March: Copepods—Keystones of Spring

Chesapeake Almanac Podcast Episode and Transcript

Episode 45

Copyright © John Page Williams, Jr. all rights reserved.

This is John Page Williams with another reading from Chesapeake Almanac. This entry is from March and it's entitled "Copepods: Keystones of Spring."

It's a real drag to tow a plankton net behind a canoe--the fine mesh acts like a small sea anchor. Pulling one against the tide in a narrow creek is even tougher; but it's a good way to stay warm on a chilly March day. Besides, at the end of five minutes, the bottle at the bottom of the net will be full of tiny crustaceans. These are called copepods, true harbingers of spring. Their population explosions at this time of year produce important fuel for the Bay's new life as the water warms.

Copepods are among the most abundant multi-cellular animals on earth. They  are generally regarded as the most numerous in the Chesapeake Bay system, with numbers going routinely as high as 30,000 per cubic meter of water in certain areas each spring. A five-minute tow behind the canoe means catching well over 100,000, each just barely visible to the naked eye. A 3x hand lens brings them clearly into view and a 10x dissecting microscope gives a detailed look.

The general shape of copepods gives away their ancestry. They're closely related to shrimp, lobsters, and crabs. Like shrimp and lobsters (and crabs, though we're not usually aware of it), they have two pair of antennae and large thoraxes, or body trunks, that hold vital organs. Behind the animal's thorax is a segmented tail. Both the thorax and the tail have many paired legs covered with short bristles.

Copepods mature in one to three weeks, depending on water temperature, so it is common to find females carrying twin egg sacs like saddle bags, one on each side of their tails. A springtime tow with a net will catch a mixture of mature individuals, naupleus larvae, and juveniles, called copepodites.

Like many other crustaceans, copepods are omnivores, eating both plant and animal material. The arrangement of legs allows several of the important species of copepod in the Bay to filter small particles out of the water or to catch larger ones. The animals move some of their legs to set up water currents that bring food to their mouths.

So much for scientific description. In plain language, what does an animal this small eat? It's hard for us to imagine, but here's the story. The early March sun is still weak, but the days are getting longer, so there is more light available. Skim ice that forms at night thaws during the day, and the surface layer reaches water's densest temperature, 4 degrees Celsius or 39.6 Fahrenheit, causing it to sink. This sinking forces up bottom water and some bottom sediments. The stirring action is increased by spring wind and rain. The result is a bloom of phytoplankton, microscopic (that is much smaller than copepods) single-celled plants (we call them algae) that thrive on the sunlight and the nutrients brought to the surface with the bottom water.

Springtime concentrations of all types of phytoplankton are not as great as it will be later on, but they still run well over a million cells per liter, and there are a thousand liters in a cubic meter. It's crowded water, even if it looks clear. The phytoplankton are a major source of food for copepods, which filter them from the water.

The copepods' other major source of food comes from the marshes. Now is a good time of year to think about this phenomenon, with both salt and fresh marshes looking like nothing more than brown stubble. In the fall, perennial marsh grasses like saltmarsh cordgrass, cattails, and yellow pond lilies die back to their root systems. The leaves and stems wither and begin to decay. At same time, annual plants like wild rice and smartweeds die altogether, leaving their tons of seeds to carry on the following year.

All this withered plant material is broken up by winter weather : rain, wind, freezing, thawing, blankets of snow. Bacteria decay it further. Then spring rains and tides wash it out of the marshes into the creeks and rivers. Doesn't sound very appetizing, does it? Or nutritious, at least to us. But this mixture of detritus (a general term for decayed plant material) with associated bacteria is like a rich vegetable broth with "bacon bits" that would be the bacteria) mean food to lots of Bay creatures, including copepods. The thaw and rains that we usually get in late February produce a veritable stewpot to fuel March's copepod blooms.

The timing is apt. Yellow perch are making the spawning runs now at the heads of many rivers and creeks. White perch will follow at the end of the month and in April; then will come rockfish. Silversides and anchovies, two important forage species for larger fish, are also getting ready to spawn. In all these cases, the larval fish hatch out with yolk sacs to nourish them for a week or so. After that time, they're on their own. For these young fish, copepodites (those juvenile copepods) are perfect food. As the fish grow, they begin to feed on adult copepods. The little shellfish produce an essential link in the Bay's spring food web.

As the season progresses. the number of spring copepods drops. One species, Eurytemora affinis, is the dominant animal in early spring, followed as the water warms by another species,  Acartia tonsa, especially in brackish and salty waters.

By now, the Bay and its rivers are full of menhaden, shoals and shoals of them. These fish, though larger, are filter feeders too. They swim along gulping water so that it passes over specially adapted gill filaments. Copepods are a major food for them. Menhaden play many roles in the Bay, but the most important to us, anyway, are that they're primary forage for bluefish, rockfish, sea trout, and other predators, and that they furnish us with tens of millions of pounds of crab bait every year. Again we can thank copepods, at least in part.

You may have noticed earlier that there are no common names given for Eurytemora affinis or Acartia tonsa, those two important copepods. They don't have any. We tend not to give everyday names to creatures we do not see regularly or even are aware of. (Scientists, of course, look at them regularly, and they are the ones who need the Latin terminology.) Discovering that creatures as important as those two copepods do not have common names is a humbling experience for us laymen. There's a lot out there in that Bay that we don't know about. But all that unknown is part of the mystery that keeps us fascinated with it. It's especially exciting when the pieces start to fit together, linked by creatures like copepods, the keystones of spring.

For more happenings around the Bay in March see our other Chesapeake Almanac podcasts and read our blog posts "Tiny, Timid, Timberdoodle Dancers," March's Lion Has Lost His Teeth," and "Watching Northern Water Snakes in March."

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