Macroinvertebrate richness and density increase over time with rebounding stream pH – a case for recovery?

Acidification inhibits macroinvertebrate ion balance causing decreased diversity. Despite widespread increasing stream-water pH following reduced industrial emissions, little is known about how macroinvertebrates recover from acidification. Our objective is to assess changes in macroinvertebrate structure and function in response to rebounding stream pH. We predicted that richness and density of acid-sensitive taxa and traits over time would increase with increasing pH. For example, Ephemeroptera and gilled taxa densities were predicted to increase over time with the alleviation of ionoregulatory stress associated with low stream pH. Taxa and traits were compared for 13 streams in 3 geologic classes (4 siliciclastic, 6 granitic, and 3 basaltic) over 30 years in Shenandoah National Park, Virginia, USA. As predicted, macroinvertebrate richness and density increased over time. Across all streams, richness increased (mean ± cv) from 16 ± 23% to 30 ± 20% taxa and density increased from 45 ± 108% to 133 ± 46% (individuals/m²) from 1987 to 2017. Ephemeroptera genera, Epeorus and Cinygmula, densities increased. Desiccation resistant, weak-flying, and soft-bodied taxa also became more common. Observed changes in taxa and trait composition suggest potential recovery from acidification that could increase ecosystem resilience to future stressors and functional capacity (e.g., secondary production).