Findings overturn old theory of phytoplankton growth, raise concerns for ocean productivity

Findings overturn old theory of phytoplankton growth, raise concerns for ocean productivity
A north Atlantic Ocean view of the spring phytoplankton bloom, as seen from a satellite image. (Image courtesy of Oregon State University)

A new study concludes that an old, fundamental and widely accepted theory of how and why phytoplankton bloom in the oceans is incorrect.

The findings challenge more than 50 years of conventional wisdom about the growth of phytoplankton, which are the ultimate basis for almost all ocean life and major fisheries. And they also raise concerns that global warming, rather than stimulating ocean productivity, may actually curtail it in some places.

This analysis was published in the journal Ecology by Michael Behrenfeld, a professor of botany at Oregon State University, and one of the world's leading experts in the use of remote sensing technology to examine ocean productivity. The study was supported by NASA.

The new research concludes that a theory first developed in 1953 called the "critical depth hypothesis" offers an incomplete and inaccurate explanation for summer phytoplankton blooms that have been observed since the 1800s in the North Atlantic Ocean. These blooms provide the basis for one of the world's most productive fisheries.

"The old theory made common sense and seemed to explain what people were seeing," Behrenfeld said.

"It was based on the best science and data that were available at the time, most of which was obtained during the calmer seasons of late spring and early summer," he said. "But now we have satellite remote sensing technology that provides us with a much more comprehensive view of the oceans on literally a daily basis. And those data strongly contradict the critical depth hypothesis."

That hypothesis, commonly found in oceanographic textbooks, stated that phytoplankton bloom in temperate oceans in the spring because of improving light conditions - longer and brighter days - and warming of the surface layer. Warm water is less dense than cold water, so springtime warming creates a surface layer that essentially "floats" on top of the cold water below, slows wind-driven mixing and holds the phytoplankton in the sunlit upper layer more of the time, letting them grow faster.

There's a problem: a nine-year analysis of satellite records of chlorophyll and carbon data indicate that this long-held hypothesis is not true. The rate of phytoplankton accumulation actually begins to surge during the middle of winter, the coldest, darkest time of year.

The fundamental flaw of the previous theory, Behrenfeld said, is that it didn't adequately account for seasonal changes in the activity of the zooplankton - very tiny marine animals - in particular their feeding rate on the phytoplankton.

"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 new theory that Behrenfeld has developed, called the "dilution-recoupling hypothesis," suggests that the spring bloom depends on processes occurring earlier in the fall and winter. As winter storms become more frequent and intense, the biologically rich surface layer mixes with cold, almost clear and lifeless water from deeper levels. This dilutes the concentration of phytoplankton and zooplankton, making it more difficult for the zooplankton to find the phytoplankton and eat them; thus more phytoplankton survive and populations begin to increase during the dark, cold days of winter.

In the spring, storms subside and the phytoplankton and zooplankton are no longer regularly diluted. Zooplankton find their prey more easily as the concentration of phytoplankton rises. So even though the phytoplankton get more light and their growth rate increases, the voracious feeding of the zooplankton keeps them largely in-check, and the overall rise in phytoplankton occurs at roughly the same rate from winter to late spring. Eventually in mid-summer, the phytoplankton run out of nutrients and the now abundant zooplankton easily overtake them, and the bloom ends with a rapid crash.

The processes at work there are somewhat similar to the importance of wind-driven upwelling in Pacific Northwest ocean waters, which brings up nutrients and enhances phytoplankton growth. However, in the north Atlantic Ocean these other, complex processes are also at work, including the dilution of phytoplankton and the ultimate control of them by aggressive zooplankton grazing.

"What the satellite data appear to be telling us is that the physical mixing of water has as much or more to do with the success of the bloom as does the rate of phytoplankton photosynthesis," Behrenfeld said. "Big blooms appear to require deeper wintertime mixing."

That's a concern, he said, because with further global warming, many ocean regions are expected to become warmer and more stratified. In places where this process is operating - which includes the North Atlantic, western North Pacific, and Southern Ocean around Antarctica - that could lead to lower phytoplankton growth and less overall ocean productivity, less life in the oceans. These forces also affect carbon balances in the oceans, and an accurate understanding of them is needed for use in global climate models.

Worth noting, Behrenfeld said, is that some of these regions with large seasonal blooms are among the world's most dynamic fisheries.

The critical depth hypothesis would suggest that a warmer climate would increase productivity. Behrenfeld's new hypothesis suggests the opposite.

