Appendix D. Additional details on measurements of environmental covariates at each experimental site.
Chlorophyll: I sampled chlorophyll a concentrations in the water column at two-week intervals to quantify suspended food availability. Three replicate water samples spaced approximately 30–50 m were collected 5 cm from the water’s surface. The seawater was passed through a glass fiber filter (Gelman A/E, 25 mm) by vacuum hand-pump. Chlorophyll a was determined by spectrophotometry following 24 hour extraction at 4°C in 90% ethanol and correcting for phaeopigments (Sartory and Grobbelaar 1984).
Crab Sampling: In the San Juan Islands crabs are a major clam predator, particularly Cancer productus, which forages extensively in the intertidal zone (Robles et al. 1989, Yamada and Boulding 1996, Byers 2002). Although not a heavily fished species, C. productus is occasionally harvested and also interacts with species that are more heavily fished (e.g., Cancer magister), and thus could vary in abundance with reserve status. I sampled the relative abundance, biomass, sex ratio, and size of all crabs on a monthly basis from March through September. On each sample date at each site, three baited crab traps (FTA-Fish Trap, Aquatic Eco-systems, Apopka, Florida, USA; dimensions: 24” × 18” × 8”, 1.25-cm mesh) were spaced 30–50 m apart. Traps were placed during daytime low tide directly downshore from the experimental array at a tidal height of approximately –0.5 m to keep them underwater during deployment. They remained overnight and were retrieved 24 hours later. Traps were set at two sites at a time, such that all six sites were sampled within three consecutive days. Biomass of each crab species was calculated using size–mass relationships of crabs from the San Juan Islands (Orensanz and Galluchi 1988) and summed to yield an overall crab biomass per site.
Sediment organic content: Following completion of the field experiment I extracted three shallow sediment cores (2 cm deep, diameter = 1.5 cm) haphazardly placed within 1 m of experimental array at each site. The shallow cores were expected to reflect the recent history of organics in the water column that serve as food for suspension feeding clams like Protothaca and Venerupis. Samples were frozen at -20° C until processing. Samples were dried for 24 hours at 75° C, weighed, and placed into a preheated muffle furnace at 500° C for four hours in order to combust organic material. After combustion, samples were weighed again to determine the ash-free dry mass and to calculate the proportion of organic content.
Ambient clam biomass: Suspension feeding clams in the ambient environment surrounding the enclosures have the potential to influence food available to experimental clams. Furthermore, these ambient clams also may attract predators drawn to patterns of clam abundance on a larger spatial scale. Therefore, to account for exploitative competition and predator attraction that might occur at a larger scale than the clam enclosures, I incorporated several measures of ambient clam biomass into analyses. Specifically, I used the ambient biomass of Protothaca, Venerupis, all native clams, all nonindigenous clams, and all clams (the first three of which were loge transformed), which were quantified at each site in the field survey.
Other predator notes: Birds were quantified because they have been shown to influence clam abundance in other areas (Richardson and Verbeek 1987, Peterson 1991). However, birds were rarely seen preying on adult bivalves at any of the survey or experimental sites. Carnivorous moon snails, Polinices lewisii, are an important clam predator in other areas of Washington and the Pacific coast in general, but are exceedingly rare or absent in the San Juan archipelago (M. Dethier and E. Kozloff, personal communication) and were never observed.
Byers, J. E. 2002. Physical habitat attribute mediates biotic resistance to non-indigenous species invasion. Oecologia 130:146–156.
Orensanz, J. M., and V. F. Gallucci. 1988. Comparative study of postlarval life-history schedules in four sympatric species of Cancer (Decapoda: Brachyura: Cancridae). Journal of Crustacean Biology 8:187–220.
Peterson, C. H. 1991. Intertidal zonation of marine invertebrates in sand and mud. American Scientist 79:236–249.
Richardson, H., and N. A. M. Verbeek. 1987. Diet selection by yearling Northwestern crows (Corvus caurinus) feeding on littleneck clams (Venerupis japonica). Auk 104:263–269.
Robles, C., D. A. Sweetnam, and D. Dittman. 1989. Diel variation of intertidal foraging by Cancer productus L. in British Columbia. Journal of Natural History 23:1041–1049.
Sartory, D. P., and J. U. Grobbelaar. 1984. Extraction of chlorophyll-a from freshwater phytoplankton for spectrophotometric analysis. Hydrobiologia 114:177–187.
Yamada, S. B., and E. G. Boulding. 1996. The role of highly mobile crab predators in the intertidal zonation of their gastropod prey. Journal of Experimental Marine Biology and Ecology 204:59–83.