Phytoplankton Decline Threatens to Worsen the Effects of Climate Change
By Anonymous
Why are Phytoplankton so Important?
Phytoplankton play a key role in ocean ecosystems. As primary producers, phytoplankton obtain energy from sunlight to convert carbon dioxide into oxygen and organic forms of carbon through photosynthesis. These organic forms of carbon can be used by other organisms and provide them nutrients. For this reason phytoplankton are drivers of marine food webs and all other organisms depend on them. They also perform a critical service to the rest of the planet by removing carbon dioxide from the atmosphere. Carbon dioxide is a greenhouse gas, meaning that it traps heat in the atmosphere and causes an overall warming of the planet. The atmosphere and the ocean interact and exchange gas at the surface of the water, so when carbon dioxide enters the ocean, phytoplankton remove some of it. This process also lowers the saturation of carbon dioxide in the ocean, allowing more carbon to enter the water to maintain equilibrium between the air and water. The ocean is a major sink for carbon, taking up about ¼ of the anthropogenic carbon emissions that would otherwise stay in the atmosphere and worsen the greenhouse effect[1].
How are Phytoplankton Expected to be Affected by Climate Change?
Changing Distribution:
Virtually all organisms are expected to be affected by climate change and the warming of the ocean. Over 90% of the ocean is expected to warm by 3 to 6° C by the end of the century[2]. Organisms are well adapted to their environments and function best at an optimal temperature, so increasing the temperature of the ocean could affect where phytoplankton species are able to live. In the tropics, the expected temperature rise exceeds the optimal temperature range for the existing species of phytoplankton. This means the current phytoplankton abundance may decrease and species may not be able to survive the new conditions. In the polar regions of the ocean, there is currently limited phytoplankton because most species cannot survive the cold waters and limited sunlight from ice. Therefore, the increase in temperature in the poles is expected to foster phytoplankton growth[3].
Oligotrophic Gyres:
Climate change may also increase the size of low productivity regions. Gyres are areas of circulation caused by wind and the earth’s rotation. Inside the gyres, the water is relatively calm. Due to gravity, nutrients tend to fall down the water column and may settle in the lower regions of the ocean. Ocean mixing is an important part of bringing nutrients back up to organisms at the surface. For this reason, gyres tend to be considered nutrient limited and low in productivity, or oligotrophic. Ocean warming is worsening this effect and causing gyres to expand in size at an accelerated rate[4]. Warming starts at the surface of the water so this creates more variability between the surface and deep waters. Warm, low salinity water tends to float and remain at the surface, while cool, salty water is more dense and stays at the bottom of the water column. This leads to stratification that prevents mixing and reduces the nutrient availability for organisms in the upper levels of the ocean. Since 1998, the gyres have been measured to expand at a rate of 0.8 to 4.3% per year. Of the high chlorophyll regions of the ocean, 0.8 million km2/yr has already been replaced with inactive, oligotrophic areas[4]. The expansion of these areas of low productivity further limits the areas that phytoplankton can occupy.
What Affects Could this Potentially Have?
If the reduction of phytoplankton due to the tropics becoming inhabitable and the expansion of the gyres is not balanced by the increase in phytoplankton growth in the poles, the net abundance of phytoplankton will decrease. This would lead to a decrease in photosynthesis rates and the ocean would not take up as much atmospheric carbon as it currently is. Of the carbon dioxide that enters the ocean, less would be removed from photosynthesis, meaning the ocean would be saturated with carbon dioxide more often. Ingassing would be prevented and the ocean will not be as effective as a carbon sink. Currently, anthropogenic emissions over 2010 to 2019 are averaging around 40 Gt/CO2 per year. The ocean takes up about 9.2 Gt/CO2 a year, which is almost a fourth of the total emissions[1]. This is an extremely large portion of carbon dioxide and greatly lessens the effects of climate change. If the ocean can no longer perform this service, the extra carbon dioxide would likely remain in the atmosphere worsening the greenhouse effect. This will accelerate the harmful effects of climate change and could be disastrous to the planet.
There is still great uncertainty around the future of phytoplankton. There are many factors to consider, such as if phytoplankton strains will be able to adapt to changing temperatures or migration patterns of plankton species. Research is currently being done to form stronger predictions about how phytoplankton will be affected by climate change and how effective phytoplankton will remain at removing carbon from the atmosphere.
[1] Friedlingstein, Pierre, et al. “Global Carbon Budget 2020.” Earth System Science Data, vol.12, no. 4, 2020, pp. 3269–3340., doi:10.5194/essd-12–3269–2020.
[2] Dutkiewicz, S., Hickman, A.E., Jahn, O. et al. Ocean colour signature of climate change. Nat Commun 10, 578 (2019). https://doi.org/10.1038/s41467-019-08457-x
[3] Thomas, M. K., et al. “A Global Pattern of Thermal Adaptation in Marine Phytoplankton.” Science, vol. 338, no.6110, 2012, pp. 1085–1088., doi:10.1126/science.1224836.
[4] Polovina, Jeffrey J., et al. “Ocean’s Least Productive Waters Are Expanding.” Geophysical Research Letters, vol. 35, no. 3, 2008, doi:10.1029/2007gl031745.