Limno in the Lab: Measuring Tiny Helmets and Microscopic Tail Spines

Sure, we get to do some awesome fieldwork in the name of science,  but what we do with all those samples and data sets once we’re back in the lab is just as important.To give you a fuller sense of what it means to study limnology, we bring you a new series complementing our stories of Limnology in Action – Presenting: “Limno in the Lab.”

Jake Walsh puts a water sample of daphnia pulicaria under the microscope.

Jake Walsh may have spent his summer out on Lake Mendota collecting zooplankton samples, but his real work begins when he gets all those tiny animals into the lab and under a microscope. Walsh, a graduate student in the Vander Zanden lab, is currently looking at the dynamics between the algae-grazing zooplankton, daphina pulicaria, and one of its principal predators, the invasive spiny water flea.

Daphnia are an important source of food for aquatic invertebrates and Lake Mendota’s smaller fish and also one of the prime drivers of the “clear water phase” the lake experiences each Spring. But daphnia are under attack by a larger and formidable invasive zooplankton called the spiny water flea. Originally brought to the Great Lakes in the ballast tanks of ocean-going ships, spiny water fleas have steadily moved to inland waters, turning up in Lake Mendota (and surprising an undergraduate Zoology class) in 2009.

Undergraduate student, Carly Broshat, counts daphnia pulicaria samples taken from different depths in Lake Mendota to calculate their abundance at different levels of the water column.

But dahpnia pulicaria isn’t completely defenseless against its predators. For one thing, Walsh says, daphnia can change their range in the water column, dropping to deeper, darker waters to avoid sight-based predators like fish and the spiny water flea. Walsh currently has undergraduate student, Carly Broshat, working this summer on counting daphnia samples from different depths in the water column to see if daphnia heads down when spiny water flea numbers start to rise.

A sample of daphnia under the microscope (and projected on a computer monitor) allows Walsh to measure body size, as well as helmets and tail spines.

The microscopic creature can also grow hard “helmets” and “long” tail spines in response to heavy predation, features that make it less palatable for fish and, Walsh says, “harder to handle” for predators like the spiny water flea. Walsh has been looking for such changes in the daphnia’s morphology and is now exploring “whether those changes have anything to do with the [rise in abundance of] the spiny water flea.”

In Lake Mendota, “spiny water flea pops up in July and becomes abundant in September,” Walsh says, leading him to wonder if a similar rise in the proliferation of bigger daphnia with larger helmets and tail spines occurs as summer moves into fall. If so, it might be a sign that one of Lake Mendota’s most important zooplankton isn’t going down without a fight. Good news for both the food web and the water clarity of Madison’s largest lake.