Chironomid-climate continentality conundrum

Abstract

It is predicted that continentality, a climate parameter representative of a region's annual temperature and precipitation range, will undergo significant changes in the future. The lack of past continentality reconstructions makes it impossible to decipher any long-term patterns of continentality changes. Here, we investigate the extent to which continentality influences modern chironomid assemblages and evaluate their ecological relevance for palaeolimnological data-based reconstructions of past continentality. We selected 53 lakes along a longitudinal gradient covering the East European Plain (Western part of Russia, Estonia, Latvia) and southern Scandinavia (Sweden and Norway). We analysed the dependency of chironomid assemblages on a variety of environmental parameters including two continentality indices (annual temperature range (ATR) and the Kerner Oceanity Index (KOI)), growing degree days at base temperature 5 °C, mean air temperatures of July, April, and October, number of ice-cover days, lake-water pH, loss-of-ignition and water depth using redundancy analysis. Correlations between all variables were tested to check for possible confounding effects. KOI had the highest explanatory power of 18.4% in the dataset and an absence of collinearity (correlation index < 0.7) with all the other tested variables. Further, we estimated weighted average optima to investigate the distribution of the morphotypes along the continentality gradient in the dataset. Glyptotendipes pallens-type, Neozavrelia, Polypedilum sordens-type, and Microchironomus showed a preference for a continental climate, while Paratanytarsus penicillatus-type, Pseudorthocladius, Thienemannimyia, and Limnophyes were found mainly in samples from oceanic areas. Weighted averaging-partial least squares regression was used for a trial test of the data, resulting in a promising KOI-based model performance with R2 = 0.73 and RMSEP = 5.1. Despite the relatively small dataset, our study suggests that chironomid data have the potential for further development as a tool for reconstructing palaeocontinentality.

Fig 1. Map of sampled lakes with respect to mean July air temperature (MJAT; °C), ice-cover duration (days), annual temperature range (ATR; °C), and Kerner Oceanity Index (KOI; continental area (orange) is KOI −10 - 0, transitional (green) is KOI = 0–10, and oceanic (blue) is KOI 10-20).

Fig 2. (A) Principal component analysis (PCA) with total variation of 27.6% and (B) Spearman correlation matrix of the climatic and environmental variables in the dataset: bedrock type; lake-water depth (m); longitude; lake-water pH; lake ice-cover (days); growing degree days with base temperature 5°C (GDD5); October (Oct T), January (Jan T), April (Apr T) and July (Jul T) mean air temperatures (°C); Kerner Oceanity Index (KOI); and annual temperature range (ATR); soil base saturation (soil BS); soil clay content (soil clay); soil sand content (soil sand). Continental sites (orange) are KOI −10–0, transitional (green) are KOI = 0–10, and oceanic (blue) are KOI 10–20.

Fig 3. Chironomid morphotypes with abundances in the dataset of at least 2% in one sample.

Species are arranged in abundance according to the longitudinal gradient. Continental sites (orange) are KOI −10–0, transitional (green) are KOI = 0–10, and oceanic (blue) are KOI 10–20.

Fig 4. Redundancy analysis (RDA) plot showing the climate and environmental variables, revealed the significant dependency in the dataset: lake-water depth (m); longitude; lake water pH; bedrock type; soil base saturation (soil BS); soil clay content (soil clay); soil sand content (soil sand); July (Jul T), January (Jan T), October (Oct T), and April (Apr T) mean air temperatures (°C); lake-ice cover (days); growing degree days with base temperature of 5°C (GDD5); Kerner Oceanity Index (KOI); and annual temperature range (ATR).

Variables explain 44.6% of variation in total with a p-value of 0.001. Continental sites (orange) correspond to −10–0 KOI, transitional sites (green) to 0–10 KOI, and oceanic sites (blue) to 10–20 KOI.

Fig 5. Weighted-average based Kerner Oceanity Index (KOI) optima and tolerances for morphotypes revealed as indicators by INDVAL.

The continentality group affiliation identified by INDVAL is marked in brackets: oc – oceanic, tr – transitional, cont – continental; all taxa revealed statistical significance in the corresponding zone based on IndVal. The background is colored according to the KOI: continental (orange) for KOI < 0, transitional (green) for KOI 0–10, oceanic (blue) for KOI > 10.

Fig 6. (A) Diagnostic plot of cross-validated estimates of the dataset compared with observed Kerner Oceanity Index (KOI) values and (B) residuals plot from a weighted-average partial least squares (WA-PLS) model based on two components.

Continental sites (orange) correspond to −10–0 KOI, transitional sites (green) to 0–10 KOI, and oceanic sites (blue) to 10–20 KOI.