Glaciers are known to harbor surprisingly complex ecosystems. On theirsurface, distinct cylindrical holes filled with meltwater and sediments areconsidered hot spots for microbial life. The present paper addressespossible biological interactions within the community of prokaryoticcyanobacteria and eukaryotic microalgae (microalgae) and relations to theirpotential grazers, such as tardigrades and rotifers, additional to theirenvironmental controls. Svalbard glaciers with substantial allochthonousinput of material from local sources reveal high microalgal densities. Smallvalley glaciers with high sediment coverages and high impact of birds showhigh biomasses and support a high biological diversity. Invertebrate grazerdensities do not show any significant negative correlation with microalgalabundances but rather a positive correlation with eukaryotic microalgae. Sharedenvironmental preferences and a positive effect of grazing are the proposedmechanisms to explain these correlations. Most microalgae found in this studyform colonies (?25?μm), whichmay protect them against invertebrate grazing. This finding rather indicatesgrazing as a positive control on eukaryotic microalgae by nutrient recycling.Density differences between the eukaryotic microalgae and prokaryoticcyanobacteria and their high distinction in redundancy (RDA) and principal component (PCA) analyses indicatethat these two groups are in strong contrast. Eukaryotic microalgae occurredmainly in unstable cryoconite holes with high sediment loads, high N?:?Pratios, and a high impact of nutrient input by bird guano, as a proxy fornutrients. In these environments autochthonous nitrogen fixation appears tobe negligible. Selective wind transport of Oscillatoriales via soil and dustparticles is proposed to explain their dominance in cryoconites further awayfrom the glacier margins. We propose that, for the studied glaciers, nutrientlevels related to recycling of limiting nutrients are the main factor drivingvariation in the community structure of microalgae and grazers.
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