Importance of hydrogen ions and aluminium in regulating the structure and function of stream ecosystems: an experimental test

Abstract

1. Experiments simulating spring acidic snowmelt episodes were conducted to determine the effects of short‐term inputs of H+ and Al on the chemistry and biology of a poorly buffered mountain stream. HC1 and A1C13 were added in separate experiments to first‐ to third‐order reaches of a New Hampshire stream.2. Cation exchange and Aln+ dissolution reactions neutralized experimentally added H +, whereas groundwater dilution was insignificant. Mobilized Ca++, Mg++ and Aln+ concentrations progressively increased from third‐ to first‐order reaches during HC1 additions. Mobilization of Ca++ and Mg++ was greater during A1C13 than HC1 addition.3. Total phosphorus was mobilized from stream sediments during both HCI and A1C13 addition. Dissolved organic carbon (DOC) decreased during A1C13 addition in the second‐order but not in the third‐order reach. DOC concentration decreased during HCI addition only when Aln + mobilized from the stream bottom was >0.28 mg AI 11.4. Production of foam at the water surface during AlCl3 addition to a second‐order and HCl addition to a first‐order reach indicated a reduction in surface tension of the streamwater and may be related to complexation reactions between Al and DOC at low pH (4–5).5. Mayfly nymphs and blackfly and chironomid larvae drifted at greater rates from HCl‐ and AlCl3treated sections of first‐ and second‐order streams than from corresponding reference areas.6. When stream pH was lowered to 5.25–5.5 by HCI alone (15 μg monomeric inorganic Al l−1), the behaviour of aquatic invertebrates did not change, but pH reduced to the same range during Al additions (280μg Al 1−1) did affect it. Therefore, fluctuating aluminium concentrations in low‐order streams at a pH range of 4.5–5.5 may alter the biology and geochemistry of poorly buffered waters.