Researchers Get to the Bottom (& Beyond) of Lake Mendota Invasive Species

It looks like ice-off is only a couple days away on Madison’s lakes. Before we hit the completely open water season, though, here’s a look back at some semi-frozen fieldwork from early March. Graduate student, Jake Walsh, shares his experience coring Lake Mendota’s sediment layers…
by Jake Walsh
The past couple of Mondays I’ve been out coring Lake Mendota with Sam Muñoz (PhD student with Jack Williams in Geography – Sam is a paleoecologist) and Tom Shannon (undergraduate researcher in the Vander Zanden Lab).

Coring the sediment on Lake Mendota
Tom Shannon (left) and Sam Munoz, (right) prepare to take a lake sediment core sample. Photo: Jake Walsh

This was my first time leading any sort of winter sampling, so learning the ins and outs of driving the ATV, running the gas powered auger, and making sure the ice was safe (it was almost two feet last week and is still like a foot and a half this week but that doesn’t mean I’m not going to worry irrationally about ice conditions) was actually a really fun experience.
Driving through several inches of water and slush in 60 degree weather yesterday was a very strange, fun, and terrifying experience. The reason I was out in that slush was because I’m interested in looking at how long the spiny water flea (or, SWF, a tiny invasive zooplankton) has been in Lake Mendota. (This is an extension of the work from a previous blog post on sleeper cells).
We originally detected SWF in 2009 when it reached population densities higher than anything recorded anywhere else in the world, as far as I know. The water sample looked like SWF apple sauce to a class of limnology students out for a class lab with professor Jake Vander Zanden. However, my population model predicts that SWF could have been in the lake indefinitely, existing at low densities before some combination of temperature, water chemistry and other conditions allowed it to explode, “activating” this “sleeper cell” in 2009. So far, we’ve found a couple of SWF specimens in a sample from 2008 which corroborates the sleeper cell story, but this sediment coring will help us go back further.
Jake Walsh taking a sediment core on Lake Gogebic on the Wisconsin/Michigan border. Photo: Jake Walsh
Jake Walsh taking a sediment core on Lake Gogebic on the Wisconsin/Michigan border. Photo: Jake Walsh

Sediment cores are a great way to “dig” into the past. Many paleoecologists ask questions about the past using cores longer than 50 feet to observe conditions from the most recent glaciation. For example, Sam’s research in oxbow lakes (lakes formed by a bend in a river being cut off from the main river) goes back about 2000 years, or right about at the start of human agriculture. In comparison, I’m only interested in the past 20-30 years, which is definitely not very “paleo.” In other words: the way I’m using sediment coring to answer research questions is a little bit like using a multi-terabyte hard drive as a thumb drive to store a couple of small documents.
But, if I can find SWF tail spines in sediment layers that go back beyond 2009, I can further support our working sleeper cell hypothesis. When I get these cores dated using lead isotopes, I can even estimate, very roughly, when SWF established its low density population in Lake Mendota by carefully modeling sedimentation rates over the past 200+ years.
In a few weeks I’ll be heading to the University of Minnestoa’s “LacCore” facility to process my cores and prepare a couple of them for lead-210 isotope dating. I’m excited to see what I can find in the deeper core layers.
The answers to the mysterious arrival of the invasive spiny water flea may lie through this hole in the ice, buried in the bottom of Lake Mendota. Photo: Jake Walsh