Who’s eating the daphnia in Square Lake? Is it the rainbow trout stocked by the Minnesota Department of Natural Resources each spring and fall? Is it the bluegills and sunfish in the lake? Could a fly be to blame? What is a daphnia anyway, and why do I care?
Square Lake, located a few miles north of Stillwater in Washington County, is considered the clearest of 950 metro area lakes. Over the past ten years, however, the Carnelian-Marine-St. Croix Watershed District has noticed that the lake’s water clarity is steadily declining. Lake monitoring reports show that there hasn’t been a noticeable increase in phosphorus, which is commonly to blame for feeding algae and impacting water quality in other local lakes. Researchers note, however, that there are less daphnia then there ought to be and that seems to be a problem.
Daphnia are from a group of zooplankton known as cladocera, which are known for eating algae. Zooplankton, if you are wondering, are little tiny aquatic animals that are often too small to be seen with the naked eye. The name was created by combing the Greek terms for animal (picture animals in the zoo) and wanderer (picture evil plankton from Sponge Bob toting his little, itty-bitty suitcase). The main point here is that something seems to be eating the daphnia, and as a result they aren’t able to graze on algae as proficiently as they should, and therefore more itty-bitty algae organisms are floating around and making the water cloudy.
A study conducted in 2004-2005 was unable to conclusively determine if rainbow trout in Square Lake were pigging out on daphnia, so researchers this year are taking the next step to figure out who is eating whom…gastric lavage.
There was an old lady who swallowed a spider, that wiggled and wiggled and tickled inside her.
Gastic lavage may sound fancy, but the poor students collecting data for the study will tell you differently. Their job (this is for real) is to catch all sorts of fish at Square Lake, and then squirt water into their mouths to make them (hopefully the fish and not the students) puke so that they can then analyze the contents of the fish’s stomachs and find out how many daphnia each is eating. Researchers are also collecting the larva of a small fly called Chaoborus and dissecting them to see if they’ve been eating daphnia (I guess you can’t make fly larva puke).
There was an old lady who swallowed a goat. Just opened her throat and swallowed a goat!
As ridiculous as it is to contemplate, so called top-down food web effects can be a big problem in our lakes. There is a delicate balance underwater, and the littlest change can throw things off and make a good lake go bad. Too few predators like walleye could mean too many sunfish eating too many daphnia, resulting in too much algae and, consequently, unhappy, green children swimming. On the flip side, introducing predator fish in a lake where they don’t really belong could spell disaster too, as they chow down on all sorts of smaller fish, leaving none around to eat the algae. Things get even more complicated when a bottom feeder like carp is thrown into the mix. It’s not so much what they eat, as how they eat it. Swimming along the bottoms of shallow lakes, the carp stir up settled sediment and nutrients, making the water murky and making phosphorus more easily accessible for the algae.
There was an old lady who swallowed a cow. I don’t know how she swallowed a cow!
For most local lakes plagued with algal blooms, the surrounding landscape is to blame for sending excess nutrients from yard waste, fertilizers and manure into the water every time it rains. The tangled food web in Square Lake shows us, however, that keeping our water clean is sometimes more complicated than just turning off the runoff switch. Furthermore, food web effects and runoff effects are not mutually exclusive. Polluted runoff full of excess nutrients can feed algae and reduce oxygen levels in lake water. Low oxygen levels and algae blooms, in turn, can kill fish in the lake, which of course disrupts the aquatic food web.
There was an old lady who swallowed a horse. She’s dead, of course.
Throughout the east metro area, resource managers are working hard to identify the causes of murky water and excess algae on many local lakes, as well as to implement projects that will fix these problems. Sometimes the work is glamorous, raingardens bloom and birds and butterflies flock. Other times they’re sitting in a wobbly canoe on a hot summer day, watching fish puke.