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The Lake Where You Live: More about plankton

October 04, 2019 by Ted Rulseh

I’ve written before about plankton (specifically zooplankton), the tiny critters that float and swim around in our lakes, providing a main source of food for the developing stages of fish.

Daphnia (water fleas), rotifers and copepods are the three main varieties, if we don’t count the invasive and obnoxious spiny water flea that has found its way into some lakes in Vilas County. But a couple of others, far less abundant, are also of interest.

One is Leptodora, the largest of the zooplankton in the cladoceran family (same family as the Daphnia). They can survive in a wide range of water conditions, from very clear, nutrient-poor waters to lakes rich in algae and plant life.

They are almost completely transparent with a single large compound eye. They are 5 to 12 millimeters long, versus no more than 1 or 2 millimeters for Daphnia. They use that size to advantage, feeding voraciously on other zooplankton. They have a pair of long and powerful legs that grasp and hold prey; they then devour it by biting into the shell and gobbling up the insides. 

One antenna, in males, is used in reproduction. A second antenna with long hairlike structures called setae is used for locomotion. For most of the year Leptodora reproduce asexually, but in fall the population of males rises and sexual reproduction occurs. Females then produce and release resting eggs that sink to the lake bottom to await hatching in spring. Upon hatching they go through a simple metamorphosis to take their adult form. 

Another form of zooplankton is Holopedium, which looks quite a bit like a Daphnia and in fact is described as a kind of water flea. The creature is surrounded by a jelly-like coating called a mantle that actually increases its volume by up to eight times. The legs stick out so they can swim — upside down — while feeding on algae.

The mantle helps protect the organism from predators. Scientists believe it also might help reduce its density and slow the rate of sinking, keeping it in the “food zone” for longer times. Like many kinds of zooplankton, Holopedium migrate toward the water surface around sunset, stay there for the night, and move to deeper water as a new day begins. 

During their lifecycle they shed their mantle, and multitudes of these little jelly balls can be blown toward shore, where swimmers observe and feel them as pea-sized objects. Holopedium can reproduce sexually or asexually. They are all but absent in lakes in winter and spring. The population then rises, peaking sometime between June and October. 

Zooplankton aren’t microscopic in size, but most can’t be seen with the naked eye as much more than specks in the water. If you want to see what these critters look like, you can find them just be doing an internet search. It’s fascinating to know these creatures exist and understand the roles they play in our lake ecosystems. 

Ted Rulseh resides on Birch Lake in Harshaw and is an advocate for lake protection and improvement. His Lakeland Times and Northwoods River News columns are the basis for a book, “A Lakeside Companion,” published by The University of Wisconsin Press. Ted may be reached at [email protected]

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