(*This brief page was originally posted early in 2000, late in 2002 evidence from NOAA/NASA confirms a global declining trend in marine phytoplankton - see: )

What is Plankton?

bowhead whales

Plankton refers to the whole assortment of tiny, often microscopic, living things in the ocean. There are two main types: the tiny plants (“phytoplankton”) and the tiny animals (“zooplankton”). (I believe there are also a few ambiguous forms that are hard to classify.) The more plankton in the water, the cloudier or murkier it is - harder to see through. If there is a lot of phytoplankton in the water, it generally takes on a greenish color. Many fish and a few mammals (some whales) survive by eating plankton. It is the starting point, or “bottom” of the marine food web, and the “health” of the whole system ultimately depends on “healthy” plankton levels.

(The picture? These are NOT plankton! They are endangered bowhead whales, that feed primarily on plankton - photo NOAA)

Are Plankton levels in the ocean currently "healthy" or "normal?"

I suspect not. This is a difficult question to answer with numbers, since solid, scientific measurements of plankton levels have never been done until fairly recently (the last few decades). I was told by a marine biologist in Atlantic Canada that plankton levels are “as healthy as ever,” but I was hardly reassured since I was given no evidence to back up the statement.

Let’s take a closer look at the plankton. The relationship between the phytoplankton and the zooplankton looks like this: the little animals eat the little plants. (Of course, some of the little animals also eat little animals, but that really does not affect the argument.) Some of the little animals are actually newly hatched “larval forms” of bigger fish that will grow up and move down to live in the deeper water, no longer qualifying as “plankton.” The decline in species such as cod would result in a decline in their larval forms, so this portion of the plankton, at least, can be assumed to be diminished. But there are zooplankton that spend their whole lives as plankton, and there are also the phytoplankton. For these two types, are the numbers up or down?

The measurements of zooplankton that have been recorded in the last few decades actually show a significant decrease in that time. A 70% drop in zooplankton levels has been detected in waters off the U.S. west coast since 1950. That is horrendous. (How much they might have dropped PREVIOUSLY from “historical levels” is anybody’s guess.) So why would we assume that phytoplankton levels are “normal” or “healthy?” The little plants not only feed the little animals, they also depend on the little animals to be present to return the “fertilizer” that they need. If zooplankton levels are declining, then phytoplankton levels must be dropping as well. (The interdependence between these two is described in more detail in “the STARVING MARINE ECOSYSTEM”). How do scientists currently estimate levels of phytoplankton in the ocean? One main strategy is the analysis of recent satellite photos..."greeness" or chlorophyll content is thereby estimated, but no information about numbers of zooplankton is provided. (See phytoplankton update from NASA 2002)

Conclusions about plankton levels can be better drawn by looking at some of the other creatures in the sea, specifically those whose growth and survival depends on the presence of plankton for food. Small filter feeders like scallops are known to be growing at slower rates now, especially in areas farther from the shorelines. Large filter feeders are in SEVERE trouble (for an example, see the NORTHERN RIGHT WHALE). These trends and others indicate the presence of less plankton. whale shark photo

IF the phytoplankton level was decreased in the ocean, what other consequences would be expected? Besides the feeding consequences for the whole marine system, decreasing phytoplankton levels would have major implications for the concentrations of oxygen and carbon dioxide, both in the ocean and in the whole atmosphere of the planet. (Through the process of photosynthesis, these little plants absorb carbon dioxide and give off oxygen gas - their evolution is believed to have been the main determinant of the favorable (to us) makeup of the atmosphere.) Have there been changes in the levels of these important gases?

First, regarding the concentration of oxygen in the water (a critical thing for fish), it has been noted that levels are dropping in some places (e.g. the deeper water in the Gulf of St. Lawrence). This is suspected to be a factor in the failure of the cod to “recover,” since apparently cod eggs are very sensitive to oxygen levels and the current level seems to be approaching the range where they cannot live and hatch. That is scary, but it gets worse...

If the phytoplankton in the ocean were depleted, and their CO2-O2 gas exchanging ability was thereby decreased, a rising level of CO2 and declining level of O2 would be expected in the atmosphere. This exactly what has happened, and it has been very well documented. Rising CO2 levels are generally accepted to be the cause of “global warming.” Most people “know” this fact. But do they know that the steady upward trend in carbon dioxide levels in the air started centuries before the invention of the internal combustion engine? This means that the whole problem cannot be blamed on “burning fossil fuels.” Major amounts of FISHING were done in the centuries before the industrial revolution. I do not offer this as “proof,” I only see circumstantial evidence and merely have a strong suspicion that is it true. What do you think? Your opinions are very welcome in the guestbook.

"Framed" photo above is a whale shark, the largest shark in the world, it feeds exclusively on plankton...and is now VERY rare - photo Gregoire Phillippe.

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