Training

See also: Tidal Wetlands, Habitat Restoration

We developed, refined and implemented tools to help coastal managers understand and manage for salt marsh sustainability in the face of threats from anthropogenic changes to sediment inputs and water quality resulting from land use changes, and sea level rise associated with global climate change.  Salt marsh sustainability is an intricate balance between relative sea level rise and marsh platform elevation.  Given ample sediment supply and marsh elevations supra-optimal for maximum productivity, increases in inundation result in enhanced sediment deposition and increased production.  However, where tidal marshes lack elevation capital,  existing marsh plains are only able to keep pace with sea level rise given sufficient sediment supply. In past centuries, human activities at multiple scales have dramatically influenced this balance in North American estuaries. 

            The project was developed in response to needs from end-users engaged in the Tidal Wetland Project (TWP), a collaborative ecosystem-based management initiative launched and directed by NERR staff at Elkhorn Slough.  The TWP stakeholders comprised the primary audience for the marsh sustainability visualization tools resulting from this project.  More broadly, this project also serves as a demonstration of the benefits to coastal management of using the NERRs as “sentinel sites” for global climate change.  Elkhorn Slough NERR has pioneered the collection of monitoring data relevant to investigations of marsh sustainability, and with this CICEET project has demonstrated how such data can be successfully incorporated into a collaborative framework for understanding marsh sustainability and migration to support development of improved salt marsh conservation strategies.

We used rapid, shallow subsurface stratigraphic assessments at an unprecedentedly high scale of spatial resolution to map the ancient, natural extent of marshes at Elkhorn Slough.  We found that marsh extent was somewhat dynamic over time, with at least two previous episodes of marsh deterioration recorded by the sediment record during pre-historic times.  Although our analysis was restricted to the portion of the estuary that has never been reclaimed for methodological reasons, results strongly suggest that the current aerial extent of marsh is lower than it was during early European-American settlement, but somewhat greater than the extent supported by the estuary over the past 200-3,000 years.  We shared these reconstructions of past habitat extent through maps presented to TWP stakeholders.  The results were surprising to this audience, and represent a paradigm shift.  Aiming for marsh extent documented in the earliest maps or aerial photos might not be within the most typical natural range for the estuary, and may  be neither feasible nor sustainable given the watershed sediment supply and low turbidity of the estuary. TWP stakeholders will be developing a new strategic plan for estuary-wide priorities in coming years, and this CICEET-generated information on ancient marsh extent will be used as one component of setting restoration goals for the estuary.

We collected data on sediment accumulation and erosion, and marsh platform elevation processes within differing geomorphic contexts by conducting radiometric dating and analysis of sediment composition for 10+ shallow sediment cores, completing terrestrial laser scanning, acoustic profiling, monitoring sediment elevation tables and feldspar marker horizons and deploying litterbags to measure decomposition rates.  These data were synthesized to obtain a better understanding of the processes that generate marsh stability at Elkhorn Slough, and were also used to parameterize and evaluate predictive models.  The results of these analyses, in concert with monitoring and modeling efforts suggest that extensive marsh drowning observed at Elkhorn Slough over the past sixty years has complex causes.  The creation of an artificial mouth to the estuary to accommodate Moss Landing Harbor is one major contributing factor, leading to dramatic initial die-back in the 1950s and decreasing long-term resilience of the marshes to sea level rise, by lowering their position in the tidal frame.  However, this factor alone does not appear to explain the on-going marsh loss observed in past decades.  Rather, subsidence of the marsh plain appears to be a major contributor to current marsh drowning.  This subsidence may be driven, at least in part, by decreased root biomass or increased below-ground decomposition rates resulting from poor water quality related to high nutrient loading and eutrophic conditions.  The identification of subsidence and the development of hypotheses involving eutrophication also represent a major paradigm shift, which will generate more science and inform marsh restoration strategies in coming years.  There has been some resistance to the new ideas, and additional data and experiments are still needed.  So management actions will not change overnight.  However, it appears likely that consideration of issues related to subsidence will be part of future marsh restoration planning for the estuary, as a result of this CICEET project.

We developed geospatial models of marsh sustainability and parameterized these models with the newly collected data on sediment accumulation as well as our existing data on marsh elevation, vegetation cover, and tidal inundation. We explored and adapted a range of marsh sustainability and migration scenarios with TWP stakeholders using visualization tools.  The modeling suggests Elkhorn Slough marshes are very sensitive to environmental changes.  Under a scenario of 1 m of sea level rise, 90% of the estuary’s marshes would disappear.  This result underlines the need for some targeted restoration projects where marsh elevation can be sustained and enhanced, and also the challenge of larger scale marsh conservation in the estuary.  Predictive modeling will be critical as TWP stakeholders move forward with strategic planning of restoration projects in general, and with the recently initiated sediment addition project in particular – elevations of the new marshes must be sufficient to account for the observed subsidence and likely sea level rise rates.   Predictive modeling also was used to explore with stakeholders the potential for marsh migration.  Due to the steep topography adjacent to current marshes, there is very limited potential for such migration.  Stakeholders expected that the head of the estuary, at the Carneros Creek floodplain, might prove a valuable marsh migration corridor, but the modeling revealed limited scope for future marsh habitat there.  The greatest extent of marsh migration is theoretically possible in the southern estuary, along the old Salinas channel and Tembladero floodplains.  However, since these are high productive and valuable farmlands, and since they are beyond the current focus area of the land trusts active in the Elkhorn Slough area, enabling marsh migration to the south may be very challenging.  Nevertheless, some workshop participants expressed interest in exploring potential incentive programs for landowners.

Our geospatial visualization tools were also shared with two different groups of end-users through Coastal Training Program (CTP) workshops.  We presented climate change predictions and sea level rise scenarios to a group of landowners around Elkhorn Slough and regional conservation interests to gauge the needs of this community.  We determined that their needs – including analyses focusing on human infrastructure and economic analyses – were more appropriate to other regional climate change initiatives than to this CICEET project.  We also presented our SLAMM models and some of the underlying data at a workshop for managers and scientists from the San Francisco Bay region.  The ESNERR CTP program conducted a needs assessment of coastal managers and found a need to better link those doing monitoring, modeling, and management of salt marshes in the Bay region.  In partnership with San Francisco Bay NERR and USGS, we hosted a workshop to identify management needs and compare approaches for monitoring and modeling marsh sustainability in the face of sea level rise.  Our CICEET-funded approach was highlighted to this broader audience of stakeholders, and will continue to shape the development of a regional approach to marsh sustainability science and management.