Tom Wessels, MA
Department of Environmental Studies
Many studies have examined the relationship between macroinvertebrate abundance and pH, often concluding that pH exerts an indirect effect on macroinvertebrate abundance and diversity—for example, by influencing fish abundance—which causes the observed trends in community composition. This project took a regional approach to the question, with the goal of isolating the effect of pH on odonate species richness from the other potentially confounding variables. Odonate (dragon- and damselfly) nymphs are one group of large, predaceous insects that may be potential indicator species in aquatic systems. During May and June of 1999, the nymphal odonates in depths of =1m were sampled at 21 ponds in the Ipswich and Parker River drainage basins (Essex County, MA), using a quantitative sampling technique. They were identified to as low a taxonomic level as possible (usually species), and their distribution data was analyzed to determine if odonate species richness is varying along a pH gradient in these ponds. Other physical parameters of the ponds—alkalinity, turbidity, transparency, maximum depth, pond area, number of distinct littoral habitats, insectivorous fish abundance—were also measured and assess for potential effects on odonate species richness. Analysis of variance revealed a significant positive relationship between odonate species richness and pond pH (range: 4.88-6.60). Ponds with a lower pH had significantly fewer species of odonates. Overall, turbidity was found to exert a significant, negative effect on odonate species richness. The ponds with higher (circumneutral) pH separate readily into two groups with regard to species richness; the ponds with lower species richness have significantly higher turbidity values than the ponds with higher species richness. Pond area and insectivorous fish abundance were not found to significantly effect odonate species richness in this study.