Invertebrate community change in acidified rivers and lakes in Southern Norway

Abstract

International policy action has led to significant reduction of sulphur emissions, and subsequent deposition, which resulted in widespread improvements in the water chemistry of acidified regions. However, biological recovery appears to be much slower than chemical recovery. The aim of this study was to analyze temporal changes of invertebrate communities in acidified rivers and lakes, and disentangle the environmental causes of the observed changes. The analysis was performed using long-term data from rivers (25-30 years) and lakes (~15 years) in three catchments along the west coast of Southern Norway, which represent a gradient of non-marine sulphur deposition. We performed statistical analysis using both biological indices and ordination methods. First, our results show that invertebrate communities in rivers shifted from a state with low or absence of acid-sensitive taxa to an alternate state with higher richness and abundance of acid-sensitive taxa. These biological shifts coincided with previously defined chemical thresholds. Second, there was no invertebrate community change recorded in the lakes, even though chemical recovery was comparable to the rivers. This finding may relate to limited variation in habitat and refuges, which may make them less susceptible to re-colonisation of acid-sensitive taxa, or it may relate to the taxonomic resolution. Third, our result indicate that the recovery of invertebrate communities in rivers is primarily related to reduced sulphur deposition, and associated water chemistry. Superimposed on the long-term trends, temperature fluctuations and sea-salt episodes have caused short-term variability in the invertebrate community. We did not identify impacts of temperature rise on the long-term invertebrate community trends. We conclude on the importance of a continuous research effort to disentangling the complex link between acidification, and climate on invertebrate community change.