Peril in the Ponds: Deformed Frogs, Politics, and a Biologist's Quest

4. Wading in and Listening

{ 68 } CHAPTER 4 WADING IN AND LISTENING 7 Before coming to the MPCA, I observed first-hand the vulnerability of wetland organisms to damaging pollutants when I participated with EPA scientists in a research project. Their goal was to field-test the impact of organophosphorus insecticides on the biota of natural wetlands. Would the results from these more complex environments line up with their laboratory toxicity tests, conducted on just a few species under simpler conditions? To sample the study sites, located in the Crow Hassan Reserve near the Twin Cities, I walked happily up a hill through a restored prairie area where bobolinks nested and tree frogs called from nearby woods. My task: to analyze the zooplankton, such as Daphnia, and other tiny crustaceans, before and after the wetlands were sprayed from a helicopter with insecticides. Another crew sampled the larger aquatic invertebrates, such as immature insects, snails, and clams, while EPA staff monitored the water chemistry, acidity, and daily swings in oxygen levels and temperature in the water. Our field results were stunning. The insecticides killed the zooplankton outright; the other invertebrates, like juvenile insects, declined over time. I began to learn that the ecology of wetlands differs considerably from that of streams. Mayflies, certain midges, and caddis flies were already known to indicate good water quality in streams, but what about dragonflies, crustaceans , snails, leeches, beetles, bugs, and other invertebrates that are more { 69 } WADING IN AND LISTENING abundant in wetlands? Oddly, some aquatic species that succeed in unpolluted wetlands, like certain types of leeches and midges, are considered pollution indicators in healthy streams. Stream-dwelling invertebrates depend on a steady supply in flowing water. Wetlands invertebrates are better adapted to survive big changes in the levels of dissolved oxygen in the water, which fluctuate daily with afternoon highs (oxygen released from photosynthesis by pond algae and plants) and overnight lows (oxygen removed after dark by microbial decomposers). Some, such as Daphnia and certain midge larvae, make a special red hemoglobin that helps them hold oxygen when it’s low in the water. I’d seen red Daphnia in sewage ponds and at the Crow Hassan wetlands, indicating oxygen was low. So the day the EPA crew’s test minnows went belly up in their floating cages, I wasn’t surprised. If forced to pick one group of species to indicate wetland water quality , I would probably suggest the presence of diverse, immature dragonflies. Ferocious underwater predators, juvenile dragonflies consume mosquito and midge larvae, small minnows, and many other prey. They are known to accumulate pollutants like lead, DDT, and toxic PCB oils from the organisms they eat (Corbet 1999). In Japan the exquisite, bug-eating adults are venerated as symbols of strength, courage, and happiness, perhaps because they zoom over rice paddies and are considered harbingers of rain. However, to develop a robust method for scoring water quality, whether in streams or wetlands, one needs to survey many different species of invertebrates, not just one “indicator ” type. My earlier work on zooplankton was conducted from docks the EPA had constructed in the three ponds we had studied. But when I started researching wetlands invertebrates at the MPCA in the early 1990s, I wanted to analyze the broader community of species. To do this, I used a long-handled dip net to sweep through the water and underwater vegetation in a repeatable manner . And I deployed underwater funnel traps overnight to capture the more active swimmers, with the help of intrepid assistants like Kyle Thompson and Cade Steffenson. This meant wading into wetlands. Early in my life, I wore high-top tennis shoes to fish for trout in rocky streams set in the balsam-scented, forested mountains of southern Vermont. I had little experience wading in mud-bottomed wetlands, shallow depressions that at best are fringed with diverse aquatic plants, at worst have shorelines packed with dense stands of cattails. At the MPCA, I started out using a clumsy pair of rubberized men’s chest high waders that were so stiff I could barely raise a leg to step over anything. I stomped eagerly into a wetland, { 70 } CHAPTER 4 gear in hand to test my methods. Unexpectedly, an underwater log snagged my boot and I fell forward, face down in the water. My hands futilely sought leverage in the muck. Rotten odors surrounded my face. I wallowed...