by Jake Walsh
There is a fortunate silver-lining to those of us prone to making mistakes – they can be a powerful teaching tool. At a minimum, our mistakes teach us things we shouldn’t be doing. In the best cases, they help us better understand ourselves and how we work.
Scientists are no strangers to mistakes. In fact, with some good background knowledge and a fair amount of luck, mistakes can set up experiments that teach us more about the how the world works.
Take, for example, the introduction of invasive species into freshwater ecosystems – a series of multi-billion dollar mistakes – have helped teach us about how lakes work. In a recent study in the journal, Limnology & Oceanography, Jake Vander Zanden, Dick Lathrop, and I used the spiny water flea invasion into Madison’s Lake Mendota to better understand the factors that influence water quality in the lake. What we found was, well, a little surprising.
We already knew that the spiny water flea has led to murkier conditions in Lake Mendota. The spinies prey on a native zooplankton called Daphnia, a small, nearly microscopic animal that eats enough algae to keep the water clear. As spinies eat a ton of Daphnia out of the food web, we’ve seen a dramatic decline in water clarity.
But what else, we wondered, has the spiny water flea changed? And could those changes help us better understand what drives water quality in Lake Mendota?
Thanks to a little luck and a lot of long-term monitoring, we were able to find some answers. And, of course, a few new questions.
Our study confirmed a lot of what we already knew about water quality. Nutrients and high summer water temperatures can lead to more algae. Daphnia (when their populations are robust) control the algae by eating it. That often means the water stays clear, instead of green.
But, we discovered, the story is a little more complicated than that.
For starters, Daphnia seem to prefer certain kinds of algae. In particular, they chow down on smaller algae like diatoms in the spring and early summer, but they struggle to make a dent in populations of larger and less edible algae, like cyanobacteria, in the late summer and fall. In other words, while our lakes are greener in spring and early summer, that isn’t the case the rest of the year. In fact, cyanobacteria populations have stayed about the same in the nearly ten years since spiny water fleas invaded Lake Mendota.
Since cyanobacteria can produce toxins that are harmful to humans and animals and big blooms of them often lead public health officials to close beaches, controlling them is important. In Lake Mendota and, really, most lakes, cyanobacteria populations usually increase in response to the amount of nutrients, especially phosphorus, that are available in the water.
Since the spiny water flea invasion was first detected in 2009, there has been no change in the amount of nutrients like phosphorus running off of the surrounding landscape and into Lake Mendota. Our watershed is primarily agricultural, and efforts to control fertilizers and cow manure from entering our waterways are still ramping up. That means U.S. Geological Survey stream monitors still show a lot of nutrients flowing into the lake.
But, when we tested the water in Lake Mendota, we found that the nutrient concentrations have mysteriously declined in the lake itself. Even more confusing, cyanobacteria population haven’t really changed in response to that decline in nutrients. Their numbers are staying steady.
To confuse things ever further, when we looked at the number of days that Dane County public health officials had to close Lake Mendota beaches in response to toxic cyanobacteria blooms, we found that those numbers are down. Despite the fact that there is still the same amount of cyanobacteria floating in our lake, on average, we’ve enjoyed an additional 46 beach days per year across Lake Mendota’s 12 beaches and swimming areas since 2009.
For reasons we don’t yet completely understand, nutrients concentrations have been lower and beach conditions have actually been better since spiny water flea showed up in Lake Mendota. Somehow, there seem to be fewer nutrients available to cyanobacteria to feed really big blooms, which means we likely have as much control over preventing summer blooms now as we did before the invasion by reducing nutrient concentrations in the lake.
Getting to the bottom of these new findings can help us get more of something we enjoy in Lake Mendota – good water quality.
Of course it’s unlikely that we’re done making mistakes, particularly when it comes to invasive species. Invasive zebra mussels were detected in Lake Mendota in 2015, six years after spiny water flea’s detection. We’re already learning more about both zebra mussels and Lake Mendota as they grow in abundance and transform the lake bottom.
But, like the spiny water flea, there’s still that silver lining – mistakes teach. If we continue to study lakes carefully over long periods of time, we can uncover more about how they work and use that information in positive ways. And, just maybe, we’ll not only learn from these mistakes but also learn how to stop making them.
You can find the full text of our paper by clicking here.