The color of the ocean is changing

W whichever way you look at it, the green still wins….
Jan Brzechwa Flea Shahrayka

Our planet is called blue. These are the oceans, covering 70 percent of the its surface, give the Earth, viewed from space, that color. But satellite observations also show that the color of the ocean is changing. As researchers report in a paper published in Nature [1], over the past two decades, more than 56 percent of the ocean surface has changed color to an extent that is beyond the norm due to natural processes. This is an area larger than the area of all land. But, after all, it is not the color of the water itself that is at issue here, but the changes taking place in these ecosystems almost before our eyes. Could the mythical big blue soon turn into a big emerald?

Different colors of water

We usually perceive the color of the ocean as blue because water acts as a filter for sunlight, absorbing electromagnetic radiation from the range of visible light corresponding to the color red (red light is absorbed by water about 100 times more strongly than blue light). A different color of water can be caused by contaminants such as organic waste and humic substances, industrial effluents or particles from soil erosion. The presence of iron salts makes the water green-blue, iron and manganese – yellow to brown, sulfur – blue, hydrogen sulfide – emerald. The green color of the water usually indicates the presence of phytoplankton.

Scientists have long pointed out that today’s oceans look different than they did just a few decades ago, which is interpreted as an effect of climate change. One of the most visible manifestations of this phenomenon is the change in their color. As experts note, waters in lower latitudes – close to the equator – are becoming greener, while other areas – especially those farther from the equator – are taking on a bluer hue as temperatures rise.

An ocean greener

Changes in the color of the oceans are so discrete that they are difficult to pick up with the naked eye. However, the growing use of hyperspectral optical data has significantly improved observational capabilities, and thus our understanding of marine ecosystem processes. Scientists led by oceanographer Dr. B.B. Cael have used data from NASA satellites to analyze color changes in Earth’s oceans. Their analysis was based on 20 years of data (from 2002-2022), acquired from the MODIS spectrophotometer aboard the Aqua satellite.

MODIS measures in seven visible wavelengths, a more complete color spectrum than those captured in earlier studies based on computer models. Statistically validated analysis results indicate that over the past two decades, more than 56 percent of the oceans have changed color to an extent that far exceeds natural processes [1].

Based on satellite data, the scientists created a simulation model that allowed them to develop two scenarios of Earth’s history – one without climate change, the other with it. A comparison of these versions showed that the color changes in the oceans are consistent with climate simulations. This allows them to be directly linked to each other. Interestingly, these trends were not linked to changes in water surface temperature, but were due to other factors, such as changes in the depth of stratification and the thickness of mixed layers in the upper ocean zones. They are known to affect the taxonomic structure and biomass of plankton.

Phytoplankton is to blame for everything

Although the exact causes of the changes taking place in the oceans are not yet clearly identified, scientists agree that phytoplankton, which is a major component of the marine food web, plays a huge role. Different phytoplankton taxa use distinctive combinations of pigments for photosynthesis, which absorb light at different wavelengths, producing different reflections. The fact that the color of the oceans near the equator has become decidedly greener, according to scientists, is due to the dominance of small phytoplankton, which are better able to cope with the reduced availability of nutrients due to marine heat waves at low latitudes. Fluctuations in food availability affect the taxonomic composition of the phytoplankton assemblage, which in turn causes the water to change color.

The huge scale of changes in chlorophyll concentrations in the ocean is also indicated by the European State of the Climate (ESOTC) report, published in April 2024. by the EU’s Copernicus Climate Change Service (C3S). He revealed that in April 2023. The concentration of this dye in the Norwegian Sea and North Atlantic was 200-500 percent. higher than the 1998-2020 average, while west of the Iberian Peninsula by 60-80 percent. lower (compared to results from the same time frame). In the Mediterranean Sea in May and June 2023. Chlorophyll levels exceeded the multi-year average by 50-100 percent.

However, the change in water color can also be due to other factors, such as an increase in zooplankton density or dissolved organic matter. Regardless of the immediate cause, any change in the optical properties of water changes the light conditions, and these in turn affect phytoplankton assemblages and the functioning of the entire ecosystem.

