Density-dependent effects of an introduced oyster, Crassostrea gigas, on a native intertidal seagrass, Zostera marina 

Abstract

Seagrasses and bivalves co-occur worldwide, and each plays a role in the structure, function, and services of coastal ecosystems. While seagrasses are declining, bivalve aquaculture is expanding, and impacts from culture practices, as opposed to the cultured organisms themselves, need to be distinguished. In 2 experiments, we tested the effects of live Crassostrea gigas, an introduced oyster, on Zostera marina, a native seagrass, in Willapa Bay, Washington. Expt 1 involved adult oyster addition across a large range in density (0 to 70% cover), and also considered the effects of the physical components of an oyster: its shell, and fertilizer mimicking biodeposition. Expt 2 simulated a crop cycle with addition of newly-settled oysters at relevant commercial density (≤20%), which then grew over 3 yr. In Expt 1, seagrass shoot density and size declined (at least 50% and at most 40%, respectively) with oyster addition. Steep declines indicating density-dependent space competition occurred at different thresholds after 1 (1.3% oyster cover), 2 (12.4%), and 3 yr (21.9%). Live oysters changed sediment characteristics, increasing organic content, silt:sand ratios, and levels of porewater ammonium, but these changes did not affect eelgrass. Nutrient addition did not affect density or growth, although it augmented porewater ammonium. In Expt 2, no significant reductions in density, size, or growth of seagrass occurred. Our results suggest that the effects of introduced oysters on seagrass occur through space competition and can exceed the footprint of the oysters, generating strong impacts above a threshold of ~20% cover.

Light indirectly mediates bivalve habitat modification and impacts on seagrass

“…Similarly, by fertilizing porewaters, tulip mussels (Modiolus americanus) can improve turtlegrass (Thalassia testudinum) leaf nutrient condition (Peterson and Heck, 1999) and growth (Peterson and Heck, 2001a,b). However, other studies have shown that enhancement of porewater ammonium by blue mussels, geoduck clams (Panopea generosa), and cultured oysters (Crassostrea gigas) does not affect eelgrass leaf nitrogen or growth (Worm and Reusch, 2000;Ruesink and Rowell, 2012;Wagner et al, 2012), despite low ambient nutrient concentrations (b30, b 100, and b20 μmol NH 4 + [L porewater] − 1 , respectively).…”

“…However, other studies have shown that sediment enrichment by mussels can inhibit the growth of seagrasses by increasing concentrations of toxic sulfides Vinther et al, 2012). In other instances, bivalves may have mixed (Reusch and Williams, 1998) or no (e.g., Worm and Reusch, 2000;Wagner et al, 2012) effects on seagrass growth. These variable and inconsistent effects cannot be satisfactorily explained by nutrient availability, suggesting more complex interactions.…”

Oysters and Eelgrass: Potential Partners in a High pCO2 Ocean

“…It is unknown whether filter-feeding molluscs may reduce concentrations of LZ in the water column, or if this process may be compromised at high pCO 2 . Many species of oysters, including Pacific oysters, frequently co-occur with eelgrass, either in naturalized populations or in aquaculture (Wall et al 2008, Wagner et al 2012 making them an obvious candidate for investigating this hypothesis.…”

Effect of oyster aquaculture on seagrass Zostera marina at the estuarine landscape scale in Willapa Bay, Washington (USA)

“…Wagner et al (2012) showed that competition for space between oysters and seagrass did not follow a 1:1 relationship and a threshold of about 20% shell cover existed, above which Z. marina density declined exponentially, at least at small experimental scales. With the exception of areas where oysters are allowed to form reefs or hummocks near the south end of Willapa Bay, however, cultured oyster density rarely reaches this threshold , 2011, Tallis et al 2009, and eelgrass commonly co-exists with oysters throughout the growout cycle, albeit at reduced density.…”