Experimental test of oyster restoration within eelgrass

Abstract

1. Both seagrasses and oysters are foundation species valued for their wide range of ecosystem services, but their space competition sets a constraint on joint benefits. A reserve for native Olympia oysters (Ostrea lurida) was established in lower Hood Canal (Washington State, USA) more than a century ago but is now devoid of that species and dominated by native eelgrass (Zostera marina). This situation sets up a conservation conflict because management activities for one species are at odds with the protection of another.2. In experimental enhancement plots, Olympia oysters were outplanted at low density, which successfully maintained eelgrass density and production. One method was used in 2013 (seeded cultch, 8% cover) and two additional methods in 2015 (anchored cultch and single oysters, the latter at 4% cover).3. For all outplant methods, oysters experienced 99% annual mortality, associated with the attraction of non-native and native predators. Shell cover remained steady for a year and then declined rapidly, as shell accumulation did not exceed sedimentation rates.4. Eelgrass per se does not preclude Olympia oysters, given that the two species were observed to co-occur at a coastal estuarine site (Willapa Bay, Washington). However, even when sociopolitical constraints on restoration activities were overcome, ecological constraints remained from predation. Competition between these two protected species was avoided, but it may be the case that top-down control on oysters was particularly acute owing to low oyster density and/or the environmental conditions of eelgrass beds.

Habitat structure influences the seasonality of nekton in seagrass

“…1b), leaf area index and above-ground biomass were similar (Gross et al 2017). In Washington State, spring-season eelgrass biomass is typically depressed by half relative to summer (Thom 1990;Ruesink et al 2010;Ruesink and Rowell 2012;Valdez et al 2017).…”

“…What we address here is whether this modification changes seasonally, and if so, can we infer why? We expected a spatio-temporal (season × habitat) dynamic for nekton simply because the habitats themselves become more distinct as eelgrass increases its canopy height and above-ground biomass in late summer in Washington State (Thom 1990;Ruesink et al 2010;Ruesink and Rowell 2012;Valdez et al 2017). Consequently, summer seagrass provides the greatest contrast with nearby unvegetated areas, which may cause seasonal changes of nekton in seagrass to be more consistent than in unvegetated habitats (Cote et al 2013).…”

Scales of recruitment variability in warming waters: Comparing native and introduced oysters in Hood Canal, Washington, USA

“…At the same time, appearance of new recruits of Olympia oysters at all sites suggests the importance of restoring habitat, which generally requires hard substratum for recruitment in moist or submerged areas where water protects the oysters from temperature extremes, rather than needing to enhance broodstock by outplanting oysters. Predators may also restrict Olympia oyster recovery (Valdez et al., ; Wasson et al., ). In the comparison of native and non‐native oysters, from the relative positions of their thermal performance curves, we expect warming temperatures to favor the non‐native, whose optimal temperature appears still higher than currently occurs in summer in Hood Canal.…”

“…Pacific oysters recruit naturally in all these locations, based on the presence of reefs above mean lower low water (MLLW). Scattered populations of adult Olympia oysters have been observed throughout main stem and lower Hood Canal, except recently at Belfair (Valdez, Peabody, Allen, Blake, & Ruesink, ).…”

Oyster aquaculture impacts Zostera marina epibiont community composition in Akkeshi-ko estuary, Japan

“…To characterize density we enumerated shoots per quadrat and for morphology we measured leaf height and blade width from the first five shoots in each quadrat ( sensu [ 40 ]). Above-ground biomass was obtained by manually removing all epibionts from grass blades and drying at 60°C to constant weight.…”