The increase in temperatures will stress many species of the planktonic community. Like corals in reef ecosystems, organisms in the open ocean have certain tolerances for heat. “A big part of the problem is that we don’t know the optimal temperature ranges for probably 99% of the organisms out there,” says Roopnarine. “We know they have them, but it’s a very difficult thing to measure.”
The sea has absorbed about 90% of the excess heat that humanity has pumped into the atmosphere, and it shows. In 2014, half of the world’s ocean surface was experiencing temperatures that were previously considered extreme, a number that increased to 57% in 2019. In other words, extreme heat has become the new normal.
Twenty years ago, we were talking about dramatic things starting to happen in 2050 and that we would have problems in 2080 or 2100,” recalls Roopnarine, “but literally every year for the last 15 years, things are happening that tell us that our models they’ve been too slow. The speed at which this has been happening, I think, is pretty amazing.”
How warming affects seawater
Heat alone is not the only concern. When the oceans warm, some things happen physically and chemically to the surface waters these organisms call home. The hotter the sea water gets, less oxygen it can hold. As the planet warms, scientists have found that oxygen levels in the oceans drop steadily, and in some cases precipitously: the loss is as much as 40% in tropical regions. This, of course, deprives the organisms of the oxygen they need to survive.
In second place, the hotter the water gets, less dense it becomes. On the surface there is a strip of warm water and in the depths, colder waters, which is known as stratification. “If you’ve ever swum in a lake in the summer, at the surface the water is warm and then you go under and it cools off quickly,” explains Michael Behrenfeld, an ocean ecologist at Oregon State University, “that’s the layer of stratification that you go through,” he clarifies.
In the ocean, this warm water acts as a plug, disrupting critical ecological processes. Nutrients normally rise from the depths, providing food for phytoplankton floating on the surface. Stratification prevents it. In addition, the winds usually blow on the surface and mix the water with the depths, also providing nutrients. But with stratification, the contrast between the surface layer of warm water and the underlying layer of cold water is so strong that it is very difficult for wind energy to mix the two.
Fish may get fewer nutrients
Taken together, all this means that phytoplankton in a warmer ocean are deprived of the nutrients they need. In response, they produce less of the pigments they use to convert sunlight into energy. “The phytoplankton will decrease their photosynthetic pigments because they are becoming more nutrient-stressed,” says Behrenfeld. “They don’t need to harvest as much light because they don’t have enough nutrients to photosynthesise as much as before”; Behrenfeld can see the transformation in the satellite images.
They also reduce their pigment production due to their increased exposure to light. Without the wind mixing the water, they’re stuck in that layer of warm water on the surface for longer. Because they have access to more light, they need less pigment to do the same amount of photosynthesis.