plankton trapped at front
carbon cycle

February 2012: Plankton.

Seeing plankton

In “Life on the Dock” (LOTD) page 94, I said that “The only practical way to study this group (the plankton) is to pull a net through the water and look at the trapped animals”. This is true for you and me, but if you have the right equipment you actually can look at plankton, e.g. copepods, as they swim and drift around. Davis et al. [1] describe the use of a towed video microscope. The camera has some advantages over a net. They could see delicate structures, e.g. copepods clutching eggs, marine snow (fecal pellets), fragile gelatinous tunicates (copepod predators). They also observed that the bodies of most floating copepods are vertical, with the head down. You couldn’t see all of this by catching them in a net.

Plankton at fronts

A more esoteric way to “see” plankton, e.g. copepods, is to use very high frequency (short wavelength) sonar. This is even more powerful than video microscopy for following the movement of copepods, because they can be tracked over a relatively large 3D space (hundreds of meters). This approach was used by Genin et al. [2] to document the concentration of copepods at water fronts.

A water front (seen on the surfac)e is a line where two regions of water with different densities (due to temperature or salinity) meet. The water on each side of the front usually moves (converges) toward the front, and then plunges down. Thus a front is actually an underwater surface. Floating debris and foam on the water surface is swept to the front and trapped along the line of convergence because it can’t sink with the water. The debris, foam, and a line of turbulence if the currents are swift, make it easy to spot vigorous fronts. Fronts with lower water velocities are often seen as a band of smooth water (a slick). This is the result of oils and fats (or molecules that have an oil-like part), which concentrate on the surface, being swept to the front. These molecules decrease the surface tension of the water and suppress the formation of waves; thus the slick.

Copepods attempt to maintain a constant depth, closer to the surface at night and deeper during daytime. When copepods are swept to a front they swim against the downward current and thus are concentrated on both sides of the front. The copepods attract small fish that prey on them, which in turn attracts larger fish that feed on the small ones. The fish attract water birds, which thus are often seen along a front. Good fishermen learn to recognize fronts and remember the locations and conditions that generate them.

Often there is a front close (a hundred meters) to my Dock. That is due to a swift tidal current across the mouth of the small bay in which my Dock is located. The current causes water in the bay to rotate, and warm water from the interior of the bay is pushed next to cold water from the ocean. Your dock may well be different. I suspect that most floating docks are not close to fronts.

In the open ocean phytoplankton growth is limited by minerals

As I say in the book (pg 8), around our Dock there is plenty of nutrients in the water for phytoplankton to grow because we are in an estuary surrounded by people that dump trash (processed sewage, fertilizer) into the water. Far away from people, on the surface of the open ocean, phytoplankton have usually incorporated most of the minerals. There is always some residual phytoplankton growth which can rapidly increase if water currents bring minerals up to the surface. Mullineaux [3] has written a useful summary of phytoplankton growth, and its importance to planet earth.

Dump iron into the ocean to stop global warming?

Behrenfeld et al. [4] present evidence that dissolved iron is the major limiting nutrient in the open ocean (as I mention on pg 8 of LOTD). This claim is the basis for one suggested remedy for global warming caused by the increasing CO2 concentration in the atmosphere. The idea is that by fertilizing the ocean with millions of tons of iron we could increase the growth of phytoplankton which would in turn consume the CO2 humans discharge into the atmosphere. However, it is not accepted universally that iron is the limiting nutrient, or that if supplied by humans, phytoplankton growth wouldn’t then be quickly limited by other nutrients, e.g. nitrogen.

This is a complicated issue, but one point is obvious. For carbon to be removed from the atmosphere, a large fraction of the phytoplankton must sink to the bottom of the ocean where they are effectively sequestered for thousands or millions of years. Otherwise, the phytoplankton are eaten by zooplankton which then convert the carbon back to CO2 which diffuses back to the atmosphere. This is the normal carbon cycle.

The last three references describe the vertical transport of minerals, especially silicon, in the oceans. Remember that on pg 17 of LOTD I mention that the shells of diatoms are made of silicon, and diatoms represent the majority of phytoplankton.

What does all this have to do with the Dock?

It’s only related indirectly. I just assume that if you are interested in the Dock, you may be interested in the bay and ocean that it is connected to. The water is one world after all.


1. Microaggregations of oceanic plankton observed by towed video microscopy; Davis et al., Science v257,p230 (1992)

2. Swimming against the flow: a mechanism of zooplankton aggregation; Genin et al., Science v308, p860 (2005)

3. The plankton and the planet; Mullineaux, Science v283, p801 (1999)

4. Widespread iron limitation of phytoplankton in the S. Pacific ocean; Behrenfeld et al., Science v283, p840 (1999)

5. Mesoscale eddies drive increased silica export in the subtropical Pacific Ocean; Benitez-Nelson et al., Science v316, p1017 (2007)

6. Eddy/wind interactions stimulate extraordinary mid-ocean plankton blooms; McGillicuddy et al., Science v316, p1021 (2007)

7. Comment on “Eddy/Wind Interactions stimulate extraordinary mid-ocean plankton blooms; Mahadevan et al., Science v320, p448b (2008)

Access to articles in Science via the Internet: The present policy is that the abstracts of all articles are free, and the entire contents are free if the article has been published more than one year earlier, but not before 1997. Why the 1997 limit? Sounds crazy to me, but I don’t have any influence with Science.