Kerstin Wasson

Biological invasions of estuaries without international shipping: the importance of intraregional transport

Increased awareness of the problem of introduced marine species has led to recent surveys of several large bays with international shipping. To our knowledge, no thorough search for introductions has been carried out in an embayment not connected to an international harbor. In 1998, we investigated the macroinvertebrate fauna of Elkhorn Slough (ES), an estuary in central California. Fieldwork and a literature review revealed 56 known exotic species at ES, a surprising diversity considering the rather modest search effort, the relatively natural setting of this estuary, and the lack of international shipping. While some exotic species at ES were probably introduced directly from distant waters with cultivated oysters, others likely arrived more indirectly via San Francisco Bay or other regional ports with thriving populations of invaders, travelling for instance as adults fouling boats or as larvae on currents. The effect of international shipping, including ballast water dumping, is thus not limited to areas with major harbors, but rather reverberates up and down the coast to seemingly isolated embayments.

Ballast water deoxygenation can prevent aquatic introductions while reducing ship corrosion

One of the most important mechanisms for the introduction of aquatic nuisance species is transport in ship ballast waters. Although several ballast tank treatments to prevent transport of aquatic organisms appear promising, all existing approaches will result in significant costs to the shipping industry. This study describes a treatment that can dramatically reduce the survivorship of most organisms found in ballast waters while providing economic benefits to ship owners. Purging of oxygen from ballast tanks with nitrogen was recently found to be a cost-effective technique for reducing corrosion and therefore extending ship life. We tested the tolerance of larvae of known invasive invertebrate species to low levels of oxygen, comparable with those resulting from the anticorrosion treatment, and detected significant levels of mortality. Two separate literature reviews further support the conclusion that few organisms will be able to withstand extended periods of exposure to nitrogen-treated ballast water. This novel deoxygenation technique may therefore have direct benefits to both marine conservation and the shipping industry.

Habitat Differences in Marine Invasions of Central California

We carried out a two-part investigation that revealed habitat differences in marine invertebrate invasions. First, we compared invasion levels of hard vs soft substrata in Elkhorn Slough, an estuary in Central California, by comparing abundance and richness of native vs exotic species in quantitative samples from each habitat type. Our results revealed that the hard substrata were much more heavily invaded than the soft substrata. Nearly all the hard substrata in Elkhorn Slough, as in most estuaries along the Pacific coast of North America, are artificial (jetties, rip-rap, docks). Some exotic species may by chance be better adapted to this novel habitat type than are natives. Two major vectors responsible for marine introductions, oyster culturing and ship-hull fouling, are also more likely to transport species associated with hard vs soft substrata. Secondly, we compared estuarine and open coast invasion rates. We examined species richness in Elkhorn Slough and adjacent rocky intertidal habitats along the Central California coast. The absolute number of exotic species in the estuary was an order of magnitude higher than along the open coast (58 vs 8 species), as was the percentage of the invertebrate fauna that was exotic (11% vs 1%). Estuaries on this coast are geologically young, heavily altered by humans, and subject to numerous transport vectors bringing invasive propagules: all these factors may explain why they are strikingly more invaded than the open coast. The finding that the more species rich habitat – the open coast – is less invaded is in contrast to many terrestrial examples, where native and exotic species richness appear to be positively correlated at a broad geographic scale.

Historical Ecology of a Central California Estuary: 150 Years of Habitat Change

We investigated the historical ecology of Elkhorn Slough, a 1,200 ha tidal wetland system in central California. The goal of this study was to identify patterns of change in the extent and distribution of wetland habitats during a 150-yr period and to investigate the causes of these changes. Using a geographic information system (GIS), we interpreted historic maps, charts, and aerial photographs. We created a series of summary maps to illustrate and quantify changes in tidal flow and habitat types at six representative historical periods. With the aid of custom software tools, we performed semi-automated spatial analysis of historic aerial photographs to quantify changes in marsh cover at fixed quadrats and tidal creek width at fixed cross sections. Our multiscale analysis documents dramatic shifts in the distribution of habitat types resulting from anthropogenic modifications to the hydrology of the slough. More than half of the marshlands were diked, and more than two thirds have either degraded or been converted to other habitat types. The construction of an artificial mouth abruptly transformed the wetland system from depositional to highly erosional, enlarging channels, widening creeks, and converting marsh to intertidal mudflat or open water. Increased tidal amplitude and velocity are the likely causes. In recent decades, levee failure and intentional breaching have restored the acreage under tidal influence to nearly historic levels, but recolonization of former wetlands by salt marsh vegetation has been minimal. Degraded former marshland and unvegetated mudflat are now the dominant habitat types at Elkhorn Slough. The rate of habitat change remains high, suggesting that a new equilibrium may not be reached for many decades. This study can help tidal wetland managers identify patterns and mechanisms of habitat change and set appropriate conservation and restoration goals.

