Seasonal effects of clams(Panopea generosa)on eelgrass(Zostera marina)density but not recovery dynamics at an intertidal site

Abstract

Today's marine ecosystems experience multiple stressors that complicate management and pose risks to resource protection. Clam aquaculture has several potential environmental effects including disturbance, space occupation, and modification of nutrient availability, which were assessed through a crossed experimental design of eelgrass (Zostera marina L.) removal and geoduck clam (Panopea generosa Gould) addition in 1 m2 plots, in an eelgrass meadow in Puget Sound, Washington, USA. Gaps recovered via clonal growth, with gap edges requiring more than 1 year and gap centres almost 2 years. Clams neither increased nor impeded eelgrass recovery, and clams did not reduce winter density of eelgrass. During summer, clams reduced eelgrass density by 30% but increased shoot length by 13% and clonal branching by 9%. Eelgrass growth rates (~1 mgDW shoot-1 d-1) and C:N ratios (~11) did not differ across clam-addition treatments, indicating no enhancement by fertilization. At the end of the 2-year experiment, clams were harvested, and this disturbance reduced eelgrass density by 70% in the geoduck plots. As managers consider expansion of intertidal geoduck aquaculture regionally, these data indicate only a minor effect on eelgrass from the presence of clams themselves, but harvest disturbance is comparable with other studies of fishing in eelgrass. Nets and tubes that serve as predator-exclusion devices during the early stages of the 5-year crop cycle for geoduck clams remain to be examined for their ecological effects. Copyright © 2012 John Wiley & Sons, Ltd.