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

Groundwater remains an underexplored habitat for protistan diversity and community dynamics relative to better-studied surface aquatic environments. To address this knowledge gap, we compared protistan communities at two sites, a wetland surface water body and a nearby shallow aquifer, using molecular analysis, to shed light on environmental drivers of both total and active protist communities. The 2-year time series with monthly sampling provided insight into seasonal patterns, and barcoding enabled taxonomic assignment. Our study identified differences in community composition and trophic modes associated with habitat type. Protistan communities in shallow groundwater exhibited pronounced seasonal dynamics, apparently temporally linked to their surface water counterparts. Higher absolute water levels in the backwater than in groundwater, along with a significant fraction of phototrophic protists sampled from the shallow aquifer, are consistent with groundwater recharge from surface water influencing the groundwater protistan community composition.