Understanding threats to estuarine ecosystems

Estuaries – where rivers meet the sea – are biologically rich habitats, hosting migratory shorebirds, nurseries for commercially valuable flatfish, and distinctive estuarine endemics such as eelgrass, salt marsh and oysters. Estuaries are considered the most human-altered ecosystems on earth, because human population growth, industries, and harbors are often focused on these productive, sheltered land-sea interfaces. The goal of my current research program is to characterize the impacts of anthropogenic threats to estuarine ecosystems, to prioritize among them, and to develop and test restoration strategies that decrease these threats.

Invasions by exotic species

Invasions of non-native species are now considered second only to habitat loss in decreasing global biodiversity, and estuaries are the most highly invaded marine habitats. We have examined invertebrate invasions of Elkhorn Slough, and became interested in how transport mechanisms may explain variable invasion levels in estuaries along this coast (Wasson et al. 2001). Island biogeographic or metapopulation models may prove useful in understanding the dynamics of estuarine colonization by invaders. We have examined differential vulnerability of habitats to invasion at two scales in central California , demonstrating that hard substrates are much more highly invaded than soft substrates, and that estuaries are much more invaded than the open coast (Wasson et al. 2005). This contrast between estuaries and coasts appears to be a global trend, but varies greatly in strength by region (Preisler et al. 2009).

Seeking solutions for the problems posed by invaders, we have examined ballast water deoxygenation as a technique for minimizing transport of invaders between regions, while at the same time benefiting the shipping industry by decreasing corrosion of tanks (Tamburri, Wasson and Matsuda 2002). We have developed and obtained grant funding for an early detection program for “least wanted” invaders to Elkhorn Slough and the Monterey Bay. In January 2002, we hosted a national workshop with invasion biology experts, federal agencies, and national organizations to develop a vision for a coordinated national program for estuarine invasion monitoring, using the Reserve system as a platform (Wasson et al. 2002).

In addition to marine invasions, estuarine ecosystems are threatened by invasions of upland plants. We have conducted restoration experiments that have revealed remarkable recovery of native salt marsh following removal of invasive ice plant (Wasson and Woolfolk in prep).

Hydrological alterations

Tidal and freshwater exchange and associated sediment transport are the life-blood of estuaries. However, most estuaries have undergone extensive hydrological alterations including freshwater diversion, restriction of tidal exchange, “reclamation” of former tidal wetlands, and dredging of harbors. At Elkhorn Slough, we have found that about 50% of historical salt marsh has been lost as a result of such alterations (Van Dyke and Wasson 2005).

We have recently investigated the ecological impacts of structures that control tidal exchange on estuarine communities (Ritter et al. 2008). We found that contrasting estuarine conservation targets are optimized under different levels of tidal exchange. In a study of the biologically rich but very narrow and fragile salt marsh – upland ecotone, we found that both tidal restriction and cattle grazing decreased native marsh plant diversity (Wasson and Woolfolk in press). In a manipulative experiment examining multiple human stressors to the ecotone, we further confirmed the negative effects of tidal restriction and trampling, and uncovered an interaction between them - diked marshes are much slower to recover from trampling than are ones with natural tidal exchange (Martone and Wasson 2008).

We are currently engaged in an ecosystem-based management initiative involving over 100 stakeholders, attempting to build a vision for the future of Elkhorn Slough's wetlands and to develop the most effective restoration strategies for achieving it. Click here to read more.


Many estuaries are highly polluted due to adjacent human land uses. In Elkhorn Slough, we have documented extremely high levels of nutrient loading resulting from agricultural inputs. We have conducted experiments that revealed that nutrient enrichment has significant impacts on salt marsh, and have explored the potential of hyperspectral imagery for detecting nutrient-loading in wetlands (Siciliano et al. 2008). We are launching a new investigation of potential indicators of eutrophication in Elkhorn Slough, examining patterns of hypoxia and algal biomass.

Estuarine endemics and threatened species

In addition to focusing on community-level impacts of human activities, we are involved in studies of particularly vulnerable species in coastal ecosystems. We have conducted surveys of the habitat requirements and population sizes of California red-legged frogs, native oysters, brackish snails (Tryonia imitator), and eelgrass in the Elkhorn Slough watershed, and plan to expand this research in the future. We recently compared habitat correlates of the threatened native California red-legged frog  and a non-native congener, the American Bullfrog, and found the former to be more sensitive to human alterations of the landscape, including roads, agriculture, and pollution (D'Amore, Hemingway and Wasson 2010). We also examined the factors that limit the distribution and abundance of native Olympia oysters in the estuaries. While current restoration efforts on the Pacific coast focus on the addition of hard substrates, our results suggest that a more effective approach for enhancing Olympia oysters as well as overall ecosystem integrity would be to focus on improvements to water quality and decreases in artificially high sediment inputs from adjacent land uses (Wasson 2010).


