Experimental acidification effects on aquatic macroinvertebrates in a tropical stream

Experimental acidification of a tropical stream was conducted using CO₂ tanks in order to measure the effects of declining pH on aquatic macroinvertebrates. Streams are characterized by having highly variable levels of temperature, discharge, and pH. Of these physiochemical factors affecting macroinvertebrates, the one of growing concern is pH. Previous studies have attempted to model stream acidification using hydrochloric acid; however, this study models stream acidification with the addition of gaseous CO₂. This method is a more natural means of modeling stream acidification conditions arising from increased atmospheric levels of CO₂. If experimental acidification is expressed most strongly below the injection site and produces a pH gradient downstream, macroinvertebrates should respond to the gradient and enter into drift to escape adverse conditions. Three macroinvertebrate sampling strategies were used: direct measurement using drift nets, indirect measurement using leaf pack samples, and indirect measurement using benthic samples. Each method was conducted at sampling sites at 4m upstream and 4m, 10m, and 29.5m downstream of the CO₂ injection site. A decline in stream pH along a gradient was observed associated with the injection of CO₂. No significant results were obtained from drift net sample analysis, possibly because of low stream flow, although analysis of both the leaf pack samples and benthic samples showed a trend indicating lower biomass at areas of lower pH. The leaf pack samples in particular show significant macroinvertebrate sensitivity to the most severe pH decline at the injection site, which could indicate a possible effect on leaf litter decomposition.