Recovery of a top predator mediates negative eutrophic effects on seagrass

Significance Many coastal vegetated ecosystems have declined, affected by human alterations to “bottom-up” forces such as nutrient loading from agriculture and by “top-down” forces such as overfishing of predators. Examining the interactions between such bottom-up and top-down changes is challenging, because top predators have disappeared from many of these ecosystems. A highly nutrient-loaded estuary in California recently colonized by a recovering sea otter population provided an unusual opportunity to examine these interactions. We demonstrate that top-down effects of sea otters mitigate negative effects of nutrient loading, enhancing growth of seagrass. Grazers that remove algae from seagrass are favored by decreased crabs resulting from otter predation. Recovery of top predators can thus support resilience of coastal vegetated ecosystems. A fundamental goal of the study of ecology is to determine the drivers of habitat-forming vegetation, with much emphasis given to the relative importance to vegetation of “bottom-up” forces such as the role of nutrients and “top-down” forces such as the influence of herbivores and their predators. For coastal vegetation (e.g., kelp, seagrass, marsh, and mangroves) it has been well demonstrated that alterations to bottom-up forcing can cause major disturbances leading to loss of dominant vegetation. One such process is anthropogenic nutrient loading, which can lead to major changes in the abundance and species composition of primary producers, ultimately affecting important ecosystem services. In contrast, much less is known about the relative importance of apex predators on coastal vegetated ecosystems because most top predator populations have been depleted or lost completely. Here we provide evidence that an unusual four-level trophic cascade applies in one such system, whereby a top predator mitigates the bottom-up influences of nutrient loading. In a study of seagrass beds in an estuarine ecosystem exposed to extreme nutrient loading, we use a combination of a 50-y time series analysis, spatial comparisons, and mesocosm and field experiments to demonstrate that sea otters (Enhydra lutris) promote the growth and expansion of eelgrass (Zostera marina) through a trophic cascade, counteracting the negative effects of agriculturally induced nutrient loading. Our results add to a small but growing body of literature illustrating that significant interactions between bottom-up and top-down forces occur, in this case with consequences for the conservation of valued ecosystem services provided by seagrass.

Testing local and global stressor impacts on a coastal foundation species using an ecologically realistic framework

Despite the abundance of literature on organismal responses to multiple environmental stressors, most studies have not matched the timing of experimental manipulations with the temporal pattern of stressors in nature. We test the interactive effects of diel-cycling hypoxia with both warming and decreased salinities using ecologically realistic exposures. Surprisingly, we found no evidence of negative synergistic effects on Olympia oyster growth; rather, we found only additive and opposing effects of hypoxia (detrimental) and warming (beneficial). We suspect that diel-cycling provided a temporal refuge that allowed physiological compensation. We also tested for latent effects of warming and hypoxia to low-salinity tolerance using a seasonal delay between stressor events. However, we did not find a latent effect, rather a threshold survival response to low salinity that was independent of early life-history exposure to warming or hypoxia. The absence of synergism is likely the result of stressor treatments that mirror the natural timing of environmental stressors. We provide environmental context for laboratory experimental data by examining field time series environmental data from four North American west coast estuaries and find heterogeneous environmental signals that characterize each estuary, suggesting that the potential stressor exposure to oysters will drastically differ over moderate spatial scales. This heterogeneity implies that efforts to conserve and restore oysters will require an adaptive approach that incorporates knowledge of local conditions. We conclude that studies of multiple environmental stressors can be greatly improved by integrating ecologically realistic exposure and timing of stressors found in nature with organismal life-history traits.

