Elkhorn Slough Salt Marshes: Past, Present and Future

Overview
Salt marshes are a vital part of estuarine ecosystems. At Elkhorn Slough, marshes have been naturally dynamic in the past. Over the last centuries, they have been strongly affected by human alterations. In the future, they are likely to continue to respond to human-induced factors such as sea level rise and sediment changes. This webpage summarizes what our ESNERR research team, together with collaborators, has learned recently about salt marsh at Elkhorn Slough. We have used a combination of paleographic and stratigraphic approaches with field monitoring of current conditions and modeling of future scenarios to inform coastal decision-makers about marsh sustainability.

A complete list of references cited is available for download at the end of this article.

Our team and funding
Most of the research summarized here was conducted by Eric Van Dyke and Beth Watson (now with EPA). ESNERR collaborators include Kerstin Wasson, Andrea Woolfolk, Grey Hayes and Bryan Largay. Other partners include Ivano Aiello (Moss Landing Marine Labs), Matt Levey (Sea Spatial Consulting), Ana Garcia (UCSC), and Joanna Nelson (UCSC).

We conducted a major marsh sustainability investigation to inform coastal management from 2009-2012, funded by the Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET), a partnership between NOAA and the University of New Hampshire. Funding from the Estuarine Reserve Division of NOAA also supported the research. A summary of the CICEET-funded research is available (Wasson et al. 2012).

Background on salt marshes

Salt marshes are among the most productive ecosystems on earth. They can contribute heavily to estuarine food webs, and provide foraging and resting habitat for waterbirds. Marshes provide shoreline protection, and can improve water quality by taking up nutrients.

At Elkhorn Slough, pickleweed dominates the low and mid marsh. Only in the highest marsh, near the upland transition zone, are other marsh species found, such as salt grass and alkali heath. At Elkhorn Slough and elsewhere, salt marshes are found in a narrow zone of tidal elevation, between Mean High Water and Mean Higher High Water. In order to track rising sea levels, marshes need to increase elevation, through accumulation of sediment and organic material. They are thus sensitive to human perturbations which affect water levels or sediment accumulation rates (Wasson 2011).

Elkhorn Slough salt marshes long ago: 3000-150 years before present
Paleoecological and stratigraphic analyses (Levey et al. 2011, Watson 2011, Watson et al. 2011a, Watson et al. 2011 b) indicate that marshes are an ancient part of the system, and reveal that the estuary had at least moderate marine influence, because marshes were dominated by salt-tolerant species even many miles from the coast. Pollen data suggest that there was greater freshwater influence in the past, with more representation of rushes, cattails and sedges, perhaps clustered around freshwater seeps at marsh margins. Rates of sediment accumulation were sufficient to track sea level rise, and showed evidence for episodic flood events. Marsh extent was dynamic over time, with periods of expansion and contraction. Acreage of marsh along the main channel was typically lower than what was documented in the earliest historical maps, although it was high and expanding a few hundred years ago. Restoration targets 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.

Elkhorn Slough salt marshes 150 years ago to present
The earliest historical maps of Elkhorn Slough show extensive salt marshes. Historical ecology analyses reveal that many of these have been lost over the past 150 years (Van Dyke & Wasson 2005). The most significant cause of marsh loss has been artificial tidal restriction: diking and draining of wetlands. Even after tidal exchange was returned to some of these areas, such as the Parsons Slough complex on the Elkhorn Slough Reserve, marsh did not return, because elevations were too low as a result of subsidence during the diked period.

Extensive marsh die-back has also been documented along the main channel of Elkhorn Slough over the past 60 years, in areas that were never diked. Marsh cover has decreased, unvegetated pannes have opened, and tidal creeks have widened. Our stratigraphy, field monitoring, and modeling suggest that this “marsh drowning” has complex causes (Van Dyke 2011, Watson 2011, Van Dyke 2012). 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. The harbor has resulted in tidal scour of banks and mudflats adjacent to the marshes (Aiello & Endris 2012). However, the harbor 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 (Gillespie et al. 2011, Wasson et al. 2012). 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. We are currently further exploring the relative role of different factors causing current marsh drowning at Elkhorn Slough, in order to inform development of robust marsh conservation and restoration strategies (Watson et al. 2011c, Watson 2012). A working group of experts recently put together a consensus statement about factors that contribute to salt marsh loss along the main channel of Elkhorn Slough (Callaway et al. 2012).

Future of Elkhorn Slough’s salt marshes
Our modeling results indicate that Elkhorn Slough’s marshes are very sensitive to environmental changes, including the interaction between sea level rise, marsh plain subsidence, and sediment availability (Van Dyke 2011, Watson 2012). For instance, under a scenario of 1 m of sea level rise during the coming century, 90% of the estuary’s marshes would disappear. This result underlines the need for targeted restoration projects where marsh elevation can be sustained and enhanced, and also highlights the challenge of larger scale marsh conservation in the estuary.

Our results suggest that potential for salt marsh migration to track sea level rise is very limited, due to the steep topography adjacent to most current marshes. 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 highly 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.

Audience and end-users
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 staff at Elkhorn Slough National Estuarine Research Reserve. 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. ESNERR has pioneered the collection of monitoring data relevant to investigations of marsh sustainability, and with this project has demonstrated how such data can be successfully incorporated into a collaborative framework for informing marsh conservation.