Behrenfeld said that oceans are very complex, water mixing and currents can be affected by various forces, and more research and observation will be needed to fully understand potential future impacts. However, some oceanographers will need to go back to the drawing board.

"With the satellite record of net population growth rates in the North Atlantic, we can now dismiss the critical depth hypothesis as a valid explanation for bloom initiation," he wrote in the report.


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Image: Phytoplankton Bloom in the North Atlantic

More information: Paper: ir.library.oregonstate.edu/jspui/handle/1957/16618
Citation: Findings overturn old theory of phytoplankton growth, raise concerns for ocean productivity (2010, July 16) retrieved 16 June 2019 from https://phys.org/news/2010-07-overturn-theory-phytoplankton-growth-ocean.html
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Jul 16, 2010
They need to make up thier minds. One guy says that global warming is going to end the world because of increased frequency and magnitude of storms, and the other guy says that global warming is going to end the world because the seas will not get mixed up enough.

Jul 16, 2010
They need to make up thier minds. One guy says that global warming is going to end the world because of increased frequency and magnitude of storms, and the other guy says that global warming is going to end the world because the seas will not get mixed up enough.

Neither is saying that global warming will end the world. Both are saying that global warming will really put a damper on how good a chance you and I stand on being able to live here if it continues.

You're fighting a two headed monster, not two seperate monsters.

Jul 16, 2010
You know what I mean about "the end of the earth". Perhaps I should have said that they are predicting that the sky will fall. It was a figure of speech.

They are proposing off-setting effects. You can't have rougher seas and still have increased stratification. You have to pick one because supposed rough seas would counter the supposed increase in stratification. One of those two factors has to over-power the other, so you really have to pick which one you think is stronger in regard to plankton blooms. It should only take one good storm to really mix up the layers well, and the storms in the north atlantic can be REALLY good.

And if you don't like that arguement then ptbtbtbtb. (raspberry sound with tounge out)

Jul 16, 2010
They are proposing off-setting effects. You can't have rougher seas and still have increased stratification. You have to pick one because supposed rough seas would counter the supposed increase in stratification. One of those two factors has to over-power the other, so you really have to pick which one you think is stronger in regard to plankton blooms. It should only take one good storm to really mix up the layers well, and the storms in the north atlantic can be REALLY good.
Well that's not really accurate either. Some of the most stratified waters are found at the poles, consequently this is also where some of the most powerful storms originate. The Currents and local geography hold great sway in this regard.

There are a great many more factors, and as we both know, local weather, and oceanic climate can be entirely different when sampled at differing locations.

Jul 16, 2010
Yes, I over-simplified it to increase the appearance of irony. The two effects are somewhat opposing though. The degree of opposition is debateable and hard to quantify, but wave action and the inversion layer created by cold precipitation would increase mixing if storm action increases. And the bulk of the storm season just happens to coincide with the fall/winter time period this guy is talking about as being critical. All kidding aside, if storm action increases, then mixing has to increase. I seriously think that the increase in stratification is questionable.

In fact you may get a feedback loop effect, where the mixing increases, scattering the phytoplankton, then increased stratification helps to protect them from zooplankton, leading to even bigger blooms.

In the end they can say whatever they want because it's untestable in any practical way.

Jul 16, 2010
Well also don't forget the depths we're talking about. Coastal waters are heavily mixed by storms, however, the type of mixing and stratification spoken of within this article is far deeper. phytoplankton utilize minerals and nutrients in abundance on the sea floor. Once you leave the continental shelves we're talking miles upon miles of water and great pressure that must be overcome in order to provide sufficient mixing. As we both agree, the geographic attributes of the locality are certainly at play when determining how this information can be applied.

This may be why you have two gentlemen providing what appear to be disparate statements.

Jul 16, 2010
The only part of the ocean we're talking about here is the photic zone. That's about 10-100 meters depth in open ocean, with the shallower portion of that being most important. That's well within the range of storm action. That's where almost all the algea and plankton grow.

By the way, this dispute isn't new. They have been arguing over the critical depth theory for more than a decade. The original arguements didn't involve CO2 though.

Jul 16, 2010
The only part of the ocean we're talking about here is the photic zone. That's about 10-100 meters depth in open ocean, with the shallower portion of that being most important. That's well within the range of storm action. That's where almost all the algea and plankton grow.


