Salt marsh as a filter for the coastal oceanElkhorn Slough National Estuarine Research Reserve, Monterey Bay, California
I am the lead researcher in a project investigating agricultural nutrient runoff on land, sea-level rise in the ocean, and what happens when those two impacts collide at the land-sea edge in salt marshes. Coastal salt marshes are among Earth’s most productive ecosystems and provide a number of benefits, including interception of watershed-derived agricultural and urban fertilizers before they reach nearshore oceans. Marshes catch agricultural nitrogen (N) headed downstream, but marsh plants can be killed by the prolonged inundation of rising seas. When salt marsh takes up N into plant tissue, it serves as a coastal filter, protecting the ocean from N pollution. However, its ability to do so will be constrained by how and where it can live, given sea-level rise, which is changing the character and location of the coast. In contrast to my previous field experiment, I found that plants exposed to higher N do not take up any more N: they are at their maximum capacity. Although biological systems are doing a great deal to buffer human impacts, we must look upstream to improve the sustainability of land-use practices.
Questions we're asking
- Are salt marsh plants taking up more nitrogen at sites where more nitrogen (with terrestrial origins) is delivered from the estuary?
- If so, do they provide more and more protection as we escalate the threat and consequences of nutrient pollution, due to mainstream farming practices? Or are salt marshes at a saturation point as a buffer between land and sea?
- Are there trade-offs between how much nitrogen marsh plants take up (a local buffer for the ocean) and how much carbon they store (a global climate-change mitigation)?
Problems we're solving
Nitrogen (N) pollution, typically due to run-off from agricultural and urban lands, has increased exponentially in recent decades, and it can lead to harmful blooms of one-celled algae (phytoplankton) or visible algal mats; changes in food webs and biodiversity; and hypoxic (low-oxygen) or anoxic (no-oxygen) ocean regions, also called ‘‘dead zones.” In the United States, three quarters of all major estuaries have hypoxic dead zones. Elkhorn Slough has some of the highest nitrogen concentrations, and highest variability in concentrations, of any temperate estuary in the world. Recent research connects low-oxygen events in the Slough with lower commercial fish catches a year later in Monterey Bay.* Salt marshes are living, self-regulating systems that intercept N and protect the nearshore oceans from dead-zone status. At the same time, they buffer storm surge, store carbon, and serve as nursery habitat for juvenile fish. Quantifying how much N salt marsh plants take up before that N can hit the ocean indicates the extent to which biological systems are buffering threats.
*Reference: Hughes, B. B., et al. (2015). “Climate mediates hypoxic stress on fish diversity and nursery function at the land–sea interface.” Proceedings of the National Academy of Sciences.
Project participants: J.L. Nelson, Elkhorn Slough National Estuarine Research Reserve postdoctoral fellow, now independent researcher; K. Wasson, Elkhorn Slough National Estuarine Research Reserve, California; E.B. Watson, U.S. Environmental Protection Agency, Rhode Island; E. Zavaleta, UC Santa Cruz, California.
Photo credit: JL Nelson 2007, Coyote Marsh, ESNERR, California