By Tom Baird
As the weather cools, the waters of the bay and Gulf will become clearer. This isn’t so much from a lack of suspended sediment, but from fewer microscopic life forms adrift in the water column. There exists in the oceans of the world, in bays, lakes, and in nearly every other natural body of water, a population of organisms so immense that it defies counting. Everyone while swimming has brushed up against millions of these creatures without being aware of their presence. No doubt you swallowed a large number while learning to swim.
Although limited studies of this population were made before 1887, it was not until that year that oceanographer Victor Hensen first proposed a name for this vast assemblage…plankton. The term refers to those plants and animals, mostly, but not entirely, microscopic in size, that are made “to wander or drift,” under the influence of ocean currents. The same root word gives us the name planet, because to early astronomers the planets seemed to wander against the more fixed background of the distant stars.
Although most planktonic organisms have the ability to swim, their efforts are too feeble in the presence of oceanic water movements. One definition of plankton is the inability to swim faster than one knot. Because of their vast numbers, wide distribution and beneficial biologic activities, plankton are considered the most important inhabitants of the marine world, with all forms of life directly or indirectly dependent on them.
Planktonic creatures are basic to the food chains of all marine life. Sponges, tube worms, scallops, clams, oysters and sea squirts are just some of the animals that filter sea water to gather them. Herring fisheries use plankton indicators to predict potential catch. Giant baleen whales, over 100 feet in length and reaching fantastic weights of 150 tons, feed almost exclusively on plankton. Diatoms, one type of plant plankton, are a major source of vital oxygen in the atmosphere and for proteins that animal life cannot synthesize but require. Without plankton the seas would be a wet desert.
Not all planktonic creatures are microscopic. Jellyfish, as large as cannon balls, are moved about the oceans by currents. Comb jellies (Ctenophora), depending on the species can range from the size of a hickory nut to 4 – 6 inches. Many plankters are visible to the naked eye. Copepods (crustaceans) are the insects of the sea, and are about the size of a lower case o on this page. They are the most abundant and universally distributed animals in the plankton.
Animal members of this floating, drifting world are named zooplankton. With the exception of marine mammals and reptiles, nearly every creature in the sea spends either a part or all of its life drifting about. Eggs, larvae and juveniles of most invertebrates and fishes, and some adult forms are common. If currents take them into suitable habitat, the juveniles will settle out and attach, much like oysters or larval scallops. If they survive and reproduce, they will then send forth their eggs and larvae into the currents. Those that don’t drift into suitable environments may be eaten or die and settle to the sea floor. Since far fewer invertebrates and fish are spawning when the water is cold, the water is clearer.
Phytoplankton is the name for the plant part of the whole plankton community and they are more numerous than their animal counterparts. Using dissolved chemicals in the ocean and the energy of the sun, phytoplankton produce energy-storing sugars and oils and provide most of the oxygen in our atmosphere.
The most common members of the phytoplankton are diatoms. These are literally the pastures and forests of the sea. Immediately recognized because of their delicate geometric shapes – circles, triangles, rods – diatoms are encased in a glass-like silica capsule. Because of this diatoms are often referred to as nature’s aquatic gems, and they are among the most beautiful plants on earth, although one needs a microscope to appreciate their delicate beauty. The hard glass-like case often remains long after the inner cellular material dies. As these cases settle to the ocean bottom over eons, thick deposits build up. These deposits are called diatomaceous earth or diatomite. Uplifting of the sea floor has made this material available to humans, and it is added to many commercial products, including dynamite, detergents, polishes, paint removers, fertilizers, and toothpastes. Some swimming pools use diatomaceous earth filters. In addition, diatomaceous earth is frequently a component in insulation and sound-proofing materials.
Plant plankton is also comprised of small single-celled algaes, and colonial green algaes. Another important group in the phytoplankton is the dinoflagellates. Many of these tiny organisms are capable of luminescence and cause the gulf to glow or “fire” under the right conditions. This is also the group that has caused toxic red tide blooms along our coast. The first documented fish kill associated with red tide along the Florida Gulf coast occurred in 1844 and the continued occurrence of these blooms is a natural phenomenon. Phytoplankton production is especially high in the estuaries along our coast because of high temperatures, light, and nutrients brought in by rivers.
The phytoplankton is grazed upon by zooplankton, such as the larvae of many sea creatures or those minute animals like copepods that permanently live in the plankton. The zooplankton is fed upon by school fishes such as herring, bay anchovy and menhaden, and by clams and oysters which strain water through their gills to concentrate the plankton. The school fishes are in turn fed upon by larger predators.
As the zooplankton feed on the plant plankton and the fishes eat the zooplankton, etc. many substances in seawater become concentrated in the tissues of consumers as one goes higher up the food chain. Pesticides, heavy metals like mercury or cadmium, and toxic organic chemicals are absorbed by the phytoplankton, which are in turn eaten. Eventually high concentrations of these chemicals are found in the organs and muscles of tuna, billfishes, eagles, osprey and pelicans, which can make the fish unsafe for human consumption or damage the reproductive cycles of the predators, causing their numbers to decline. Sewage and industrial wastes are the principal offenders in damaging plankton food chains. Dumping in creeks and rivers as far away as Pennsylvania or Minnesota brings toxins down the Mississippi drainage to the Gulf of Mexico where the rich, vital plankton food chains become contaminated and diminished. All communities, not just coastal communities, have a responsibility to avoid actions that can alter the food webs of the sea.
Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.