When I made it to Churchill in 1984 I conducted a series of experiments with Mesostoma lingua, a common member of the fauna of ponds in the granitic outcrops around Hudson Bay. If you really want all the details you can read them in the published paper (Schwartz, S.S. and P.D.N. Hebert. 1986. Prey preference and utilization by Mesostoma lingua (Turbellaria, Rhabdocoela) at a low arctic site. Hydrobiologia 135:251-257). The results of the experiment were clear to me: these benthic flatworms had the ability to shape the zooplankton community above them. They showed a distinct like for the tube-building larvae of chironomids (midges) around them, but they had no aversion to eating whatever came their way.
One easy to replicate experiment involved a cafeteria in which I offered all the different available food equidistant to the flatworm. Although the freshly killed prey (by me) were randomly placed the flatworm chose the chironomids twice as often as anything else. They must have sensed the presence of the prey and crawled to it each time. Once there, they spent much longer with the chironomid as any other prey as well. My experiments comparing chironomids in their tubes as compared to those I removed demonstrated the effectiveness of the tubes. Tube-building chironomids are common in vernal ponds and I’d suggest that predator defense at least partially explains this behavior. The test is easy enough to do yourself (maybe in a classroom with kids).
Even though Mesostoma preferred chironomids, they still ate Daphnia and probably did so regularly. The demonstration of choice among the remaining prey indicated to me that these benthic organisms could play a pivotal role in structuring the zooplankton community by selectively feeding on some species and ignoring others.
I studied Hydra at about the same time as I have described previously. It seemed to me that in shallow ephemeral ponds benthic organisms might play a major role in structuring the zooplankton community.
A few years later the point was re-enforced while I was a lecturer at the University of Houston. While there, I had looked at ponds in a reserve between Houston and Galveston. I was surprised to see amphipods (scuds) grasping, and apparently eating, mosquito larvae. This led to another set of experiments with a new predator but the same sort of zooplanktonic prey. Again, I wouldn’t be telling the story if the results didn’t make my point. In this case, it was amphipods that loved to eat Daphnia and mosquitoes and lots of them.
The moral of these stories about flatworms, amphipods and Hydra is that vernal ponds, shallow waters, are much more complex than we imagine. And the lack of depth means that animals that we consider to be planktonic, in the open water, really aren’t. The bottom is so close to the top that animals that the benthos may have profound effect on animals on the plankton. Understanding these systems takes an appreciation of the fact that there might be lateral distinctions in a pond (the particularly shallow shore vs. the slightly less shallow “pelagic” realm) but even the open water is so shallow that planktonic near the bottom are potential meals for the animals that dwell there. Daphnia and other plankton are never free of some sort of predator from below or within the water.
To me, it makes the entire system that much more exciting. People driving by or even walking through a wetland area are unaware of the drama that is played out daily in vernal ponds. We just haven’t appreciated all that is going on in these tiny ecosystems.
Next, a slight departure as I discuss why chemically at least, ponds are tiny lakes.