The Baltic is also green

Satellites are providing plenty of evidence of how phytoplankton assemblages are changing the color of surface waters. July 2018. NASA Earth Observatory (NASA) satellite images circulated in the media, showing a spectacular image of a spiral bloom of cyanobacteria in the Gulf of Finland in the Baltic Sea. The mass of microscopic organisms was captured in a large eddy created by the collision of two opposing water currents.

According to NASA, the spiral at its widest point was about 25 kilometers in diameter and was part of a much larger algal bloom that covered large areas of the Gulf of Finland. The spectacularity of this phenomenon should not overshadow its consequences for the ecosystem – the mass of microorganisms has contributed to the formation of a vast area deprived of oxygen, the so-called “oxygen deprivation”. dead zone. The accumulation of a large mass of algae near the surface cuts off light access to the deeper layers and causes a decrease in their oxygenation. This leads to the death of many organisms, whose decomposition further depletes oxygen, exacerbating the phenomenon.

As plankton blooms expand their range, dead zones also expand. According to NASA’s Earth Observatory, the dead zone in the Gulf of Finland observed in 2018 covered an area of approx. 70,000. ^km2, which is twice the area of the Mazovia province. Baltic bloom captured in 2019. covered 200,000. ^km2, which is only a little less than half the area of Sweden. NASA satellites document large cyanobacteria blooms in the Baltic Sea basically every year in late summer. Research conducted in 2023. showed that between 2003 and 2020, the average size of algal blooms increased by 13 percent worldwide.

In this region of the sea, phytoplankton organisms naturally flourish every summer when vertical water movements bring nutrients stored in lower layers to the surface. However, in recent decades there has been an increase in the frequency and extent of these phenomena as a result of the intensification of nutrient supply from human activities, such as agricultural runoff. Dead zones are not only getting bigger and bigger, they are also becoming more destructive. Survey conducted in 2018. showed that over the past century, oxygen levels in the Baltic Sea have fallen to their lowest in 1,500 years. Thanks in part to record sea surface temperatures last year as a result of El Niño, we can expect an even greater expansion of algal blooms around the world this summer.

Arctic is also changing under the influence of offshore heat waves

As experts note, while in lower latitudes, close to the equator, the waters are becoming greener, at higher latitudes their blue hue is intensifying.

Field observations and modeling suggest that marine heatwaves contribute to this phenomenon, causing a decrease in primary productivity at lower latitudes and an increase at higher latitudes [2]. These opposing patterns are strongly linked to regional levels of water abundance, but also to changes in nutrient and light availability. In the Arctic, the number of marine heat waves has increased significantly in recent decades, and their effects are expected to increase even more as temperatures rise and the thickness and area of the ice sheet decreases.

Why does the color of the ocean matter?

In a warming climate, extremes such as marine heat waves are becoming more frequent, more intense and lasting longer. Their effects on marine ecosystems include changes in the composition of biological assemblages, mass mortality of species with low tolerance to elevated temperatures, and a decline in biodiversity. Changes in the optical properties of ocean waters have potential consequences both for phytoplankton and their role in marine biogeochemical cycles (and thus in ocean carbon storage) and for their consumers at higher trophic levels, including ichthyofauna communities.

Scientists predict that as temperatures rise, phytoplankton will move northward at about 35 km/decade. This will lead to changes in the distribution of zooplankton that feed on it, and consequently fish. Species richness is expected to decrease dramatically in tropical areas, while it will increase significantly in temperate and subpolar waters. This will lead to a domino effect in marine trophic networks. People will also be affected, as it involves the reduction or loss of ecosystem services, including, among others. with a decline in productivity and even the disappearance of regional fisheries.

Blue areas of the open ocean will not change color overnight, but the changes taking place in them reveal trends that may intensify as temperatures rise. Knowing where and in what direction the surface microbial ecosystem is changing can be helpful in identifying regions of the open ocean where marine protected areas should be established.

Photo. main: Nasa/earthobservatory.nasa.gov

In the article, I used, among others. From the works:
[1] Cael B.B., Bisson K., Boss E. et al. (2023). Global climate-change trends detected in indicators of ocean ecology. Nature 619, 551-554. https://doi.org/10.1038/s41586-023-06321-z
[2] Wolf K.K.E., Hoppe C.J.M., Rehder L. et al. (2024). Heatwave responses of Arctic phytoplankton communities are driven by combined impacts of warming and cooling. Sci. Adv. 10, DOI: 10.1126/sciadv.adl5904