Climate Change: Elkhorn Slough

There are a few large and many small estuaries along the Central California coast; however, Elkhorn Slough is the only estuary located within the boundaries of Monterey Bay National Marine Sanctuary. Estuaries represent the confluence of terrestrial, freshwater, and marine ecosystems, creating multiple, unique habitats supporting highly diverse communities and providing important ecosystem services. Unfortunately, these rare, but highly productive areas are also very fragile, and human alterations and impacts can diminish their ability to provide biological services (e.g., nursery and feeding grounds for fishes and birds) and to act as environmental filters.

A 2015 assessment of the estuarine environment of Elkhorn Slough builds upon the 2009 assessment and highlights areas of concern within the estuary. Elkhorn Slough has a history of extensive alteration of physical structures and natural processes that strongly impacts water quality, habitat quality and abundance, and the structure and health of the faunal assemblage. Continued inputs of nutrients and contaminants, especially in areas of muted tidal influence, contribute to events, such as frequent hypoxia, algal blooms, and impacts to sensitive species. Historic human modifications to this system have led to substantial changes in hydrology, erosion, and sedimentation that continue to affect the abundance and quality of habitats and living resources. There is a high percentage of non-native species competing with natives and impacting ecosystem health. Some key species, such as eelgrass, native oysters, and sea otters show signs of improvement.

Recently, Elkhorn Slough has become the focus of new and on-going conservation and restoration efforts. In the next several years, these restoration projects and improvements in land management practices should result in measurable improvements in water and habitat quality in portions of the slough, and increase resilience to climate change.

Stressors on water quality continue to be measured and documented in Elkhorn Slough, with particularly high levels of agricultural inputs, such as nutrients and sediment. These pollutants may inhibit the development of assemblages and may cause measurable, but not severe declines in living resources and habitats. A main cause of water and sediment quality degradation is agricultural non-point source pollution (Caffrey 2002; Phillips et al. 2002; ESNERR et al. 2009). Relatively high levels of nutrients and legacy agricultural pesticides, such as DDT (dichlorodiphenyltrichloroethane), have been documented within the Elkhorn Slough wetlands complex, with the highest concentrations measured in areas that receive the most freshwater runoff (Phillips et al. 2002; ESNERR et al. 2009). Pathogens, pesticides, sediments, low dissolved oxygen levels and ammonia have impaired sections of Elkhorn Slough and water bodies adjacent to the slough, including Moro Cojo Slough and Moss Landing Harbor. Since 1988, the Elkhorn Slough National Estuarine Research Reserve’s (ESNERR) researchers and volunteers have been monitoring water quality at 26 sites in and around the reserve. Data collected from 2004-2009 determined if nutrient loading causes negative impacts to particular areas of the Elkhorn Slough estuarine complex. Of the 26 sites monitored, more than half far exceeded the thresholds for nitrate, phosphate, and ammonia as established by the local water agency, sometimes by two orders of magnitude, thus indicating that the Elkhorn Slough is highly impacted by nutrient loading (Hughes et al. 2010).

The abundance and distribution of major habitat types in the estuarine environment is fair to poor and has been declining. This trend was based on an analysis of a chronological series of maps and aerial photos by the Elkhorn Slough National Estuarine Research Reserve that revealed dramatic changes in the relative abundance of estuarine habitats over 130 years. In 1870, approximately 65% of Elkhorn Slough habitat was dense salt marsh, with less than 5% mud and sparse salt marsh habitat. By 2000, the amount of estuarine habitat composed of dense salt marsh had decreased to less than 20% and the amount of mud or sparse salt marsh habitat had increased to approximately 50%.

The estuary showed relative stability in the system between 2000 and 2009 with very little change in the relative abundance of estuarine habitats. Additional analyses (Wasson et al. 2013) revealed that salt marsh extent has remained stable since 2009, with minor losses balanced by gains.

Sediment budgets for Elkhorn Slough projected continued erosion and degradation of subtidal and intertidal habitats under current conditions (PWA 2009). Projected sea level rise will create an additional sediment demand that should be factored into current management and restoration planning (PWA 2009).

Since its launch in 2009, the Tidal Marsh Restoration Project developed plans to restore salt marsh at the Hester Marsh site, which subsided during a diked period. This project has increased suitable habitat through soil addition and planting salt marsh plants. Ecotone establishment is assessed using quantitative field methods and the displacement of tidal prism is assessed using LiDAR topographic measurements. These efforts will be critical to assess the achievement of goals and whether the restoration project has the intended effect of increasing tidal marsh and associated species in Elkhorn Slough.

References

Caffrey, J., M. Brown, W.B. Tyler, M. Silberstein. 2002. Changes in a California estuary: a profile of Elkhorn Slough. Moss Landing, CA: Elkhorn Slough Foundation, Moss Landing, CA. 280pp.

ESNERR (Elkhorn Slough National Estuarine Research Reserve), National Oceanic and Atmospheric Administration’s Estuarine Reserves Division, California Department of Fish and Game. 2009 Elkhorn Slough National Estuarine Research Reserve final management plan 2007-2011. 339pp.

Hughes, B., J. Haskins, K. Wasson. 2010. Assessment of the effects of nutrient loading in estuarine wetlands of the Elkhorn Slough watershed: A regional eutrophication report card. Elkhorn Slough Technical Report Series 2010:1.

Phillips, B., M. Stephenson, M. Jacobi, G. Ichikawa, M. Silberstein, M. Brown. 2002. Land use and contaminants. In: Caffrey, J., M. Brown, W.B. Tyler, M. Silberstein (eds.). Changes in a California estuary. Elkhorn Slough Foundation. Moss Landing, CA. pp. 237-257.

PWA (Philip Williams & Associates). 2009. Sediment budget estimates. Memorandum to Elkhorn Slough Foundation. September 15, 2008 (revised June 17, 2009).

Wasson, K., A. Woolfolk, C. Fresquez. 2013. Ecotones as indicators of changing environmental conditions: Rapid migration of salt marsh–upland boundaries. Estuaries and Coasts 36(3):654-664.