Salt marsh as a coastal filter: Marsh plant uptake of nitrogen at the scale of the estuary indicates saturated capacity

Background/Question/Methods

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.

Results/Conclusions

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.