MEPS 344:71-80 (2007)  -  DOI: https://doi.org/10.3354/meps06942

ABSTRACT: Seagrasses are a critical element in many estuaries and act as drivers of abiotic and biotic processes. One species, Zostera marina L., has been declining globally. A potential contributor to this decline is shellfish aquaculture, although we know little about its impacts. On the US west coast, shellfish aquaculture co-occurs with protected eelgrass habitats. Many aquaculture practices constitute a periodic disturbance, and a key concern is eelgrass recovery. We used observations and experiments to understand how oyster aquaculture practices (i.e. dredging [oysters grown on the bottom and harvested mechanically via dredging] and off-bottom longline culture [oysters suspended off the bottom on rope and harvested by hand]) influence eelgrass recovery. Studies of natural recruitment showed highest seedling densities in dredged beds (7 seedlings m^–2) and lowest under longlines (0.1 seedlings m^–2). Seed production was highest in dredged beds (295 seeds m^–2) and lowest in longline beds (52 seeds m^–2). Seed addition experiments were conducted to understand the effect of oyster aquaculture and adult eelgrass neighbors on seedling germination, growth, and survival. In March 2005, seedling germination was 146% higher in eelgrass removal treatments compared to control plots, with no difference among aquaculture and reference areas. By April 2005, there were no effects of neighbors, but reference areas had greater seed densities (11 seedlings m^–2) compared to longline areas (3.2 seedlings m^–2). By August 2005, seedling mortality in longline and reference control plots was 100%. In dredged areas, seedlings in removal plots had greater biomass (0.38 g) than seedlings in control plots (0.14 g). We propose that if eelgrass is to be disturbed by aquaculture, dredge beds may recover more successfully than longline beds.

KEY WORDS: Zostera marina · Crassostrea gigas · Shellfish aquaculture · Disturbance · Seeds · Germination · Recovery · Estuary · Willapa Bay