Proceedings of the 2023 Mississippi Water Resources Conference

Year: 2023 Authors: San Antonio K., Holifield M., Wu W., Huang H.

Sea level rise is an escalating threat to saltmarsh ecosystems as increased inundation can lead to decreased biomass, lowered productivity, and plant death. Another potential stressor is elevated nitrogen, which has a controversial impact on belowground biomass, potentially affecting the stability of saltmarshes and is relevant due to additional nitrogen brought into coastal regions via freshwater diversions. Our research objective is to examine the combined effects of inundation and nitrogen on common saltmarsh plants (Spartina alterniflora and Spartina patens). We set up two marsh organs with six rows and eight replicates in each row, one planted with S. alterniflora that occupies low, emergent marsh zones, the other with S. patens—a high marsh plant less tolerant of inundation and salinity. We randomly selected four replicates in each row to add 25 g/m2 of nitrogen in the form of ammonium nitrate every two or three weeks in the growing season. With the same frequency, we collected morphological characteristics such as plant height and leaf count to represent vegetation conditions in different dimensions over time. Furthermore, we harvested half of the marsh organ vegetation in Year 1 and the remaining in Year 2 to evaluate the short- and long-term impact of inundation and nitrogen on above- and belowground biomass (Year 2 belowground biomass still in processing). To help the inference, we developed multilevel Bayesian models. Our results show that inundation positively affected most of the characteristics of S. alterniflora measured over time while negatively affected S. patens, with both plants exhibiting some quadratic relationships for certain measurements and biomass. Exceptionally, leaf count and stem width of S. alterniflora showed maximum values at an intermediate inundation time, while stem width of S. patens reached the minimum at an intermediate inundation time. Aboveground biomass of both species responded to inundation differently in the short- and long-term. In the short-term, aboveground biomass of S. alterniflora reached the minimum at an intermediate inundation time, while there existed an optimum inundation time for long-term aboveground biomass and short-term belowground biomass. For S. patens, there existed an intermediate inundation level at which the short-term biomass reached the minimum, while inundation negatively affected long-term aboveground biomass and short-term belowground biomass. Additionally, for both species, plants with the nitrogen addition had higher aboveground biomass when compared to non-fertilized plants, while nitrogen addition had little to no impact on short term belowground biomass. This work will facilitate more-informed restoration and conservation efforts in coastal wetlands while accounting for climate change and sea-level rise.