Results from this project were also shared with coastal managers and scientists from the San Francisco Bay region. A summary of the needs (Hayes 2012a) of these end-users was conducted, revealing great interest in vegetation and sediment dynamics, but less interest in paleoecology, sea level rise modeling, and a case study approach. For those coastal managers and scientists that were interested in sea level rise monitoring, a workshop was held to compare modeling approaches and build future collaborations (Hayes 2012b). Modeling scenarios were also shared with regional landowners and conservation interests likely to be affected by sea level rise in the Elkhorn Slough region. A synthesis of the three Elkhorn Slough workshops held as a part of this CICEET-funded project is available (Hayes 2012c).

References Cited

Aiello I, Endris C. 2012. High resolution geomorphologic surveys of estuarine habitats at Elkhorn Slough, California. Report prepared for the Elkhorn Slough National Estuarine Research Reserve. (Aiello_2012_TLS_Report.pdf, 1.77MB)

Gillespie, A. A. Schaffner, E. Watson, and J. Callaway. 2011. Morro Bay sediment loading update. Morro Bay National Estuary Program, Morro Bay, CA. (Gillespie_etal_2011_MorroBay_Report.pdf, 1.5MB)

Hayes G. 2012a. Planning for salt marsh sustainability in central California: A needs assessment of coastal managers. (Hayes_2012a_salt_marsh_needs_assessment.pdf, 98K)

Hayes G. 2012b. Planning for salt marsh sustainability in central California: A summary of a workshop exploring the application of different tools for sea level rise modeling. (Hayes_2012b_salt_marsh_sea_level_rise_modeling.pdf, 96K)

Hayes G. 2012c. Coastal decision-making about marsh sustainability at Elkhorn Slough: A summary of three collaborative workshops. (Hayes_2012c_3_workshop_summary.pdf, 230K)

Levey MD, Garcia A, Watson EB. 2011. Mapping current and historical geological changes to the upper Elkhorn Slough estuary, Moss Landing, California. Poster presented at the American Geophysical Union Fall Meeting, San Francisco, CA. (Levey_etal_2011_AGU_Poster.pdf, 451KB)

Van Dyke E, Wasson K. 2005. Historical ecology of a central California estuary: 150 years of habitat change. Estuaries 28:173-189. (VanDyke_Wasson_2005_Slough_habitat_change.pdf, 3.1MB)

Van Dyke E. 2011. Modeling sea level rise and wetland habitat transition. Powerpoint presented at TWP coastal decision-maker workshop, Elkhorn Slough, CA. (VanDyke_2011_TWP_Powerpoint.pdf, 5.3MB)

Van Dyke, E. 2012 . Water levels, wetland elevations, and marsh loss. Elkhorn Slough Technical Report Series 2012:2. (VanDyke_2012_Water_Levels_Wetland_Elevations.pdf, 1.6MB)

Wasson K. 2011. Marshes at Elkhorn Slough: past, present and future. Powerpoint presented at TWP coastal decision-maker workshop, Elkhorn Slough, CA. (Wasson_2011_TWP_Powerpoint.pdf, 1.7MB)

Wasson K, Watson EB, Van Dyke E, Hayes G, Aiello I. 2012. A novel approach combining rapid paleoecological assessments with geospatial modeling and visualization to help coastal managers design salt marsh conservation strategies in the face of environmental change. Elkhorn Slough Technical Report 2012:1. (Marsh_Sustainbility_Elkhorn_Slough_Technical_Report_2012.pdf, 979KB)

Watson E. 2011. Elkhorn Slough marsh stratigraphy: prehistoric marsh extent and recent sediment accretion. Powerpoint presented at TWP coastal decision-maker workshop, Elkhorn Slough, CA. (Watson_2011_TWP_Powerpoint.pdf, 1.7MB)

Watson, E.B., K. Wasson, G.B. Pasternack, A. Woolfolk, E. Van Dyke, A.B. Gray, A. Pakenham, and R.A. Wheatcroft. 2011a. Applications from paleoecology to environmental management and restoration in a dynamic coastal environment. Restoration Ecology 19:765-775. (Watson_etal_2011a_RestorationEcology_Pub.pdf, 1.2MB)

Watson EB, Wasson K, Van Dyke E. 2011b. Elkhorn Slough tidal wetlands: past, present, and future. Poster presented at the Pacific Climate Workshop, Pacific Grove, CA. (Watson_etal_2011b_PacClim_Poster.pdf, 848KB)

Watson EB. Wigand C, Nelson J, Wasson K. 2011c. Consequences of climate change, eutrophication and anthropogenic impacts to coastal salt marshes: multiple stressors reduce resilience and sustainability. Poster presented at the American Geophysical Union Fall Meeting, San Francisco, CA. (Watson_etal_2011c_AGU_Poster.pdf, 762KB)

Watson EB. 2012. Coastal wetland sustainability: linking empirical evidence and data from field and laboratory mesocosms with predictive modeling. Invited presentation, Smithsonian Environmental Research Center, Edwater, MD. (Watson_2012_SERC_Powerpoint.pdf, 677KB)