Abstract
Both seagrasses and bivalve shellfish provide valuable ecosystem services in estuar- ies worldwide. Seagrasses are protected by no-net-loss provisions in US federal and state regula- tions, resulting in precautionary management that avoids any direct impacts from development activity, including shellfish aquaculture. Recent research suggests that oyster aquaculture has direct impacts on native seagrass (eelgrass Zostera marina) at small spatial and short temporal scales in US west coast estuaries. We quantified impacts of oyster aquaculture on Z. marina at the estuarine landscape scale in Willapa Bay, Washington. A model of Z. marina cover outside of aquaculture was created using distance to estuary mouth, distance to nearest channel, salinity, elevation, and cumulative wave stress as factors, and was then used to predict Z. marina distribu- tion within oyster aquaculture beds and compared to an inverse distance interpolation of points outside of aquaculture. The amount of Z. marina cover observed within oyster aquaculture beds was less than predicted, but represented <1.5% of the total predicted amount of Z. marina cover in Willapa Bay in any year. Type of oyster culture bed did not contribute to observed variation, but mechanically harvested beds had significantly less Z. marina cover than beds harvested by other methods. The majority of beds had 65�145% of the model-predicted Z. marina cover and exhib- ited relatively low variability between years, suggesting that Z. marina as habitat is resilient to oyster aquaculture as a disturbance and does not result in persistent effects at the landscape scale in this estuary.
Highlights
Research on oyster aquaculture in Willapa Bay, Washington on the US west coast suggests that this interaction is nonlinear and thresholds occur above which shoot density of Z. marina declines markedly (Wagner et al 2012)
Though nutrients in sediment porewater were enhanced by the presence of oysters in Willapa Bay (Wagner et al 2012) and oysters measurably cleared the water (Wheat & Ruesink 2013), only eelgrass shoot size was affected by oysters at the local scale (Wagner et al 2012), suggesting that seagrass response differs depending on estuarine conditions
Willapa Bay provides a unique opportunity to examine this interaction at the estuarine landscape scale because shellfish aquaculture within eelgrass was not restricted prior to 2007 when a new permit (US Nationwide Permit 48) was issued by the US Army Corps of Engineers (US ACOE) with protection for native eelgrass Z. marina
Summary
Both seagrasses and reef-forming bivalves serve a variety of important ecological functions in estuaries, including: enhancing biodiversity and providing structured nursery habitat and refuge from predators for fish and invertebrates (e.g. seagrasses: Bostrom et al 2006, Gillanders 2007, Heck et al 2008; bivalves: Coen et al 1999, Gutierrez et al 2003, Grabowski et al 2008), water column filtration and water property enhancement ( bivalves as phytoplankton grazers; Prins et al 1998, Newell 2004, Ferreira et al 2011), sediment accretion and erosion control through current modification Willapa Bay provides a unique opportunity to examine this interaction at the estuarine landscape scale because shellfish aquaculture within eelgrass was not restricted prior to 2007 when a new permit (US Nationwide Permit 48) was issued by the US Army Corps of Engineers (US ACOE) with protection for native eelgrass Z. marina. This permit did not influence the placement or activities on privately owned or leased aquaculture beds, but if these activities occurred in eelgrass, ‘pre-construction notification’ was required and any expansion of aquaculture must leave buffers for eelgrass. We further that effort for this estuary by first constructing a model to predict seagrass distribution outside of aquaculture using several factors that we suspected
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