Kerstin Wasson

Kerstin Wasson, P.I.Kerstin Wasson (CV)

Adjunct Professor; Department of Ecology and Evolutionary Biology, University of California, Santa Cruz

Research Coordinator, Elkhorn Slough National Estuarine Research Reserve


 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.


Salt marsh sustainability

A major focus of our current research is on salt marsh sustainability. In the past century, about half of Elkhorn Slough’s salt marshes have been lost to anthropogenic alterations (Van Dyke and Wasson 2005). Salt marsh extent has been dynamic over the past thousands of years and today’s acreage still falls within this baseline (Watson et al. 2010). While much of the marsh loss in the last century was due to diking, currently undiked marshes appear to be “drowning”, failing to track sea level rise. Through experiments, monitoring and geospatial studies, we are attempting to unravel the mechanisms behind this loss. While drowning occurs at the scale of the entire marsh and seems to start in interior pannes, marsh loss is also occurring at bank edges, where erosion rates are high. We are exploring how biological factors such as algal wrack and crab burrowing may accelerate marsh loss at edges.

In addition to examining the lower edges of marshes, we have intensively investigated the upper edges, the transition zone from marshes to uplands. This marsh-upland ecotone is biological rich, but very narrow, and threatened by invasive upland species, cattle trampling, and tidal restriction (Martone & Wasson 2008, Wasson and Woolfolk 2011). We have monitored the location of the marsh-upland boundary since 2001, and have discovered that it is very responsive to interannual variation in inundation, and thus serves as a sensitive indicator of climate change (Wasson et al. 2013). For more detail on our marsh ecotone work, click here.


Native oyster restoration

Another major current focus is native oyster restoration. We have found that oysters are rare at Elkhorn Slough (Wasson 2010), and have many years of near-zero recruitment, thus facing risk of local extinction from this estuary, as occurred earlier at Morro Bay. We have conducted extensive restoration experiments, attempting to find the physical conditions that optimize native oyster cover relative to that by invasive species. As a part of a large interdisciplinary, collaborative study, we have identified key environmental conditions that support sustainable oyster populations in central California, and prioritized sites in San Francisco Bay and Elkhorn Slough for restoration (Wasson et al. 2014). For more on our Olympia oyster work, click here.


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). 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.

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 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.



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 have conducted an assessment of eutrophication in the estuary, revealing that stagnant conditions behind water control structures are responsible for the greatest eutrophication problems (Hughes et al. 2011). We detected significant improvements to water quality in wetlands adjacent to restoration project, a cause for optimism (Gee et al. 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.

Ecosystem-based management

In 2003, we launched an ecosystem-based based management initiative for Elkhorn Slough, the Tidal Wetland Project. With funding from the Packard and Resources Legacy Fund Foundations, NOAA, and other partners, we were able to bring together over 100 stakeholders to jointly explore management alternatives for the estuary. In November 2012, the decision-making panel for this initiative, the Strategic Planning Team, selected restoration alternatives and mapped out future strategies for the estuary (Wasson et al. 2012b).


Selected publications

Jeppesen, R., M. Rodriguez, J. Rinde, J. Haskins, B. Hughes, L. Mehner, and K. Wasson. 2018. Effects of hypoxia on fish survival and oyster growth in a highly eutrophic estuary. Estuaries and Coasts.41:89-98.

Santana, R., Lessa, J. Haskins, Wasson, K. 2018.Continuous monitoring reveals drivers of dissolved oxygen variability in a small California estuary. Estuaries and Coasts 41:99-113.

Wasson, K., Jeppesen, R., Endris, C., Perry, D.C., Woolfolk, A., Beheshti, K., Rodriguez, M., Eby, R., Watson, E.B., Rahman, F., Haskins, J., Hughes, B.B.  2017.  Eutrophication decreases salt marsh resilience through proliferation of algal mats. Biological Conservation212: 1-11.

Eby, R., Scoles, R., Hughes, B.B., Wasson, K. 2017.  Serendipity in a salt marsh: detecting frequent sea otter haul outs in a marsh ecosystem.  Ecology98:2975-2977.

Bible, J.M., Cheng, B.S., Chang, A.L., Ferner, M.C., Wasson, K., Zabin, C.J., Latta, M., Sanford, E., Deck, A. and Grosholz, E. D.  2017. Timing of stressors alters interactive effects on a coastal foundation species. Ecology 98:2468–2478.

Wasson, K., Hughes, B.B., Berriman, J.S., Chang, A.L., Deck, A.K., Dinnel, P.A., Endris, C., Espinoza, M., Dudas, S., Ferner, M.C., Grosholz, E.D., Kimbro, D., Ruesink, J.L., Trimble, A., Vander Schaaf, D., Zabin, C.J., Zacherl, D.  2016. Coast‐wide recruitment dynamics of Olympia oysters reveal limited synchrony and multiple predictors of failure. Ecology97:3503-16.

