Friday, December 21, 2012

Life Under the Ice

Nets in hand, a line of Master Naturalist students ventured onto the dock. Snowflakes danced on the wind as a prelude to the big snowstorm yet to come. For now, the thin ice in the marina by Barker’s Island in Superior only held a layer of snow one flake thick. Dr. Joel Hoffman of the EPA had broken open a narrow channel in the ice parallel to the dock, and was about to help us discover a hidden world. 

Underneath the snow-free ice, he explained, sunlight penetrates enough that life continues as usual. But what is usual for a harbor connected to Lake Superior? Our nets were made of 150 micron mesh. One micron (or micrometer or ┬Ám) is a millionth of a meter. We sure were not trying to catch fish! A Northland College student sampled with one diamond-shaped net. This plankton net is designed to be let down to the bottom of the lake and drawn straight up. It provides a cross-section of the entire water column. John Kudlas, a steadfast Museum volunteer and educator, used the other net on a long handle to sample one layer of water with a horizontal sweep.

The particles in the bottom of the nets did not look particularly lively to our naked eyes. Most of the organisms that Joel expected us to find were only about 1-2 mm long. We brought them in out of the chill, rinsed the samples onto petri dishes, and put them under dissecting microscopes. 

The makeshift office of the brand-new Lake Superior National Estuarine Research Reserve (NERR) erupted with enthusiasm. “Wow! Cool! Oh my gosh, look at this! Joel, what’s this? Did you see that? Check out what I’ve got? Here’s a good one! Oh it just moved. You’ve gotta see this!”

Copepods, cladocera, and rotifers, oh my! Tiny aliens swam, spun, and ate before our eyes. Joel had drawn diagrams of what we might see on the board, and now they came alive.

Copepods were some of the most common critters in our samples. These tiny crustaceans live in the sea and in nearly every freshwater habitat (including the lake right outside your door!). Their teardrop-shaped bodies are covered by an exoskeleton so thin it is transparent, and are adorned by large antennae and a single red eye. Bristle-like setae do most of their sensing, and can differentiate patterns in the water flow around the body caused by approaching predators or prey. Some copepods have extremely fast escape responses when a predator is sensed, and can quickly jump a few millimeters.

Such small creatures do not need a circulatory system. One group, the Calanoid copepods, have a heart, but not blood vessels. Most lack gills and let oxygen absorb directly into their bodies. The copepod larval form is even simpler, consisting only of a head and a small tail, with no thorax or abdomen. The larvae must molt 5-6 times before emerging as a larvae with all the parts. After 5 more molts it reaches adulthood. This process can take anywhere from a week to a year, depending on the species, temperature, and food availability.

Some scientists say that copepods form the largest animal biomass on Earth. Since copepods use carbon to create their exoskeletons (discarding 10 or more exoskeletons over their lives), and release carbon into the ocean through respiration and scat, they are important to the global carbon cycle. The upper layers of the ocean are the world’s largest carbon sink, and can absorb 1/3 of human’s annual carbon emissions.

Several copepods ate their carbon-based lunch as awed students spied through the scopes. Their mouthparts swished the water to create currents that drew food toward their bristly setae. Large particles were individually caught by “fling and clap” movements where appendages grasped both the particle and a packet of water surrounding it, and removed the water by an inward squeeze.

All too soon, the students returned the samples to the lake. The snow fell harder. With less sunlight filtering through the ice and snow, life in the lakes will slow down a bit for the winter. Some critters enter resting stages, others just slow their metabolisms. As you look out over the frozen lakes, can you imagine tiny, translucent copepods living in their hidden world?

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