Epilithic Algal and Meiofaunal Assemblages of Arctic/Alpine Lakes and Streams Along Latitudinal and Environmental Gradients in Scandinavia

ABSTRACT Epilithic biofilms along a latitudinal gradient of Arctic/alpine lakes and streams were analysed for their composition of algae (incl. cyanobacteria) and meiofaunal assemblages and correlated these with environmental variables. The composition of algal, meiofauna and nematode assemblages differed between lakes and streams. Algal biovolumes were higher for lakes (373–6456 mm 3 /10 cm 2 ) than for streams (7.1–8763 mm 3 /10 cm 2 ), as were their carbon concentrations (32.9–393 and 10.7–211 mgC/gDW, respectively). Stream assemblages were frequently predominated by green algae, that is, > 48% of total biovolume, whereas lakes were predominated by diatoms ( n = 3 lakes) or cyanobacteria ( n = 8 lakes). Meiofauna biomass averaged 486 ± 90.3 μgDW/10 cm 2 for lakes and 262 ± 94.2 μgDW/10 cm 2 for streams and was numerically predominated by nematodes, followed by rotifers. Harpacticoid copepods and cladocerans, largely absent in streams, were most abundant in lakes. Nematode species richness ranged 7–25 for lakes and 3–20 for streams and showed a strong predominance of bacteria‐feeders (> 80%). Algal assemblages did not correlate with environmental variables, but lakes split into two distinct subgroups, separated by nutrients and TOC. Abundances of cladocerans, nematodes and cyclopoids correlated with nutrient concentrations and TOC, while nematode composition also correlated with N, Cl and with diatom biovolume. Estimates of meiofauna biomass were similar to or exceeded those of epilimnetic zooplankton, while the biovolume of benthic algae was more than 1000× higher than that of epilimnetic phytoplankton in similar lakes. Combining these patterns emphasises the importance and complexity of the epilithic biofilm habitat and its role in lake food webs. Our study revealed biofilm assemblages of lakes and streams are highly diverse and distinctly different. This unique dataset provides insight into the limited latitudinal changes in meiofauna assemblages of northern lakes and streams and establishes a baseline to gauge the effects of future climate‐induced change.