Microbial Community of a Sandy Beach Subterranean Estuary is Spatially Heterogeneous and Impacted by Winter Waves

ABSTRACT Subterranean estuaries (STEs) are critical ecosystems at the interface of meteoric groundwater and subsurface seawater that are threatened by sea level rise. To characterize the influence of tides and waves on the STE microbial community, we collected porewater samples from a high‐energy beach STE at Stinson Beach, California, USA, over the two‐week neap‐spring tidal transition during both a wet and dry season. The microbial community, analyzed by 16S rRNA gene (V4) amplicon sequencing, clustered according to consistent physicochemical features found within STEs. The porewater community harbored relatively abundant Proteobacteria, Verrucomicrobiota, and Bacteroidota, as well as members of the archaeal DPANN superphylum and bacterial Candidate Phyla Radiation (CPR). Tidal conditions were not associated with microbial community composition; however, a wave overtopping event significantly impacted the beach microbiome. As a baseline for environmental change, our results elucidate the unique dynamics of a STE microbiome with unprecedented temporal resolution, highlighting the transport of cellular material through beach porewater due to waves. Porewater samples were collected from a high‐energy beach subterranean estuary at Stinson Beach, California, USA, over the two‐week neap‐spring tidal transition during both wet and dry seasons. Shifts in microbial community composition, analyzed by V4 16S rRNA gene sequencing, were contextualized with extensive hydrological and physicochemical measurements. The vertical depth structure of the community remained consistent over tidal cycles and seasons but was disturbed by a set of overtopping waves in winter.