April 14, 2021

The sea is changing color | Science

The sea is changing color | Science

Explosion of microscopic algae in the Bay of Biscay.

By the end of the century, most of the sea will have changed color. Marine phytoplankton, the base of the oceans, is suffering the impact of climate change, altering its composition and distribution. These organisms use chlorophyll to synthesize solar energy, being responsible for the green portion of the water. Now, a study has modeled how the color of the oceans will be throughout the century as the phytoplankton go. With warming, the seas will remain blue or green, but with new tonalities. And the change of color, indicates a whole chain of changes in marine life.

The sea is blue because it reflects blue light. When the sun's rays hit the water molecules, and there are many in the sea, most of the light's spectrum (the rainbow in which it decays) is absorbed. Only the blue band (around 443 nanometers, nm, of the radiation) bounces and, as with the sky, the sea looks blue. But it is not a pure color, in fact everything is shades of blue to greenish, with turquoise in between. And in the sea there is not only water, there are also plants, microorganisms and organic matter that give it its color palette.

The phytoplankton was until recently a conglomerate of microscopic algae that, like the rest of plants, have a green pigment, chlorophyll, to carry out photosynthesis. And this makes the light that reflects most green, hence the greenish tones of many parts of the seas. Although now biologists have complicated things and in that conglomerate there would also be cyanobacteria and protists, all these microorganisms live on the energy they get from light and solar and, to synthesize it, they also use chlorophyll, reinforcing the green tones. For a few decades, observation from satellites has served to infer the presence of chlorophyll, as an indicator of marine biodiversity.

The satellites use the color of the sea to infer the concentration of chlorophyll and, therefore, of phytoplankton

Now, a group of researchers from universities in the US and Europe have modeled how climate change is affecting phytoplankton and, therefore, the color of the sea. Most of the global warming is being absorbed by the oceans. It is estimated that, if nothing is done to reduce CO emissionstwo, the global average temperature of the marine surface increases by 3º by the end of the century. If so, there would be a series of impacts on the cycle of the ocean life basis, phytoplankton. Well, they would already be producing.

"The warming of the oceans alters the oceanic circulation and the portion [de aguas] of the deep ocean that emerges to the surface. Phytoplankton need light (its source of energy) and nutrients. And most of those nutrients come from the depths, "explains a researcher at the Massachusetts Institute of Technology (MIT) and the study's lead author. Stephanie Dutkiewicz. "The changes induced by the warming are causing less nutrients to reach the surface layer, so it is likely that phytoplankton will decrease in many parts of the ocean," adds this expert in the biogeochemistry of the sea.

One of the biogeochemical processes most affected by climate change is that of ocean circulation: taking advantage of temperature differences, the waters move both vertically (downward) and in latitude (towards and from the poles). With heating, this circulation is slowing down, the stratification of the water column increases and the mixing of deep and shallow water is reduced. All this explains that the contribution of nutrients, in particular the macronutrients, is being reduced.

The oceans will remain blue, although there will be variations in the tone between blue, turquoise and green

"Temperatures also affect how fast phytoplankton grows, some species adapted to hot water do it faster than others adapted to the colder ones, so with a warmer ocean in regions where there are more nutrients, some more can increase the amount of phytoplankton, "recalls Dutkiewicz. So there will be regional changes in the composition, quantity and distribution of communities of marine microorganisms, which color the water.

According to the results of the study, published in Nature Communications, a good part of the ocean is already changing color and, by 2100, they estimate that even more than 50% of the marine surface could have another color. "The changes will be very subtle, the human eye probably will not see them, but the optical sensors will," clarifies the MIT researcher. "Yes, the sea will remain blue, some regions, large areas to the north and south of the equator, the subtropical turns, will possibly be even blue," he adds. Meanwhile, green will become more present in polar waters and in tropical coastal waters where phytoplankton carry heat better.

The model they have used to study the evolution of color has been used to predict changes in phytoplankton, such as local algae explosions, or ocean acidification. Now, in the parameters they have included, they have added other elements present in the water, in addition to chlorophyll. In particular, detritus and other dissolved organic matter. They recognize, however, that in order to better match the color of the sea in the future, other microscopic constituents of seawater must be included, such as bacteria, minerals or the sea's own salinity.

The marine biologist at the University of California at Irvine, Jefferson Keith Moore, published last year a study in the magazine Science on the effects of climate change on phytoplankton and the overall consequences of its reduction. He also published it summarized as an article published on the website of the World Economic Forum, The plants of the sea, as it calls to the phytoplankton, need, besides sun, nutrients like nitrogen or phosphorus. Without ocean circulation slowed by global warming, these nutrients will not reach the surface. Although the study is placed in the somewhat distant temporal scenario (year 2300), their results show that, having fewer plants, there will be less zooplankton (microscopic animals) than can feed small fish, which will reduce their populations, putting them in trouble. Larger predators, dolphins, sharks or humans. And everything will start with a change in the tone of the blue of the sea.


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