Bad news for oceans: Food chain might be at greater risk

For more than 50 years, scientists thought they knew why ocean phytoplankton — tiny plants that form the foundation for the entire marine food chain — appeared to bloom wildly in the spring.

Not only have they been wrong, new research suggests, but the real cause of phytoplankton blooms comes with a disturbing implication, one that suggests oceans warmed by climate change could end up being far less productive in years to come.

“The old theory made common sense and seemed to explain what people were seeing,” said Michael Behrenfeld, a botanist at Oregon State University whose new study is published in the journal Ecology. However, far more detailed, year-round data from satellites indicates that old theory is based on an incomplete picture of what really happens in the oceans.

The 1953 “critical depth hypothesis,” long a fundamental concept in ocean science, stated that phytoplankton begin blooming in the late spring and early summer because longer, brighter days warm the sea’s surface. Because that warm layer floats above colder water, the theory went, less water is mixed vertically by the wind and phytoplankton can thrive at the surface.

Only that’s not what happens, Behrenfeld has found. Considered one of the world’s leading experts in using remote sensing technology to examine ocean productivity, Behrenfeld analysed nine years’ worth of satellite data to determine that the phytoplankton population actually begins growing rapidly in mid-winter when conditions are still cold and dark.

The population booster for phytoplankton, Behrenfeld says, isn’t a top layer of warm sunny water but, rather, the fact that winter storms churn up the seas and mix the top layers with cold, near-lifeless water from below. That dilution helps put a greater distance between the phytoplankton and the zooplankton — tiny marine animals — that eat them, allowing the microscopic plants’ numbers to grow.

After the storms of winter come to an end, the phytoplankton do benefit from the warmer, sunny waters. However, they’re also once again in closer proximity to the predator zooplankton, which continue munching away at the same rates they did in colder weather.

“To understand phytoplankton abundance, we’ve been paying way too much attention to phytoplankton growth and way too little attention to loss rates, particularly consumption by zooplankton,” Behrenfeld said. “When zooplankton are abundant and can find food, they eat phytoplankton almost as fast as it grows.”

The summertime phytoplankton blooms end with a crash when the nutrients they need run out and the remaining populations are devoured by zooplankton.

Behrenfeld calls the process the “dilution-recoupling hypothesis.” And, if it’s right, it could mean we’ll start seeing far fewer fish and other food sources coming from the oceans as the planet keeps warming.

Where the old phytoplankton theory suggested ocean productivity would increase as the water got warmer, the new theory points to the opposite result. More stratified, warmer ocean water would mean less wintertime mixing … which, according to the dilution-recoupling hypothesis means less phytoplankton and less food for everybody up the marine food chain.

And that could be a big problem, considering that some of the oceans regions with large seasonal phytoplankton blooms are also among the world’s most productive fisheries.

Knowing for sure what future fishery production might be like will require more study and research Behrenfeld said. But anyone who’s been basing their predictions on the old theory of why phytoplankton bloom will have to re-start from square one.