Salinity gradients in coastal river corridors impact organic matter processes and ecosystem function

Background/Question/Methods

Tidally-influenced portions of watersheds are among the most biogeochemically active and diverse natural transition systems on Earth and influence global carbon and nutrient budgets. The dynamic mixing of fresh and saline waters in both surface and subsurface domains in tidal river corridors, however, are prone to future disturbance events like droughts, storm surges, intense rain events and coastal flooding, with 88,000 miles of coastal shoreline in the conterminous U.S. predicted as vulnerable. However, we have limited mechanistic and predictive understanding between these episodically reduced or elevated freshwater input and the links among salinity, sediment characteristics, nutrient dynamics, and microbial ecophysiology in coastally-influenced watershed components. A distributed, community-enabled sediment sampling campaign was conducted across 24 locations (12 non-tidal freshwater streams and 12 tidally-influenced saline regions of tidal rivers/creeks) in the United States. Using microcosm batch reactors, sediments were subjected to six salinity treatments to determine how altered salinity levels impact biogeochemical processes. Bulk biogeochemical responses, including dissolved oxygen consumption, carbon dioxide (CO2) production, and redox ion concentrations in the reactors, chemistry of dissolved organic matter using Fourier Transform Ion Cyclotron Resonance Mass Spectroscopy (FTICR-MS) and metabolite profiles of dissolved organic matter using Liquid Chromatography Mass Spectroscopy (LC-MS) were evaluated.

Results/Conclusions

We found that metabolic efficiency (defined as the total energy needed for the synthesis of a unit C-mole of biomass) of dissolved organic matter decreased in field sediments from non-tidal river systems from the western to eastern United States but saline sediments did not show any difference. LC-MS features showed distinctly separate metabolite profiles for field tidal saline and non-tidal fresh sediments. Lab data showed conserved trends of increasing CO2 flux with increasing salinity for freshwater locations for most sites and showed no or slight increase for saline sediments. Complemented with microbial community characteristics, results will be used to develop predictive understanding of sediment biogeochemical responses to salinity perturbations.