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).