Acid rain became a serious threat to freshwater ecosystems in the industrial era, because emissions from internal combustion engines burning various hydrocarbon fuels produce oxides of nitrogen and sulfur, which react with water in the atmosphere to form nitric and sulfuric acids, respectively.

Aquatic plants can exert control over the pH of their medium in several ways. Photosynthetic uptake of CO2 or HCO3- raises pH, while respiratory release of CO2 lowers pH. Uptake of negatively charged nitrate ions raises pH, while uptake of positively charged ammonium ions lowers pH. Consequently, the effects of acid rain on freshwater ecosystems may be moderated or exacerbated by these plant-driven processes.

We will test several hypotheses in experimental systems with aquatic weeds: 1) Photosynthesis raises pH and respiration lowers pH; 2) nitrate uptake raises pH and ammonium uptake lowers pH; 3) addition of acid lowers pH unless plant processes such as nitrate uptake and photosynthesis operate to neutralize acidity. We will address the overall question: Under what biological conditions is acid rain likely to be a threat to the health of freshwater ecosystems?

Procedure

We will use 300 ml BOD bottles as experimental vessels. Each vessel will receive the appropriate plant (+/-), nutrient (NO3, NH4, -) acid (+/-), and light/dark treatment and we will use 2 replicates of each. We will measure the pH in each vessel at the beginning and end of a 24 hour period (volunteers needed to do pH readings at the end). Data will be put into an Excel file, printed, posted, distributed, and discussed at the next lab time.

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