Impacts of coastal and watershed changes on upper estuaries: causes and implications of wetland ecosystem transitions along the US Atlantic and Gulf Coasts

Science Center Objects

Estuaries and their surrounding wetlands are coastal transition zones where freshwater rivers meet tidal seawater.  As sea levels rise, tidal forces move saltier water farther upstream, extending into freshwater wetland areas. Human changes to the surrounding landscape may amplify the effects of this tidal extension, impacting the resiliency and function of the upper estuarine wetlands. One visible indicator is the rapid conversion of some Southeast and mid-Atlantic tidal freshwater forested wetlands to ‘Ghost Forests’ in which trees die from increases in salinity. Because data on the complex causes and impacts of tidal extension are limited, this project takes an integrated, large-scale approach to research and monitoring to expand our ability to model these processes and apply them to other coastal areas along the Atlantic Coast, Gulf Coast, and internationally. Results of this effort will provide critical data to guide future decisions regarding the fate of carbon, water quality, coastal resilience, wildlife and fisheries, and effective allocation of taxpayer dollars for ecosystem restoration.

Statement of Problem: Throughout the southeastern United States, tidal freshwater forested wetlands (TFFW) found along the upper tidal estuary (where freshwater watersheds and estuaries meet), are converting to ‘Ghost Forests’ (dead trees) and then to oligohaline (low salinity) marsh. Nontidal floodplains are also converting into TFFW as tides extend farther upstream along rivers, and little scientific research has been directed toward describing or understanding the effects of this tidal extension.

Conversion is related to both changing sea level and associated salt-water intrusion and to human influences (e.g., land use change, coastal development, construction of dams, river dredging, etc). All these factors affect the resiliency and ecosystem services provided by wetlands of the upper estuary. Watershed inputs to the upper estuary (water, sediment, nutrients) also may determine the response of TFFW through complex interactions between the ecosystem, water, and landscape. Further, TFFW itself influences delivery of watershed sediment to lower tidal estuaries that are targets of restoration projects, making their function critical to ecosystem health.

diagram showing carbon movement through coastal wetlands

Diagram showing the transition from a) tidal freshwater forested wetland (TFFW) to b) tidal fresh water marsh to c) low-salinity marsh to d) saltmarsh with increasing salinity and the movement of carbon in, out, and through this system. Modified from Figure 1 in (Krauss et al., 2018)

(Credit: Ken Krauss, USGS. Public domain.)

Why this Research is Important: This project fills a large gap in integrative ecosystem research and monitoring of different coastal areas which vary in land use activity, tidal range, relative sea-level rise, watershed characteristics and changes, and delivery of ecosystem services for taxpayers. A primary outcome thus far has been the value assigned to TFFW in terms of carbon storage, sequestration, and conveyance; adding them to the list of important "blue carbon" wetlands globally.

“Blue Carbon” refers to carbon that is stored in coastal or marine ecosystems. In addition to the monetary value of the ecosystem services, these coastal wetland areas are crucial for protecting the coast from storm surge, mitigating downstream eutrophication (when a water body becomes overly enriched in nutrients), and providing wildlife and fishery habitat. By improving understanding of the processes and services provided by both healthy TFFW and those transitioned to marshes, managers and coastal inhabitants (and taxpayers) will be able to anticipate how past and current changes will affect the ecosystems in the future.

The TFFW transition/conversion phenomenon is not unique to the southeast and mid-Atlantic United States, and expansion of the project enhances our ability to (1) apply this research to other domestic and international coastal areas undergoing similar changes, and (2) add to critical information gaps on carbon sequestration to better understand the role of coastal ecosystems in the cycling of carbon and nutrients). 

Dead trees in a tidal marsh

A ghost forest in tidal freshwater forested wetlands of the Sampit River, South Carolina.
Photo taken June 17, 2015 by Dr. William Conner, Clemson University

(Courtesy: Dr. William Conner, Clemson University)

Objective(s): We hypothesize that ecosystem functions, sediment and nutrient processes, and carbon sequestration vary in predictable ways in different coastal environments as nontidal floodplains convert to TFFW and TFFW convert to marshes. Through extensive data collection over 15 years, we have quantified several processes and changes to sedimentation, nutrient uptake and release, forest condition, and balance of habitat. From analyses of these data we developed an understanding of emergent processes at multiple scales that are critical to future investigation, culminating in predictive models and advancements to ecological theory of coastal wetland change. In order to further expand our focus from TFFW stresses and degradation, our focus moves to the larger scale of the entire upper estuary landscape, guiding a series of new goals and objectives:

  1. Maintain repeated measurements at existing field sites in order to reach a full 20-year-record of change, while also incorporating new site locations farther inland, upriver, and into non-tidal zones
  2. Incorporate new studies to determine where sediment is coming from and what potential land use and management practices may influence sedimentation
  3. Continue documenting how biogeochemical cycles are affected by changes in the watershed and estuary
  4. Perform comparative carbon studies between regions to determine generality
  5. Map and describe tidal extension and landward habitat conversion to develop accurate land cover maps of tidal wetland habitat distribution
  6. Advance dynamic models to predict upper estuarine habitat change, coupled with elevated CO2 experiments simulating environmental stress to support future modeling
  7. Begin proactive efforts to compare upper estuarine wetlands of the US Atlantic and Gulf Coasts to similarly positioned habitat conditions globally (e.g., Pacific Northwest, Australia, New Zealand)

     

Methods: We use a combination of complementary field, laboratory, modeling, and remote sensing techniques to achieve our objectives. Changes in vegetation, tidal extent, surface elevation, and microtopography are measured using field data collection. For example, plots of trees and herbaceous vegetation are surveyed over time to document a record of habitat change, while direct measurements of growth records inform contemporary and historic productivity.

Water level and salinity are measured continuously with logging devices. Surfaces of hummocks and hollows are measured annually to assess changes in elevation, accretion, subsidence and general resilience to sea-level rise.  More complex methods that involve both field collection and laboratory analyses are required to measure soil biogeochemistry, nutrients, and carbon (C) storage and flux. 

We collect and analyze shallow soil cores (up to 1-meter in depth), for total organic carbon and nitrogen, and date them using Pb-210 activity to determine rates of soil C burial. In analyses of deeper cores (up to 6-7 meters depth), macrofossils, pollen, C-14, and carbon and nutrient sequestration are used to develop historic baselines prior to rising sea-level conditions. Further soil and vegetation analyses assess ground carbon stocks, belowground net ecosystem productivity, greenhouse gas production (CO2, CH4, N2O) in soils of hummocks and hollows, and transpirational demand on soils in forests vs. newly converted marsh. 

Finally, river channel bathymetry, longitudinal sampling of suspended and bed sediment, and analysis of sediment stable isotopes and metals to determine changes in mineral and organic sediment sources (“sediment fingerprinting”) are used together to determine how channels and floodplains have changed and have impacted sediment transport.  We will use remote sensing techniques to gather information on the extent of tidal flooding along upper estuarine reaches in our study areas.  Using our data, we will validate remote sensing results and apply it to regional and national mapping efforts, as well as model development. Synthesizing the various data streams is accomplished through development and application of both statistical and dynamic process models to generate new understanding and tools to predict ecosystem change in new coastal areas.