Contaminant concentrations and risks associated with the Pacific oyster in the highly urbanized San Diego Bay.

Contaminant concentrations in filter-feeding shellfish may indicate the health of coastal waters and consumption risks. Widespread expansion of the Pacific oyster (Crassostrea gigas) and its popularity as food make it a useful sentinel. We surveyed intertidal Pacific oysters in San Diego Bay, California for contaminants during summer 2018 and winter 2019. We compared contaminants in Pacific oyster to California mussel from California's State Mussel Watch Program (1993-2003) and human consumption thresholds. Contaminants such as neonicotinoid and chlorinated pesticides, selenium, and several metals were higher in Pacific oysters in summer, while PBDEs, benzylbutyl phthalate, and plastics were higher in winter. Contaminant levels were generally lower in Pacific oyster than mussel except for copper and zinc. Bay-wide PCB concentrations in oysters exceeded thresholds but individual samples (locations) also met or surpassed chlordane, PCB and PAH thresholds. Monitoring and risk assessments that consider species' biology, season, location, effects of multiple contaminants, and human consumption patterns will contribute to more effective consumption guidelines.

Natural history matters: Plastics in estuarine fish and sediments at the mouth of an urban watershed.

The extent to which small plastics and potentially associated compounds are entering coastal food webs, especially in estuarine systems, is only beginning to be realized. This study examined an estuarine reach at the mouth of urbanized Chollas Creek in San Diego, California to determine: 1) the extent and magnitude of microplastics pollution in estuarine sediments and fish, 2) the extent and magnitude of SVOC contamination in estuarine fish, and 3) whether fish preferentially ingested certain types of microplastics, when compared with the microplastic composition of creekbed sediments. Surface sediments (0-5 cm depth) contained about 10,000 small plastic pieces per m2, consisting mostly (90%) of fibers, and hard and soft pieces. Nearly 25% of fish contained small plastics, but prevalence varied with size and between species. Of the 25 types of small plastics found in sediment, fish preferred about 10 types (distinct colors and forms). Several SVOCs, both water soluble and sediment-associated compounds, were found in the two species of fish tested. This study revealed that a species' natural history may influence contamination levels, and warrants further study to better understand the pathways of plastics and associated contaminants into and throughout coastal food webs, and the potential health risks for small and/or low-trophic level organisms.

Insights into the establishment of the Manila clam on a tidal flat at the southern end of an introduced range in Southern California, USA.

Coastal ecosystem modifications have contributed to the spread of introduced species through alterations of historic disturbance regimes and resource availability, and increased propagule pressure. Frequency of occurrence of the Manila clam (Venerupis phillipinarum, Veneridae) in Southern California estuaries has increased from absent or sparse to common since the mid-1990s. Potential invasion vectors include seafood sales and aquaculture, and spread from established northern populations over decades. The clam's post-settlement habitat preferences are, however, uncertain in this region. Our project aimed to identify factors associated with established patches of the clam within a bay toward the southern end of this introduced range. During summer 2013, we sampled 10 tidal flat sites in Mission Bay, San Diego; each containing an area with and without hard structure (e.g., riprap, boulders). We measured likely environmental influences (e.g., sediment variables, distance to ocean). Manila clam densities across the bay were most strongly associated with site, where highest densities were located in the northern and/or back halves of the bay; and weakly correlated with lower porewater salinities. Within sites, Manila clam density was enhanced in the presence of hard structure in most sites. Prevailing currents and salinity regimes likely contribute to bay wide distributions, while hard structures may provide suitable microhabitats (refuge from predators and physical stress) and larval entrapment within sites. Results provide insights into decisions about future shoreline management efforts. Finally, we identify directions for future study to better understand and therefore predict patterns of establishment of the Manila clam in the southern portion of its introduced range.

Testing the effects of an introduced palm on a riparian invertebrate community in southern California.

Despite the iconic association of palms with semi-arid regions, most are introduced and can invade natural areas. Along the San Diego River (San Diego, California, USA), the introduced Canary Island date palm (Phoenix canariensis) forms dense patches among native riparian shrubs like arroyo willow (Salix lasiolepis). The structural differences between the palm and native shrubs are visually obvious, but little is known about palm's effects on the ecosystem. We tested for the effects of the palm on a riparian invertebrate community in June 2011 by comparing the faunal and environmental variables associated with palm and willow canopies, trunks and ground beneath each species. The palm invertebrate community had lower abundance and diversity, fewer taxa feeding on the host (e.g., specialized hemipterans), and more taxa likely using only the plant's physical structure (e.g., web-builders, oak moths, willow hemipterans). There were no observed effects on the ground-dwelling fauna. Faunal differences were due to the physical and trophic changes associated with palm presence, namely increased canopy density, unpalatable leaves, trunk rugosity, and litter accumulations. Palm presence and resulting community shifts may have further ecosystem-level effects through alteration of physical properties, food, and structural resources. These results were consistent with a recent study of invasive palm effects on desert spring arthropods, illustrating that effects may be relatively generalizable. Since spread of the palm is largely localized, but effects are dramatic where it does occur, we recommend combining our results with several further investigations in order to prioritize management decisions.

Which spatial heterogeneity framework? Consequences for conclusions about patchy population distributions.

Patches, gradients, and hierarchies are three common organizational frameworks for assessing the effects of spatial heterogeneity on species distributions. Since these frameworks are often chosen a priori, without knowledge of study systems, they may not correspond to the empirical heterogeneity present and may result in partial or erroneous conclusions about the forces structuring species distributions. I tested the consequences of choosing particular frameworks and whether patch heterogeneity structured patchily distributed populations of the valley elderberry longhorn beetle (Desmocerus californicus dimorphus) along four rivers in California's Central Valley (USA). A comparison of the three approaches revealed that each led to incomplete conclusions about controls on the beetle's distribution and populations. Patch analysis revealed weak effects of patch size and quality, and high unexplained variance, which likely reveals large amounts of stochasticity since replication was high. The patch analysis therefore concluded that distributions consistent with patch dynamic structures like classic metapopulation, source-sink, and mainland-island models existed in the different rivers. Conversely, gradient analyses revealed a gradient-distribution pattern responding to continuous and often large-scale variables, such as host-plant age or size, water availability, and the presence of an invasive leguminous tree; again most variance in beetle occurrence remained unexplained. Hierarchical analysis identified the natural spatial patterns of the system but gave no indication of causal processes. The combination of all three approaches explained the maximum variance in beetle occurrence, through inclusion of a comprehensive list of explanatory variables, multiple spatial scales, various types of heterogeneity, and a focus on the scales at which beetle-environment interactions were strongest. Surprisingly, these results still supported the notion that the beetle exists as a metapopulation, a structure thought to be rare because it ignores habitat quality and landscape conditions. These analyses exemplify the simultaneous importance of local patch attributes and broad-scale and/or gradient variables that are commonly overlooked in patch studies. Importantly, some patch attributes acted over inter-patch scales, affecting the perception of patch distances and distributional extents. Only through the integration of frameworks was I able to decipher the system's complexity and see that all three types of heterogeneity were acting in the system, sometimes over unexpected scales.