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From tides to seasons: How cyclic tidal drivers and plant physiology interact to affect carbon cycling at the terrestrial-estuarine boundary

Active Dates 8/15/2021-8/14/2024
Program Area Environmental Systems Science
Project Description
Coastal ecosystems are among the most biologically and biogeochemically active and diverse systems on
Earth. Because they act as important linkages between terrestrial ecosystems and the open ocean, their
incorporation in Earth system models (ESMs) is critical to predict coastal and global responses to
environmental changes. However, they vary greatly in the magnitude of tides and the volume and timing
of freshwater input from land, making it challenging to model the major biogeochemical reactions that
control productivity and greenhouse gas emissions across coastal terrestrial aquatic interfaces (TAIs).
Key characteristics that distinguish coastal wetlands, such as tidal oscillation, sulfur
biogeochemistry, and plant structural adaptations to anaerobic soil, have only very recently been
incorporated in land surface models such as ELM-PFLOTRAN. There remains large uncertainty in their
parameterization. Particularly challenging are: 1) the small-scale, dynamic, heterogeneous redox
conditions in wetland soils; 2) the aerenchyma tissue in wetland plants that greatly facilitate gas flow into
and out of sediment; and 3) the temporal and spatial variability in salinity, which is a key determinant for
plant species distribution and productivity, as well as organic matter decomposition.
Our overarching goal is to improve mechanistic process understanding and modeling of tidal
wetland hydro-biogeochemistry in coastal TAIs. Working in brackish marsh, we will combine intensive
and new spatially-explicit sediment redox measurements with continuous sediment redox, salinity, and
water table data, testing relationships between these sediment data and eddy covariance measurements of
carbon and energy exchange. Measurements will be guided by, and will inform, new developments in
ELM-PFLOTRAN designed to capture critical features of diverse coastal TAI functions.
Three questions, spanning small to ecosystem scales, and short to long-term drivers, guide our approach:
1. How do tidal water level oscillations, evapotranspiration-driven water level changes, and oxygen (O2)
transport from roots into the rhizosphere control tidal marsh redox and sediment oxygen distribution?
2. How do hydrology, biogeochemistry and plant biology interact on different timescales to influence
energy and greenhouse gas fluxes?
3. How will carbon sequestration and greenhouse gas fluxes respond to changes in climate and sea level?
We will test six hypotheses within the framework of these three questions. We plan integrated
measurements and modeling at two locations with contrasting hydro-biogeochemistry in an oligohaline
marsh in the Parker River estuary, MA. The estuary is the largest remaining tidal wetland complex in the
northeastern United States. Research at this location will be synergistic with ongoing efforts by our
modeling team at the Global Change Research Wetland GCReW.
We will use field measurements to help constrain three phases of ELM-PFLOTRAN development
designed to improve simulations of brackish marsh biogeochemistry under fluctuating oxygen availability
and salinity influenced by tides, diel and seasonal changes in plant physiology and river discharge.
Ultimately, we will be poised to combine our new process understanding and model formulation with
existing long-term data already in hand from two more saline salt marsh sites in the Parker Estuary. As
sea level rises, saline conditions will become more common in many coastal TAIs, but they will also
potentially be exposed to more flashy freshwater riverine input from intense, sporadic, storms. By having
information from marshes at both ends of the full salinity gradient, we will be able to better constrain
biogeochemical reactions in ELM-PFLOTRAN, validate ecosystem-scale modeled fluxes with eddy
covariance measurements, and simulate present and future hydrological variations and resulting carbon
dioxide and methane fluxes in tidal wetlands in the face of expected global change.
Award Recipient(s)
  • Marine Biological Laboratory (PI: Forbrich, Inke)