Salt marsh as a coastal filter: Marsh plant uptake of nitrogen at the scale of the estuary indicates saturated capacity
Coastal salt marshes provide multiple ecosystem services and are important places to study global change interactions, because multiple impacts converge at the land-sea interface. Nitrogen (N) pollution and sea-level rise are co-occurring perturbations, yet their combined effects in salt marshes are poorly understood. We investigated the role of salt marsh in a central California estuary, Elkhorn Slough, as a “coastal filter” to intercept nitrogen, using the elevation gradient in the intertidal zone – and concomitant differences in inundation – as a window into sea-level rise. Given a body of evidence that marshes provide a nitrogen-filtering function between land and sea and that marsh sustainability is at risk, both globally and in particular regions, what might happen to the ecosystem service of marsh as a coastal filter? In an observational experiment, in nine marshes distributed evenly around the whole estuary, we crossed the two factors of a) intertidal height within salt marsh and b) main-channel-Slough nitrogen concentrations (“high” and “low”). We measured above- and belowground plant biomass (where the dominant species is pickleweed, Sarcocornia pacifica), tissue N concentrations, and N sequestered in plant biomass in the spring after the peak delivery of N in the rainy season.
We found that across a range of intertidal elevations, salt marsh vegetation was not sequestering additional main-channel N into biomass, indicating saturated capacity. The one exception to saturation was an increase in tissue-N concentration in new-growth pickleweed (S. pacifica) at mid elevations. Root biomass decreased significantly at the highest main-channel N level, lending support to the hypothesis that increased nutrients can have a detrimental effect on belowground biomass and therefore organic marsh-platform building in equilibrium with sea-level rise. Aboveground plant biomass decreased significantly along a vertical gradient from high- to low marsh in the fully-tidal sites; the loss of plant biomass with greater inundation further reduces the capacity of the marsh to buffer N loading.
In these observational findings, the impact of the highest level of Slough nitrogen on the ocean – where N inputs globally and regionally are forecasted to continue to increase – is not buffered, since salt marsh N-capture is at the limits of its capacity. Climate change is expected to exacerbate existing pollution problems, emphasizing the need to reduce land-based nutrient additions to waterways.