs to match the limiting nutrient.


The biological pump is the mechanism that transports nutrients through the ocean (Fig. 5). Dead plankton and other biological materials such as faeces of zooplankton are broken down and recycled in the upper layers of the ocean, but some of them fall into much deeper water where they can be buried in the sediment. The euphotic zone life essentially pumps nutrients into deeper waters to keep nutrient levels low to balance input from the land. The biological pump is also responsible for removing iron from surface waters to maintain the 16:1 ratio produced by phytoplankton in the euphotic zone, and transferring iron to the entire ocean and depths. If there was no biological pump, ocean circulation would mix waters to cause a uniform distribution and increase nutrients in the euphotic zone (as occurred 65 million years ago with the death of phytoplankton with extinction of dinosaurs). The biological pump basically ensures that the wealth of nutrients in the ocean surface and areas close to continental shelves with high nutrients from rock weathering is mixed throughout the entire ocean. The fine balance of nutrients that all marine organisms rely on, in conjunction with the stable ocean chemistry means that attempts to artificially alter this composition to increase phytoplankton growth - leading to a green ocean - would be unwise and likely catastrophic.

So, in conclusion, we see the sea as blue because of light absorption and relative clarity due to the lack of a green sea. Why is the sea not green? Firstly, much of the world’s sea water is too dark for photosynthesis. In the euphotic zone where there is enough light for photosynthesis, the density of large rooted plants and macroalgae is limited because they can only survive in small areas that are shallow enough for them to access light and attach to substrate and cannot float. Microscopic photosynthetic organisms cannot occur in abundances high enough to colour the sea green because most seawater is too low in nutrients needed for growth, part of which is due to the action of plankton themselves through the biological pump (which pumps nutrients from the surface to the depths, and vice versa). It's probably never crossed your mind to question the colour of our oceans, but when it comes down to it, this really is a big question about our Earth that still largely remains a mystery.


Inspiration and ideas from Big Questions in Ecology and Evolution by Thomas Sherratt (2009).

Lena is studying an honours degree at AUT. Her research is comparing behavioural differences of Nemo fish in captivity and the wild. As a part time job, she teaches at the university. One of her favourite pastimes is to banter about current environmental issues. However, this tends to be only with people who agree, so it’s generally one-sided.

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