A Story Of A Lake Study

Environmental & Science Education

STEM

Nature of Science

Water & Watersheds

Clean Water

Edward Hessler

I try to attend the weekly biology seminars at Hamline.  The fall features invited speakers from local institutions: colleges (sometimes HU), the University of Minnesota, health departments, etc. In the spring (it is really winter!) students are the featured presenters. They have several choices: a report on a scientific paper, a design for a research study, and original research.

I've intended to write about several seminars that have been about a wildlife management issue and had been keeping notes. They focused on the question of stocking rainbow trout and its effect on water quality (area residents were concerned about diminishing water clarity) in Square Lake, a metropolitan lake noted for its outstanding water quality. Good intentions, I've been told more than a few times, lead somewhere. In my case I missed the boat but a newspaper article provides the prompt to write a short post about this fisheries management question.

Intern Zach Walker of the St. Paul Pioneer Press (May 13, 2019) writes about this research. Professor Leif Hembre, a member of Hamline University's Biology Department has been studying Square Lake since 2003. He has kept tabs on the ups-and-downs of the population of Daphnia or water fleas (trout like them) and the lake's overall water quality. 

Hembre's research has many dimensions, one of them, of course, is in the making of an evidence-based decision on managing fisheries in a lake. The research resulted in a short, compelling tale of how nature works and how humans often conspire against it. It is also a story about resolving competing interests, a central issue that state departments of conservation/natural resources face in most of their decisions. And it is also a story about the nature of science.

You may recall Daphnia from a bio course, high school or college but if not, here is the Wiki entry and a short video on their care and handling from Carolina Biological Supply Company.

Square Lake has had a long history of stocking trout, a favorite fish for many anglers. It is also one of only three trout lakes close to the Twin Cities of Minneapolis and St. Paul. In 2012, trout stocking stopped, a decision that has been controversial. Eventually, plans were made to renew a somewhat limited stocking program in 2017.

Fortunately, Hembre had comparative data: during stocking trout (from 2003 to 2012) and after stocking trout (from 2013 to 2015). He did not have to design an elaborate laboratory study; the lake had already done that for him. That he would be studying Daphnia to begin with is not a surprise. These critters were a focus of his graduate school work.

Below is what he found.

Following stocking the Daphnia population increased and water quality (clarity and oxygenation) increased. Daphnia are known algae foragers. There was a surprise, too (there almost always is). As Daphnia numbers increased, phosphorous levels in the lake decreased. It is suspected that phosphorous might be ending up in the bodies of Daphnia, a research opportunity for future bio majors.

Walker ends his reporting by writing, "The DNR has no immediate plans to resume trout stocking." One word in this sentence troubles me: immediate.

I hope you will read Walker's story for additional details. It is a great story well told. Unfortunately, I've yet to find it on the web. The headline is "Study: No trout helped clear up lake: Stocking halt allowed comparison."