by Jake Walsh
We (or scientists at the University of Wisconsin-Madison, at least) have known for years that the spiny water flea, a tiny, non-native zooplankton that has invaded Madison’s Lake Mendota, has a voracious appetite. But when I got together with my colleagues, Jake Vander Zanden and Dick Lathrop, at the UW-Madison Center for Limnology to try to measure that appetite, we had no idea how big that appetite would be.
Our report in the journal, Limnology and Oceanography, was made available online this week (Monday, May 10) and it shows that, If you crunch the numbers, the population of “spinies” living in Lake Mendota consumes so many native zooplankton that, in just seven years, they eat an amount of biomass equivalent to the entire human population of Madison*.
In other words, if you put seven-years’ worth of the spiny water flea’s diet on a scale, it would weigh as much as the 240,000 people who call Madison home.
That, obviously, is a lot of plankton. I know it might not quite match the recent study that found all of the spiders in the world could eat all of the humans in the world but, if you consider that a single spiny water flea is not much larger than a grain of rice, then, well, it’s kind of jaw-dropping.
It also indicates two things – one: the spiny water flea has an insatiable appetite and, two: there is a staggering amount of plankton living in our lake.
A history of human intervention
To really understand what this all means, you first need to know a little bit of not-too-distant history of human activity on Lake Mendota. Back in the late 1980’s, fisheries managers launched a campaign to improve the water quality of the lake by tinkering with the food web. At the time, native fishes like cisco were so abundant that they were eating a ton of native zooplankton, especially one called Daphnia pulicaria. While there’s nothing necessarily wrong with that, people were concerned because Daphnia pulicaria like to eat a lot of algae and, when their numbers are low, water quality suffers as more algae is able to grow and the water gets greener.
So officials hatched a plan to start stocking the lake with piscivores, or species of fish (like walleye and pike) that like to eat other fish. The idea was that those fish would eat cisco and other species that feast on Daphnia. With fewer of those planktivores around, Daphnia populations could grow and they, in turn, would eat a lot more algae.
And the plan worked. Until it didn’t.
The new fish stocking approach, combined with a cisco die-off in 1987, quickly yielded much clearer waters on the lake, a condition anyone who spent time on or near the lake would enjoy for nearly two decades. Then the spiny water flea came on the scene.
A tiny invasive with a big appetite
By using decades’ worth of data (thanks LTER!) and our own measurements we estimated that the spiny water flea has more than doubled the mass of zooplankton (like Daphnia) being eaten each year in Lake Mendota. While they don’t entirely make up for all of the zooplankton that fish like cisco used to eat before humans got so involved in the food chain, that’s still an increase from roughly (these are ballpark estimates) 1,800 metric tons per year to 4,600 metric tons per year. That’s 4,600 metric tons of zooplankton in a single lake.
To put this in perspective, Madison’s population of roughly 240,000 people is about 20,000 metric tons of human (assuming everyone is an average adult). We estimate that just the spiny water flea population in Lake Mendota eats roughly 2,800 metric tons of its food source each year and that they ate 20,000 metric tons of zooplankton from 2009 to 2015 – equal to the entire standing biomass of all of the humans living in Madison.
What’s truly surprising is that these numbers shouldn’t be surprising. If you repeat this back-of-the-envelope calculation for each of Lake Mendota’s other dominant zooplanktivores, you see that there have been times in the past 40 years when every one of them eats enough to consume 20,000 metric tons, and do it fairly quickly. See the chart to the left.
Tiny predators – whether they’re spiders on land or spiny water fleas in Lake Mendota – play a disproportionate role in consuming prey. The average spiny water flea is tiny compared to bass, perch, and cisco, but they pull their weight in terms of eating Madisonians.
For example, one spiny water flea is just 1/100,000 the mass of the average perch in Lake Mendota but, during the fall when spiny water flea are abundant, there are roughly one million spiny water fleas for every one perch in the lake.
Also, smaller animals eat much more food relative to their body size than bigger animals. An average spiny water flea would eat roughly 80% of its body weight per day while an average perch would eat “just” 3% of its body weight per day. So, if the average Madisonian ate like a perch, they would eat a whopping 5 pounds per day. But, if the average Madisonian ate like a spiny water flea, they would be eating over 140!
An extremely productive lake
While Lake Mendota is teeming with life, the average Madisonian usually only sees what we scientists call its “productivity,” when the lake is green and murky with algae or when they are reeling in a catch on a fishing line. What we don’t see is the action that led to a murky lake or the fish in our net. We don’t see the tens of thousands of zooplankton that were consumed to “build” the fish and we certainly don’t see the carbon dioxide and nutrients that went into the construction of each cell of algae in the water.
What we’re left with – what we see – is known as standing biomass. Last year, we wrote about how the zooplankton, Daphnia, helps to keep Lake Mendota clear by reducing the standing biomass, or amount, of algae in the lake. The unfortunate arrival of the invasive spiny water flea cut the standing biomass, or population numbers, of Daphnia which, in turn, increased the biomass of algae. And that hurts our water quality, not only making the water green and murky, but also eventually leading to things like harmful algal blooms and dead zones at the bottom of the lake.
But there’s more to the picture. When we say Lake Mendota is teeming with life, that implies action, and that action isn’t captured in how many fish or zooplankton there are, it’s captured by measurements of how many zooplankton are being eaten by fish. What kind of predation went in to building all of this standing biomass. Or, to ask another way, at the end of a day of fishing, when you haul in your basket holding a dozen perch, how many millions of zooplankton did your catch eat in its lifetime?
What this means for water quality in Lake Mendota
Lake Mendota – a large, 15 square mile lake – is packed with algae, zooplankton, and fishes, and it should come as no surprise that they eat each other at ridiculous (and slightly terrifying) rates. However, this provides some context and mechanism for understanding how invasive zooplanktivores, like spiny water flea, impact native organisms in lakes.
We found that when more Daphnia are being eaten, fewer are available to eat algae and water clarity is worse. In other words, spiny water flea tipped the scales in favor of Lake Mendota’s zooplanktivores over its zooplankton, which indirectly tipped them in favor of algae. And this has had a big impact – for example, we’ve recently done work that show the reduction in water quality in the lake come with a $140 million price tag.
Unfortunately, there’s no easy solution to the spiny water flea problem. Our study also revealed challenges about using spiny water flea’s predators as biological controls. First, native spiny water flea predators like white bass, yellow perch, and cisco have not been able to control spiny water flea, even during years of high predator biomass. In fact, the years of highest white bass and yellow perch biomass were also the years that spiny water flea ate the most Daphnia – compounding predation on Daphnia. Daphnia were present at record low numbers in these years. So, the second challenge: increasing the number of spiny water flea predators in Lake Mendota is also likely to harm Daphnia.
Luckily for Madison, we do have a few things that are in our control. We can control the amount of algae in the lake by reducing the amount of manure that runs into Lake Mendota from farms (manure fertilizes both crops and algae growth). Continuing to reduce the amount of farm run-off into Lake Mendota should help offset the effects of spiny water flea and reduce algae blooms.
And, we can keep spiny water flea out of other waters. When boating on lakes, clean, drain, and dry your boats, ensuring that you never move invasive species. Spiny water flea eggs can survive a long time in the sediment on your anchor or in pockets of standing water in your boat.
In other words, it’s up to us to ensure that no additional lakeside cities in North America have to live with the risk of being “devoured” by spiny water flea.
*I have to note that my “spinies eating Madisonians” angle is playing off a fun article on spiders you can find here (that is if you’re interested in knowing how quickly spiders could eat every human on the planet!)
A PDF of our study can be found here.