Most beachgoers enjoying the Sun and surf don’t think very much about the wet sand beneath their feet. But this soggy, sandy zone is of prime ecological importance. Nutrients and contaminants leach into the marine environment here, and the mix of salty water and freshwater can drive chemical reactions that affect coastal organisms. Now a new study demonstrates that this tidal zone is even more dynamic and complex than previously thought.
Quartz-rich beach sand was once thought to be rather boring biochemically because it doesn’t contain much organic matter. But Kim et al. suspected otherwise, particularly at the boundary where salty water and freshwater mix underground. As waves and tides drive salty water inland, it collides with freshwater underground, and together they flow to the sea as submarine groundwater discharge. The interaction between groundwater and ocean water can accelerate chemical processes such as oxygen consumption and nitrate removal, which produces nitrogen gas.
To determine whether this boundary was, indeed, more biochemically active, the team installed seven polyethylene multilevel sampling wells roughly 3–4 meters below the mean sea level along a 30-meter transect of a beach near Cape Henlopen in Delaware. They collected samples of water from these tubes and incubated them for 15 days, measuring how much oxygen their contents consumed and how much nitrogen gas they produced. The researchers also recorded other important hydrochemical properties, such as salinity and pH.
As expected, the data revealed hot spots of nitrogen gas emission and oxygen consumption, particularly in samples collected at locations where mixing of salt water and freshwater is known to happen because of salinity distributions, suggesting that the consumption of organic matter by bacteria is enhanced in this zone. Surprisingly, the team did not find higher oxygen consumption where chlorophyll, a sign of microorganisms called algae, was present. That result suggests the sand contains a different form of reactive carbon available for consumption that was not detected by their methods. The scientists expect that this other form of reactive carbon may play an important role in the ecology of intertidal aquifers. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2017JG003943, 2017)
—Emily Underwood, Freelance Writer