Invasions of estuaries vs the adjacent open coast: A global perspective

Invasions by alien species have been reported from every marine habitat where surveys have been conducted for them. Conspicuous examples from around the globe include the brown alga Sargassum mangarevense in tropical coral reef systems (Andrefouet et al. 2004), the bivalve Mytilus galloprovincialis along temperate rocky shores (Steffani and Branch 2003), and the reef-building polychaete, Ficopomatus enigmaticus in estuaries (Schwindt et al. 2004). Despite numerous examples of marine invaders from a variety of habitats, little is known about how invasion rates of entire assemblages of organisms compare between different marine habitat types. And indeed most marine habitats have not been thoroughly surveyed – the majority of our understanding of marine invasions comes from shallow near-shore environments. Some studies have attempted to quantify habitat differences in marine invasions, examining assemblages (both natives and aliens) at different scales. Within estuarine ecosystems, focus has been on comparisons between different salinities and substrates. (In this chapter an estuary is considered to be a ‘partly enclosed body of water by the coast in which sea water and fresh water mix‘ (Little 2000).) Wolff (1973) examined the benthic macroinvertebrates of four major estuaries in the Netherlands. He found that in the high salinity parts of these estuaries about 2% of the species were alien, in the brackish part about 20%, and in the tidal freshwater part about 8%. In non-tidal brackish waters the share of alien species was about 28%. Wolff (1999) re-analyzed these data and included three more estuaries in the northern Netherlands and Germany. He found that tidal and stagnant low salinity habitats of seven Dutch and German estuaries harbored a higher proportion of alien species (about 20%) than estuarine high salinity habitats (about 6%). This pattern was not clearly related to propagule pressure (harbors and aquaculture were not focused in the middle salinity). Lee et al. (2003) found that patterns of invasion varied along an estuarine gradient in San Francisco Bay; soft-bottom benthic communities at estuarine salinities were more invaded than communities at either brackish or marine salinities. Wasson et al. (2005) found hard substrates to be more invaded than soft substrates, and a site near the mouth of an estuary to be less

Do a threatened native amphibian and its invasive congener differ in response to human alteration of the landscape

Anthropogenic changes to habitat are a global phenomenon and the impact of these changes may act in tandem to cause loss of biodiversity. One major global change is the introduction of invasive species. In order to determine whether other human impacts might correlate with populations of invaders, we examined the habitat correlates of distribution, persistence and reproduction of a global invader, the American bullfrog (Rana catesbeiana). We then compared these correlates with those of a threatened, native congener, the California red-legged frog (Rana draytonii). We found striking differences between the two species in response to habitat fragmentation and degradation. Our work suggests that human alteration of habitat, in particular the hydrology of freshwater sites and through building roads, favors this invasive species across the landscape.

Salt Marsh-Upland Ecotones in Central California: Vulnerability to Invasions and Anthropogenic Stressors

Ecotones, zones of abrupt biological transition typically reflecting strong physical gradients, may be particularly sensitive to changes in environmental conditions. Our characterization of the ecotone between salt marshes and uplands in the Elkhorn Slough watershed in central California revealed that extent of appropriate habitat for native high marsh species endemic to this zone is extremely limited. The ecotone is highly invaded, with non-native upland weeds accounting for a significant proportion of cover. We investigated responses to two anthropogenic landscape management strategies, restriction of tidal exchange through water control structures and cattle grazing. Moderate tidal restriction resulting in muted tidal exchange dramatically decreased ecotone width, native marsh plant richness, and cover by native ecotone specialists. Even stronger tidal restriction resulting in very low tidal exchange led to a seaward shift of the ecotone into the area formerly occupied by mid-marsh vegetation; upland plants now occupy the former ecotone zone so net loss of wetland habitat has occurred. Cattle grazing led to a very substantial increase in bare ground, a significant decrease in native marsh plant richness and a significant increase in non-native plant cover. Thus, both of these management regimes can have significant negative impacts on rare salt marsh ecotone extent and biodiversity.

Detecting invasions of marine organisms: kamptozoan case histories

Detecting marine invasions can be challenging, especially for lesser-known taxa, and requires (a) thorough field surveys of the region of interest for members of the taxon, (b) systematic analyses to identify all species found, (c) literature searches for the worldwide distribution of these species and for previous records of the taxon in this region, and (d) application of rigorous criteria to assess whether each species found is native or introduced. We carried out these steps in order to detect and document kamptozoan (entoproct) invasions on the American mid-Atlantic coast. We report on the occurrence of two colonial kamptozoans (Barentsia benedeni, Loxosomatoides laevis) in Chesapeake Bay (Maryland and Virginia, USA). On the American Atlantic coast, B. benedeni had previously only been reported from Massachusetts, although this species has a worldwide distribution in bays and harbors. The genus Loxosomatoides had not previously been reported from North America and L. laevis was known only from India. Since the genus Loxosomatoides was very poorly characterized, we briefly review all four of its species, which differ only slightly from each other. We have also synonymized L. japonicum with L. laevis. We did not find any of the kamptozoan species previously recorded in surveys of Chesapeake Bay and the American Atlantic coast. This is the first detailed consideration of anthropogenic influences on kamptozoan distributions, and we emphasize that most kamptozoan species are cryptogenic pending further investigation.