@g-
No, they're talking about the N.Atlantic and N Pacific basins, not just the top 100 meters. Both areas are quite complex in terms of ocean floor/coastal geomorphology, and due to upwelling/stratification/evaporation/coriolis effect/storm activity, there is considerable overturn/mixing that occurs during the winter storm season in the north end of both oceans, and around Antarctica. And then there is the mid/low latitude summer/fall storm(hurricane/cyclone) season, primary effects of which are upon the western verges of the basins, after they move from over the ocean in the lower latitudes of formation.
Most of their mixing effects are felt coastally, and at shallow ocean depths.

Jul 16, 2010
Wow! It's really complicated. Maybe we should have some scientists do measurements and try to quantify the interactions, instead of playing "more and more of less and less" rhetorical games.

Jul 17, 2010
They need to make up thier minds. One guy says that global warming is going to end the world because of increased frequency and magnitude of storms, and the other guy says that global warming is going to end the world because the seas will not get mixed up enough.

Science is not about "making minds" it's about learning things and then relearning and reevaluating what you have learned. Scientific discoveries are more of a process not a stance. However most can be backed up repeatedly, even proven.

Jul 17, 2010
I think we will never get there with an ever increasing complex model.

Maybe it is time to figure out a simple model that predicts it. (Einstein like people please stand up)

if we would still be apes it is safe to say the the world wouldn't have changed that much compared with those times we were apes.
So how stable is a complex world, how hard do we have to push to make it jump to an other stable state.
We are part of that complex world a predefined life cycle does sound very likely for this universe.

Jul 17, 2010
there is no "end of the world". What everyone refers to with this is an end to our species reign on this planet. Frankly, it happens, it's well deserved. We are too frivilous and wasteful with the resources we've had. Had we spend less time arguing over shiny rocks, whose ancient fiction is the Truth, and imaginary lines on the ground and more time bettering ourselves, perhaps we could have established ourselves in a more sustainable, and extra-planetary way by now.

Jul 17, 2010
GSwift7 said: "In the end they can say whatever they want because it's untestable in any practical way."


Well, you are totally incorrect there, too. They are very testable, and Mother Nature is performing the test at this very moment. What we need to wish for is that both hypotheses are wrong (ie, that global warming will not knock us off). But that possibility seems less and less likely, as we learn more and more.

tkjtkj@gmail.com

Jul 17, 2010

Kedas,

Sorry dude sometimes there are no simple answers and the worst thing you can do is to continue to demand them.

Philosothink,

The human species is not under direct threat from warming. Millions of other species are but we are not. Its more a matter of what kind of world you want to live in and how much economic and political disruption you are willing to tolerate.


Jul 18, 2010
Kedas, are you aware that heat is the number one weather related killer already? And it's just going to get warmer...

Jul 18, 2010
ppnlppnl,

I didn't say make the answers simple, I said make the model simple. if every new bit of data put your model upside down then it is time to start over.
It's like programming if your code is starting to look like a collection of exceptions it is time to start over from scratch with a new design.
And the best most ingenious models or simple ones.

A simple model doesn't mean that the answer is simple they even usually give surprising answers.

Jul 18, 2010
sorry it took me so long to reply Caliban. You can look up critical depth theory and phytoplankton in a lot of places. The photic zone is defined as the part of the ocean from 10 meters depth down to the point where sunlight decreases to about 1% of surface strength. That comes to about 100 meters in most open ocean. The part where the blooms discussed in this article happen is in the top part of the photic zone, and most concentrated near the surface where the phytoplankton get the sunlight for photosynthesis. You can easily look all that up like I had to. Are you really under the impression that water from the bottom of the ocean makes its way to the surface in the time span from fall to spring? Deep ocean water is sometimes called fossile water because it takes millenia for it to resurface. You can look that up too, if you want.

Phytoplankton are plants. They don't do well in the near total darkness below 100 meters.

Jul 24, 2010
OMG! It's worse than they thought!!!

Another pile of steaming BS from the brothers at AGW Central.

Anyway what's the big deal, there won't be any fish in the Oceans in ten years anyway, let alone warming rising water, acidic water, no plankton and whatever else they're selling this week!

Jul 25, 2010
Phytoplankton are plants. They don't do well in the near total darkness below 100 meters.


@G
Yeah, I'm well aware that mixing occurs PRIMARILY in the photic zone, but that is not, by any means the ONLY layer of mixing.
The seafloor topography of the N. Atlantic, NW Pacific, and Antarctic is quite variable and complex, and you may be assured that Mixing DOES occur from depth to surface, at various places, various times, and under various circumstances. Most of the mixing occurs in the coastal upwelling zones -but not all. My point was that the system is more complex than you depicted it.

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