Growth and emergence of the stonefly Leuctra nigra in coniferous forest streams with contrasting pH

Abstract

1. Growth and emergence of the acidification tolerant stonefly Leuctra nigra were studied in four coniferous forest streams with contrasting pH. Invertebrate samples were taken bimonthly during May 1997 to April 1998. Emerging L. nigra were captured in two of the streams during April to July 1998 using pyramidal emergence traps. In addition, a laboratory experiment was conducted to evaluate growth of L. nigra in response to variations in food quality (neutral or acid conditioned leaves) and water pH.2. The invertebrate community in low pH streams had lower diversity and density compared with neutral streams. Abundance of L. nigra was also lower, despite the dominance of this stonefly in the invertebrate community in the acidic streams.3. Secondary production of L. nigra was 2.04 and 3.07 g DW m–2 year–1 in the neutral streams compared with 1.02 and 1.27 g DW m–2 year–1 in the acidic streams. The P/B ratio differed only slightly among streams.4. More adult L. nigra emerged in the neutral stream compared with the acidic stream, despite the density of the pre‐emerging fifth instar nymphs of L. nigra being similar in the two streams. Adult size (expressed as wing length) and female fitness were also significantly greater in the neutral stream.5. Laboratory growth of L. nigra nymphs was significantly lower when fed acid conditioned alder leaves rather than neutral conditioned leaves. There was no difference in growth when nymphs were fed neutral conditioned leaves in acid and neutral water.6. Our study shows multiple, sub‐lethal effects on life history of the acidification tolerant stonefly L. nigra in coniferous streams with low pH. The results suggest that these effects could be mediated primarily by low food quality. These findings add to the existing evidence that coniferous plantation forestry has a negative impact on stream ecosystems.