Beneath sandy beaches, microbes filter chemicals from groundwater and safeguard ocean health. A Stanford-led study reveals that sneaker waves provide a lens to explore the impending impacts of sea level rise on beach hydrology, chemistry, and microbiology.
A hidden world teeming with life lies below beach sands. New Stanford-led research sheds light on how microbial communities in coastal groundwater respond to infiltrating seawater. The study, published Dec. 22 in Environmental Microbiology, reveals the diversity of microbial life inhabiting these critical ecosystems and what might happen if they are inundated by rising seas.
“Beaches can act as a filter between land and sea, processing groundwater and associated chemicals before they reach the ocean,” said study co-first author Jessica Bullington, a Ph.D. student in Earth system science in the Stanford Doerr School of Sustainability. “Understanding how these ecosystems function is key to safeguarding their services in the face of sea level rise.”
The research team conducted the intensive study at Stinson Beach, north of San Francisco. Stinson Beach is representative of a “high-energy” beach, which has only a handful of previous papers on the microbiome worldwide.
Microbial guardians
Microbial communities living in groundwater within beach sand play a crucial role in maintaining coastal water quality. These microbes help break down chemicals, including excess nutrients like nitrogen, which can come from natural sources, such as decomposing plant matter, or human sources, like agricultural runoff and wastewater.
To better understand the dynamics of this microbial filtering system, the research team headed to Stinson Beach. Over two weeks, during both a wet and dry season, they collected samples from the beach’s subterranean estuary around the clock to capture changing tides. Then, the researchers analyzed the microbial DNA using advanced gene sequencing techniques. This approach — the first of its kind at such a fine time scale — provided unprecedented insight into the microbial community’s composition and stability.
The researchers found that the microbial communities remained relatively stable over changing tidal conditions and seasons. However, a wave overtopping event — when seawater surged into the aquifer due to high-energy waves — caused significant changes in the microbial makeup. Such disturbances are expected to become more frequent with rising sea levels and storm surges, making it harder for the microbes to do their water purification work.
“These microbes live in complex communities, many with specialized roles that include processing nutrients and even producing or consuming greenhouse gases,” said co-senior author Christopher Francis, a professor of Earth system science and of oceans in the Stanford Doerr School of Sustainability. “The microbial community’s resilience under typical conditions is encouraging, but disturbances like wave overtopping highlight their vulnerability to climate change,” said co-first author Katie Langenfeld, a postdoctoral scholar in civil and environmental engineering at Stanford at the time of the research and current postdoctoral fellow at the University of Michigan.
Implications for coastal resilience
The study’s findings establish a critical baseline for understanding how subterranean estuaries function and respond to environmental changes. As sea levels rise, beach sands will be forced inland or erode, altering groundwater hydrology, chemistry, and microbial composition.
The research adds a crucial piece to the puzzle of coastal resilience. By highlighting the interplay between microbial dynamics and physical processes like wave action, the study brings into question impending changes to coastal groundwater. Policymakers and coastal planners should consider the role of these hidden ecosystems when designing strategies to manage sea level rise, according to the researchers.
“We rely on these microbial communities for essential biogeochemical cycling at the land-sea interface,” said co-senior author Alexandria Boehm, the Richard and Rhoda Goldman Professor of Environmental Studies in the Stanford Doerr School of Sustainability and the Stanford School of Engineering. “If their capacity diminishes due to climate impacts, we could see cascading effects on coastal water quality and marine life.”