CO2-driven experimental acidification effects on aquatic macroinvertebrates in a tropical stream

Abstract

ABSTRACTExperimental acidification of a tropical stream was conducted to measure the effects of declining pH on aquatic macroinvertebrates. The mechanisms by which anthropogenic acidification occur in a freshwater stream system are relatively well understood, while little is known about the natural phenomena of acidification or the corresponding effects on macroinvertebrate assemblages. Previous studies have attempted to model stream acidification using strong acids; however, this is one of the first studies which models stream acidification using the addition of gaseous CO2. This method is a more natural means of modeling stream acidification conditions arising from increased levels of dissolved CO2. We hypothesized that if experimental acidification was expressed most strongly at the injection site and produced a pH gradient downstream, macroinvertebrates should respond to the gradient and employ an escape mechanism to avoid the adverse conditions. Three macroinvertebrate sampling strategies were used: drift nets, leaf pack samples and benthic Surber samples. Samples were evaluated in the lab for macroinvertebrate abundance and taxonomic richness per m3 for drift net samples and benthic samples, and per g leaf material in the leaf pack samples. A maximum decline of 2 units in pH along a gradient was observed associated with the injection of CO2. Results obtained from drift net and benthic sample analysis were inconclusive, possibly because of low stream flow, although analysis of the leaf pack samples indicates lower macroinvertebrate composition at areas of lower pH. The leaf pack samples also show significant macroinvertebrate sensitivity to the most severe pH decline at the injection site.