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 µm) 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?
No comments:
Post a Comment