Temporal trends and spatial patterns of chironomid communities in alpine lakes recovering from acidification under accelerating climate change

Abstract

Abstract The biological recovery of remote aquatic ecosystems from atmospheric acidification under ongoing climate change is a complex and poorly understood process, often developing along unanticipated trajectories. Our objective was to evaluate the interacting effects of changing water chemistry and climate on the biological recovery of chironomid communities in the Tatra Mountain lake district (Slovakia and Poland), one of the most strongly acidified alpine regions worldwide. Chironomid communities were chosen as a sensitive indicator to disentangle the effects of acidification and climate change. We used sediment records of a shallow, strongly acidified lake, Starolesnianske pleso, plus data on 77 alpine lakes sampled across the whole study region in 2000–2004 and 2010–2014. Paleolimnological data from Starolesnianske pleso revealed a dramatic effect of acidification on the chironomid communities from the mid‐20th century. A rapid decline of water pH and a depletion of the carbonate buffering system were accompanied by the establishment of a species‐poor chironomid community. The previously dominant Tanytarsus lugens‐type was replaced by the congeneric Tanytarsus mendax‐type in the late 1970s. A subsequent reversal of water chemistry from acidification starting in the early 1990s was followed by a compositional change towards a more diverse community. The effect of chemical recovery was, however, overwhelmed by rising temperatures. Tiny larvae of more thermophilous Corynoneura scutellata‐type quickly gained dominance in the lake community. Throughout the lake district, C. scutellata‐type significantly shifted towards higher elevations, by more than 100 m per decade. The absence of significant interactions between recovering water chemistry and rising temperatures suggests independent effects of these processes on chironomid communities. The observed climate‐related compositional shifts towards the dominance of small‐bodied species are in line with the metabolic theory of ecology predicting higher metabolic demands in warmer environments, which can favour smaller, faster growing organisms and may lead to future changes in the ecosystem functioning of alpine lakes in general. The combination of monitoring and palaeoecological data provided a useful tool for tracking environmental changes. This approach may also help in disentangling the roles of other multiple pressures on aquatic ecosystems. The sensitive responses of shallow Starolesnianske pleso to atmospheric acidification and climate warming suggest that small lentic waterbodies could be useful sentinels of freshwater deterioration, thus deserving special attention in the future monitoring of alpine environments.