Spring has sprung. Daffodils are blooming, birds are building nests and my kids are putting away the winter PJs. (I put a stop to that). I know, with colder temperatures threatening much of the northern US, it’s hard to believe it, but spring officially started last Sunday, with the vernal equinox.
Spring means new growth as plants respond to warmer temperatures and higher light levels. That’s true in the ocean, as well. In the spring, the population of phytoplankton (microscope algae) in surface waters grows rapidly, creating patches of green so large they can be seen in satellite photos. Unlike land plants, however, the algae aren’t just sitting there waiting for the sun’s warmth to return – in the ocean, there’s nowhere to sit! So what happens?
Springs Means Green – Even in the Ocean
When water is heated from above, it becomes stratified – separated into layers, with lighter warm surface waters floating above denser, cold water. A boundary forms between the warmer and colder waters, called the thermocline. This prevents mixing between the water layers. (If you’ve ever gone swimming in a deep lake or pond, you’ve probably experienced this effect, when the water around your feet is colder than at the surface).
The classic explanation for the spring bloom was that the thermocline allows phytoplankton to remain in the upper water levels, where there is enough light for them to grow and create large blooms. As they grow, populations of the critters that feed on them, microscope animals called zooplankton, grow as well.
Eventually, in late spring/early summer, the phytoplankton eat all the food in the upper layer, their population growth slows, and the zooplankton eat them at roughly the same rate as they grow. (So, even though the growing and eating is still going on, the big green patch disappears from satellite photos). Newer research* suggests that the phytoplankton begin to bloom during the winter, and that stratification only concentrates them at the surface where our satellites can see them – and zooplankton can find them and eat them.
Marine Food Web
Phytoplankton are the primary producers in the marine food web – they capture energy from the sun. Zooplankton feed on the phytoplankton, and are themselves food for immature fish, shrimp, small fish such as sardines and herring, and even huge creatures like blue whales and North Atlantic right whales (who feed on tiny shrimp-like krill and copepods, respectively). Small fish feed bigger fish and so on – right onto our plates.
Just like on land, recent studies** of satellite data suggest that climate change is causing the spring bloom to occur earlier in parts of the ocean, like the Arctic. While that may seem like a good thing (who doesn’t want winter to end sooner?), it’s not clear if the creatures that feed on phytoplankton will be able to hatch earlier in response. If the phytoplankton use up all the nutrients and begin to die sooner, before the zooplankton can hatch and eat them, then all the other creatures that depend on this annual event may suffer.
Scientists are watching the spring bloom closely, monitoring fish populations throughout the Arctic region, to see what will happen.
In spite of the impending cold front, I’ve been out in my garden, planting peas and potatoes, and harvesting greens from seeds that I put out in the late summer and fall, (and didn’t get around to harvesting during the winter). How about you? What does activity does the bloomin’ spring inspire in you? Gardening? Spring cleaning? A trip to the beach? Let us know in the comments below!
* Behrenfeld et al. Abandoning Sverdrup’s Critical Depth Hypothesis on phytoplankton blooms. Ecology, 2010; 91 (4): 977 DOI: 10.1890/09-1207.1
** M. Kahru, V. Brotas, M. Manzano-Sarabia, B. G. Mitchell. Are phytoplankton blooms occurring earlier in the Arctic? Global Change Biology, 2010; DOI: 10.1111/j.1365-2486.2010.02312.x
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** LIVE WEBCAST – Clearing the Air: Managing Air Quality to Benefit Heath and Climate in India.
Dr. Sarath Guttikinda, an air quality expert from Delhi, India; Dr. William Lau, a climate scientist from NASA; and yours truly will discuss the links between air quality and climate in India, and what can be done to improve both. The event is free, open to the public, and will be webcast by Johns Hopkins University.
For more information, click here.
Danielle Meitiv is an oceanographer by training, an advocate for all things marine and a writer of science fiction and non-fiction. Danielle is also a huge fan and sales affiliate of Holly Lisle’s online courses: How to Think Sideways: Career Survival School for Writers, and How to Revise Your Novel. Follow @Danielle_Meitiv on Twitter, and on Facebook: Brave Blue Words, and Danielle Meitiv.