Is climate change affecting phytoplankton?
Phytoplankton—single celled green floaters—fulfil the same role in the oceans as plants do on land. They’re the basis of the food chain, capturing energy from sunlight, and eventually feeding just about everything else. So the news that they’ve declined by about 40% since 1950 (Nature News) is rather worrying. Let’s take a look at where the number came from.
The standard way of finding the amount of phytoplankton in seawater is to measure the concentration of chlorophyll, the green pigment used in photosynthesis. Essentially, you can just test how green the water is, although modern methods are a bit cleverer. Using satellites, you can even remotely measure vast areas of ocean. But people didn’t make those measurements much until the 1960s (and not with satellites until 1979). So the researchers combined them with an even simpler method, which has been done a lot since the 1930s. The ‘Secchi disk’ is lowered into the water until it can no longer be seen, giving a measure of how clear the water is. After throwing out measurements near the shore, where mud can reduce visibility, they fit pretty well with the chlorophyll measurements.
What did they find? Well, the number the media picked up on was the global average 1% decline per year. That’s one percent of current levels, so working back gives you just under a 40% drop since 1950. That average, though, hides quite a bit of variation in the actual change:
It looks like there’s a clear decline in the Atlantic ocean and the polar oceans, a smaller decline in the Pacific, while plankton actually increased in the Indian ocean. The authors go one step further, breaking it down into large grid squares. That shows still more variation, but without a clear pattern, at least to my eye.
I mentioned a link with climate change. It works like this: when you heat a pan of water on a stove, you get convection currents, as warm water rises, cools, and sinks again. But the oceans are mostly heated from the top, by sunlight, which means a layer of warm water forms, sitting on top of a huge depth of colder water. Phytoplankton can only grow near the surface, because they need sunlight, but they quickly use up the nutrients there, and then need water mixed in from the depths to grow. As a result, regions of upwelling water, such as off the coasts of Peru and Antarctica, have particularly rich sea life. The ocean currents that drive them aren’t expected to stop any time soon, but warmer temperatures at the surface could be reducing smaller scale mixing.
This isn’t just conjecture. The scientists separated the year-by-year variation in plankton levels from the overall trends, and compared that variation to various ocean ‘oscillations’. These are roughly regular patterns in temperature and pressure, the best known of which is the El Niño/Southern Oscillation in the Pacific. In most areas with oscillations, there was less plankton in warmer years (the pattern didn’t fit for the North Indian ocean, perhaps due to the effects of the monsoon rains).
I’m a bit surprised, reading the paper, that they didn’t explore any of the other things that could be affecting the plankton. There’s a brief list of possible factors, including nutrients coming from the land, ocean circulation, and the effects of other organisms in the sea, but then only the surface temperature and the resulting ‘mixed layer depth’ are given any discussion at all.
If phytoplankton are on the decline due to global warming, that’s not just bad news for the algae. As I described above, almost* everything in the oceans ultimately relies on phytoplankton. They’re also a key part of the carbon cycle, removing CO2 from the air. That leads to a positive feedback: as we release more CO2 and warm the earth, we also slow down its absorption by life in the oceans.
Boyce, D., Lewis, M., & Worm, B. (2010). Global phytoplankton decline over the past century Nature, 466 (7306), 591-596 DOI: 10.1038/nature09268
*Treasure your exceptions: some things living at hydrothermal vents can get all their energy from dissolved chemicals.