Long-term monitoring programs

We carry out a comprehensive long-term monitoring program for Elkhorn Slough. Our goal is to detect patterns of change over time, and to discern the processes underlying them. In particular, we need to distinguish the effects of natural perturbations from anthropogenic disturbances. We participate in the National Estuarine Reserve System-wide Monitoring program, which provides a unique time series of national estuarine water-quality and weather data. We track land-use and habitat changes using aerial imagery and GIS analysis, as well as field measurements of sediment deposition, elevation, and tidal inundation. We conduct biological monitoring using a variety of indicator species, including mudflat invertebrates, migratory shorebirds, and threatened amphibians. We have found that long-term monitoring data provides a vital framework for applied conservation, identifying declines or disturbances that then can be further investigated with manipulative experiments. Short-term applied research and long-term monitoring thus complement each other and both support conservation of estuarine ecosystems.

Selected publications

Hughes, B.B,, Eby, R., Van Dyke, E., Tinker, M.T., Marks, C.I., Johnson, K.S., Wasson, K. 2013. Recovery of a top predator mediates negative eutrophic effects on seagrass. Proceedings of the National Academy of Sciences 110:15313-15318.

Wasson, K., Woolfolk A, Fresquez C. 2013. Ecotones as Indicators of Changing Environmental Conditions: Rapid Migration of Salt Marsh–Upland Boundaries. Estuaries and Coasts 36(3):654-664.

Hughes, B.B, Haskins, J.C., Wasson, K., Watson, E. 2011. Identifying factors that influence expression of eutrophication in a California estuary. Marine Ecology Progress Series 439:31-43.

Watson, E. B., Wasson, K., Pasternack, G. B., Woolfolk, A., Van Dyke, E., Gray, A. B., Pakenham, A., Wheatcroft, R. A. 2010. Applications from paleoecology to environmental management and restoration in a dynamic coastal environment. Restoration Ecology doi: 10.1111/j.1526-100X.2010.00722.x

Wasson, K., Woolfolk, A. Salt marsh-upland ecotones in central California: vulnerability to invasions and anthropogenic stressors. Wetlands 31:1-14.

Wasson, K. 2010. Informing Olympia oyster restoration: evaluation of factors that limit populations in a California estuary. Wetlands. 30:449-459

Wasson, K. 2010. Selected large benthic infaunal invertebrates: factors that control distribution and abundance in Pacific Coast estuaries and a case study of Elkhorn Slough, California. Elkhorn Slough Technical Report Series 2010:3

Hughes, B., Haskins, J., Wasson, K. 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.

Gee, A. K., Wasson, K., Shaw, S. L., Haskins, J. 2010. Signatures of restoration and management changes in the water quality of a central California estuary. Estuaries and Coasts33:1004-124

D'Amore, A., Hemingway, V., Wasson, K. 2010. Do a threatened native amphibian and its invasive congener differ in response to human alteration of the landscape? Biological Invasions. 12(1):145-153

Preisler, R.K., Wasson K, Wolff, W.J., Tyrrell, M.C. 2009. Invasions of estuaries vs. the adjacent open coast: a global perspective. Chapter 19 in Marine Bioinvasions: Ecology, Conservation and Management Perspectives , G Rilov and J Crooks (editors)

Martone, R., Wasson, K. 2008. Impacts and interactions of multiple human perturbations in a California salt marsh. Oecologia. 158(1):151-163

Ritter, A.F., Wasson. K., Lonhart, S.I., Preisler, R.K., Woolfolk, A., Griffith, K.A., Connors, S., Heiman, K. 2008. Ecological signatures of anthropogenically altered tidal exchange in estuarine ecosystems. Estuaries and Coasts 31(3):554-571

Siciliano, D., Wasson, K., Potts, D.C., Olsen, R.C. 2008. Evaluating hyperspectral imaging of wetland vegetation as a tool for detecting estuarine nutrient enrichments. Remote Sensing of Environment. 112:420-433

Van Dyke, E. and Wasson, K. 2005. Historical ecology of a Central California estuary: 150 years of habitat change. Estuaries 28(2):173-189

Wasson, K., Fenn, K., Pearse, J.S. 2005. Habitat differences in marine invasions of Central California . Biological Invasions 7:935-948

Tamburri, M.N., Wasson, K., and Matsuda, M. 2002. Ballast water deoxygenation can prevent aquatic introductions while reducing ship corrosion. Biological Conservation 103:331-341

Wasson, K., Lohrer, D., Crawford, M., and Rumrill, S. 2002. Non-native species in our nation's estuaries: a framework for an invasion monitoring program. National Estuarine Research Reserve Technical Report Series 2002:1. (available as a pdf from http://www.ocrm.nos.noaa.gov/nerr/resource.html )

Wasson, K., Zabin, C.J., Bedinger, L., Diaz, C.M., and Pearse, J.S. 2001. Biological invasions of estuaries without international shipping: the importance of intraregional transport. Biological Conservation 102(2):143-153

Kerstin in the field
Kerstin and daughter Fiona