Spatio-temporal Variation in Biomass of Herbaceous Wetlands across Distinct Hydrogeomorphic Zones in the Atchafalaya and Terrebonne Basins, LA, USA

The Atchafalaya and Terrebonne Basins (AB, TB) in the Mississippi River Delta (MRD) are examples of active and inactive coastal basins, respectively; and the latter has experienced extensive deterioration over the last century. Long-term modifications in hydrology and sediment supply continue to cause changes in vegetation dynamics, distribution, and composition threatening the MRD's long-term sustainability in inactive basins, such as TB; in contrast to active basins that are gaining land each year.

Using a combination of in-situ biomass (AGB) and necromass (AGN) measurements combined with airborne imaging spectrometry (AVIRIS-NG), we investigated how vegetation dynamics change in response to seasonality, soil properties, and salinity gradients across different hydrogeomorphic (HGM) zones (supratidal, intertidal) in the AB and TB coastal deltaic basins. In addition, we generated landscape-level vegetation maps of the two basins to identify patterns of change in AGB and C- stocks.

Total AGB increased from freshwater (FW) to saline (SL) sites along both basins and was significantly different across sites and seasons. Unlike AGB, total AGN decreased during peak biomass season and was significantly different among sites. Aboveground net primary productivity (ANPP) rates varied across basins. SL sites had the highest ANPP (919 ± 42 g m-2 yr-1), followed by FW marshes (742 ± 275 g m-2 yr-1). Species diversity increased with seasonality in FW sites across basins. Foliar nutrient concentrations (N, P) were significantly different across sites and HGM zones in both basins during early growing and peak biomass seasons. Mean δ15N of the dominant FW species in AB was significantly more enriched (9.4 ± 0.9 ‰) compared to that of FW species in TB (2.3 ± 0.4 ‰). These differences likely reflect the ability of FW species to fix N and adapt to N-limited soil conditions. Foliar δ13C values varied among zones and sites, as well as between species. Saline and brackish species had higher δ13C values compared to FW species in both basins, which is likely the result of plant adaptations to environmental stressors such as elevated salinity.

The results show the importance of in-situ and landscape-level assessment of vegetation in response to seasonality and salinity gradients across ecologically vulnerable deltaic basins in the MRD.