MEPS 468:149-160 (2012)  -  DOI: https://doi.org/10.3354/meps09952

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.

KEY WORDS: Zostera marina · Crassostrea gigas · Density-dependent effects · Shellfish aquaculture · Disturbance · Recovery