Raposa, K.B., Wasson, K., Smith, E., Crooks, J.A., Delgado, P., Fernald, S.H., Ferner, M.C., Helms, A., Hice, L.A., Mora, J.W., Puckett, B., Sanger, D., Shull, S., Spurrier, L., Stevens, R., Lerberg, S.  2016. Assessing tidal marsh resilience to sea-level rise at broad geographic scales with multi-metric indices. Biological Conservation204:263-275.

Zabin, C.J, Wasson, K., Fork, S. 2016.  Restoration of native oysters in a highly invaded estuary.  Biological Conservation202:78-87.

Hughes, B.B., Hammerstrom, K.K., Grant, N.E., Hoshijima, U., Eby, R. and Wasson, K., 2016. Trophic cascades on the edge: fostering seagrass resilience via a novel pathway. Oecologia1-11.

Wasson, K., B. Suarez, A. Akhavan, E. McCarthy, J. Kildow, K. S. Johnson, M. C. Fountain, A. Woolfolk, M. Silberstein, L. Pendleton, and D. Feliz. 2015. Lessons learned from an ecosystem-based management approach to restoration of a California estuary. Marine Policy 58:60-70.

Fork, S., Woolfolk, A., Akhavan, A., Van Dyke, E., Murphy, S., Candiloro, B., Newberry, T., Schreibman, S., Salisbury, J., Wasson, K.  2015. Biodiversity effects and rates of spread of non-native eucalypt woodlands in central California.  Ecological Applications25:2306-2319.

Wasson, K., C. Zabin, J. Bible, S. Briley, E. Ceballos, A. Chang, B. Cheng, A. Deck, E. Grosholz, A. Helms, M. Latta, B. Yednock, D. Zacherl and M. Ferner. 2015. A Guide to Olympia Oyster Restoration and Conservation: Environmental conditions and sites that support sustainable populations.  Elkhorn Slough National Estuarine Research Reserve. Available from

Cheng, B. S., Bible, J. M., Chang, A. L., Ferner, M. C., Wasson, K., Zabin, C. J., Latta, M., Deck, A., Todgham, A. E., Grosholz, E. D. 2015. Testing local and global stressor impacts on a coastal foundation species using an ecologically realistic framework. Global change biology 21:2488-2499.

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 Coasts36(3):654-664.

Hughes, B.B., Haskins, J.C., Wasson, K., Watson, E.B.  2011.  Identifying factors that influence expression of eutrophication in a central California estuary.  Marine Ecology Progress Series439:19-30.

Watson, E. B., Wasson, K., Pasternack, G. B., Woolfolk, A., Van Dyke, E., Gray, A. B., Pakenham, A., Wheatcroft, R. A.  2011. Applications from paleoecology to environmental management and restoration in a dynamic coastal environment. Restoration Ecology19:765-775.

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

Wasson, K.,Woolfolk, A.  2010.  Salt marsh-upland ecotones in central California: vulnerability to invasions and anthropogenic stressors.  Wetlands  31:1-14.

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.  Coasts and Estuaries33:1004-124.

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. and Wasson, K.  2008.  Impacts and interactions of multiple human perturbations in a California salt marsh. Oecologia158(1):151-163.

Siciliano, D., Wasson,K., Potts, D.C., Olsen, R.C. 2008.  Evaluating hyperspectral imaging of wetland vegetation as a tool for detecting estuarine nutrient enrichment. Remote Sensing of Environment112:4020-433.

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 Coasts31(3):554-571.

Wasson, K., Fenn, K., Pearse, J.S.  2005. Habitat differences in marine invasions of Central California.  Biological Invasions7:935-948.

Van Dyke, E. and Wasson, K. 2005.  Historical ecology of a central California estuary: 150 years of habitat change.  Estuaries28(2):173-189.

Wasson, K. and Lyon, B.  2005. Flight or flight: flexible anti-predatory strategies in porcelain crabs.  Behavioral Ecology16:1037-1041.

Wasson, K.  2002. A review of the invertebrate phylum kamptozoa (Entoprocta) and synopsis of kamptozoan diversity in Australia and New Zealand.  Transactions of the Royal Society of South Australia126:1-20.

Wasson, K., Lyon, B.E., and Knope, M.  2002.  Hair-trigger autotomy in porcelain crabs is a highly effective escape strategy.  Behavioral Ecology13:481-486

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.

Sterflinger, K., Hain, M., Scholz, J., and Wasson, K. 2001.  Fungal infections of a colonial marine invertebrate: diversity and morphological consequences. Facies 45:31-38.

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 Conservation102(2):143-153.

Wasson, K., Von Holle, M., Toft, J., and Ruiz, G. 2000.  Detecting invasions of marine   organisms: kamptozoan case histories.  BiologicalInvasions2:59-74.