Fig. 2. (A)Interaction web of top-down and bottom-up effects in the eelgrass study system. The top predator is the sea otter (E. lutris), the mesopredators are crabs (Cancer spp. and Pugettia producta), the epiphyte mesograzers are primarily an isopod (I. resecata) and a sea slug (P. taylori), and algal epiphyte competitors of eelgrass primarily consist of chain-forming diatoms, and the red alga Smithora naiadum. Solid arrows indicate direct effects, dashed arrows indicate indirect effects, and the plus and minus symbols indicate positive and/or negative effects on trophic guilds and eelgrass condition. C, competitive interaction; T, trophic interaction. (Original artwork by A. C. Hughes.) (B-E) Survey results testing for the effects of sea otter density on eelgrass bed community properties (Tables S2 and S3). Elkhorn Slough (sea otters present and high nutrients) eelgrass beds (n = 4) are coded in red, and the Tomales Bay reference site (no sea otters, low nutrients) beds (n = 4) are coded in blue. (B) Crab biomass and size structure of two species of Cancer crabs; (C) grazer biomass per shoot and large grazer density; (D) algal epiphyte loading; and (E) aboveground and belowground eelgrass biomass. DW, dry weight; FW, fresh weight.

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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.

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 dielcycling 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.

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