Dynamic changes of protist community composition along a surface water-groundwater transect in the Danube wetland Lobau, Vienna, Austria

Groundwater remains an unexplored habitat in terms of protistan diversity and community dynamics, when compared to better-studied surface aquatic environments. To address this knowledge gap, a comparison of protistan communities found at two differing sites, a wetland surface water body and the close-by shallow aquifer, aims to shed light on environmental drivers of both the total and active protist communities using molecular analyses. The two-year time series with monthly samplings provided insight into seasonal pattern, and barcoding allowed taxonomic affiliation. Our study revealed clear differences in community composition and trophic modes with the habitat type. Protistan communities in shallow groundwater exhibited pronounced seasonal dynamics somehow temporally linked to the surface water counterparts. Higher absolute water levels in the backwater compared to groundwater and the significant fraction of phototrophic protists sampled from the shallow aquifer point at groundwater recharge from surface water to strongly influence protistan community composition.