Effects of an experimental canopy gradient on pond ecosystems

Background/Question/Methods Forestry is a major global landuse and can affect ecosystems other than forests. Many aquatic systems are dependent on the allocthonous input of nutrients from forests, and forests also influence the shading and microclimate of aquatic systems. These parameters have been shown to be important in shaping stream communities by influencing the source (primary production or the breakdown of litter) of nutrients for consumers. Although pond and stream communities differ in many ways, canopy cover may influence these two aquatic ecosystems in similar ways. To investigate this, clusters of four cattle tanks were placed along a canopy gradient in four locations: the center of clearcut plots, clearcut edges, forest edges, and the center of forest plots at three replicate sites in the spring of 2006. Tanks were allowed to naturally colonize with invertebrates and were sampled in fall of 2007 for macroinvertebrate and other physical and chemical parameters. Results/Conclusions Oxygen levels were lower in the forest tanks than in the clearcut tanks, presumably because of high decomposition of leaf litter and low primary production in those tanks. The two most abundant types of predators had different trends in abundance in the four treatments. Odonate (primarily Pachydiplax longipennis) abundance was highest in the clearcut edge tanks, while Chaoborus sp. abundance was highest in the forest edge tanks. Chironomid abundance was higher in the forest than the clearcuts, possibly due to higher levels of leaf litter. The tanks located in the center of clearcuts were consistently low in abundance for all groups of macroinvertebrates. The abundances of invertebrates are influenced primarily by the choice of oviposition sites by adults and survival of larvae in the various biotic and abiotic conditions present in the cattle tanks. It is unclear which of these factors is resulting in low macroinvertebrate abundances in the center of clearcuts, but these results indicate that large expanses of clearcutting around ponds could be detrimental to pond communities. Overall, the changes in canopy cover and litter input from timber extraction have large effects on the structure of aquatic communities.