May 16, 2011
Recognize these babies? Planktonic snook eggs.
They spend their lives drifting aimlessly, bound for wherever fate takes them. No, this isn't about the neighbor's kid and his friends. This is about plankton.
The word is taken from the Greek “planktos,” for drifting. It refers to any plant or animal that floats in, or on, the water, and is unable to swim, or swims just a little. Although most plankton are too small to be seen with the naked eye (one drop of water can contain thousands), the term can include much larger life forms, like jellyfish, and sargassum seaweed. Without plankton, and lots of it, there'd be no life in the ocean, or the lakes and rivers, and probably none on land.
Plankton can be divided into three major groups:
Phytoplankton—algae and bacteria, including diatoms, dinoflagellates, and tiny armor-plated plants called coccolithophorids.
Zooplankton—microscopic animals like isopods, copepods and marine worms.
Macrozooplankton—animals bigger than two millimeters, such as fish eggs and larvae, shrimp and jellyfish.
Phytoplankton are unicellular aquatic plants. They come in a staggering variety of shapes and configurations, from simple triangles to forms as complex as a snowflake. They're the foundations of Florida's aquatic food pyramids. Through photosynthesis, they also supply about half of the earth's oxygen supply, even more than “the lungs of the world,” the Brazilian rain forest.
One of the most common and interesting types of phytoplankton is the diatom. These are simple, single-celled plants with shells made of hydrated silica, which has the same chemical composition as precious opal.
Another important group of phytoplanktons are the coccolithophorids, strange little plants with calcite shells. England's White Cliffs of Dover are comprised mostly of fossil coccolithophorids that settled on the ocean floor, layer by layer, for hundreds of millions of years.
While a high phytoplankton biomass can be beneficial in the open ocean, it can wreak havoc in the shallows. Some phytoplanktons thrive in polluted waters, and their presence in large numbers indicates problems in water quality. Pollution from fertilizer runoff and engine exhaust can create ideal conditions for algae growth around inshore seagrass beds. The water becomes opaque, preventing grasses from photosynthesizing, resulting in a loss of habitat for gamefish and their prey. As the bloom dies, it sinks and decomposes, depleting dissolved oxygen and suffocating fish.
Most phytoplankton blooms are simply a case of too much of a good thing. The worst case scenario is the bloom of a toxic phytoplankton, such as the dinoflagellate species Karenia brevis, or red tide.
Some species of dinoflagellates emit a faint light when they're disturbed. The light, called bioluminescence, is produced by mixing a compound called luciferin with an enzyme, luciferase. They flash the light to confuse predators. Many other marine organisms, including adult fish and squids, can also bioluminesce, and do it to startle their prey or signal potential mates.
Zooplankton (and macrozooplankton) can be divided into two broad categories: holoplankton, which spend their whole lives adrift, and meroplankton, which drift or float only at a certain time in their life cycles. Almost all fish begin life as meroplankton. Most fish eggs are planktonic—a few notable exceptions include seahorses, pipefish and “mouth-brooding” catfish.
The vast majority of zooplankton are crustaceans, tiny insect-like critters called copepods, which are closely related to crabs and lobsters. It's estimated there are over 10,000 species of copepods distributed worldwide, found everywhere from frigid mountain lakes, to steaming geysers, to the abyssal zones of the oceans.
Fish, crabs, lobsters, squids, barnacles, corals, scallops and sea stars are all plankton when they're young, as are tens of thousands of other crustaceans, mollusks and marine worms. Filterfeeders like clams and worms begin life as plankton themselves. As adults they burrow into the sand, extend one or more siphons, and extract phytoplankton and zooplankton from passing currents.
Recently scientists have begun to study how global warming influences plankton growth. But there's a frustrating lack of data to work with, particularly here in Florida. That makes it tough to predict how our fish populations will be affected.
“There aren't any easy answers,” says Dr. Sharon Smith, a research scientist at the University of Miami's Rosenstiel School of Marine and Atmospheric Sciences. “We know that the change (in the earth's climate) is accelerating. There's a lot more fresh water coming into the Atlantic now, more evaporation in the tropics. We're seeing things like the expansion of ranges...a slow northward movement of organisms formerly associated with more southerly areas.”
And global warming may have other consequences as well. The buildup of greenhouse gases like CO 2 could actually change the alkalinity of the world's oceans, making it difficult for coccolithophorids to form their shells, and corals to make reefs.
Over three-quarters of the earth is covered with water, and almost everything that lives in it depends, ultimately, on plankton for survival. The survival of the earth's plankton depends on us, and the steps our generation takes to control pollution and climate change. Whatever harms our plankton, harms Florida's fisheries.