Importance of eelgrass early life history stages in response to oyster aquaculture disturbance

Authors: WISEHART, L - OREGON STATE UNIVERSITY; Dumbauld, Brett; RUESINK, J - UNIV OF WASHINGTON; HACKER, SALLY - OREGON STATE UNIVERSITY

Submitted to: Marine Ecology Progress Series
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2007
Publication Date: 8/23/2007
Citation: Wisehart, L.M., Dumbauld, B.R., Ruesink, J.L., Hacker, S.D. 2007. Importance of eelgrass early life history stages in response to oyster aquaculture disturbance. Marine Ecology Progress Series.344:71-80.

Interpretive Summary: Seagrasses are among the most productive plants on earth and provide critical structure and habitat for numerous estuarine and marine species. The effects that numerous human activities including aquaculture operations may have on seagrass populations are of increasing concern especially with seagrass declines in many parts of the world. On the U.S. West coast shellfish aquaculture has been an important component of the local economy for many years and co-occurs with protected eelgrass (Zostera marina) in many locations. This study examined seed production, germination, and seedling growth and survival of eelgrass under different oyster aquaculture practices including dredge harvest and off-bottom long-line culture. Germination of experimentally added seeds was highest in dredge harvested areas where adult shoot densities were lowest and also greater in plots where adult plants had been experimentally removed. Natural seedling recruitment and seed production by those adult plants present was also higher in dredge harvest areas than in long-lines. Thus dredge harvest techniques may enhance or at least maintain seed density and seed germination suggesting that eelgrass recovery after dredge harvest is rapid and that harvest dredge sites may serve as sources of eelgrass seed. Aquaculture operations form a mosaic of disturbance and habitat types within an estuary however and future studies should examine the site dependent and landscape scale effects of culture operations on eelgrass population dynamics.

Technical Abstract: Aquaculture is increasing worldwide, however we have little understanding of its impacts on marine communities. A critical element of many marine communities are seagrasses, a group of globally distributed marine angiosperms that are drivers of many abiotic and biotic processes in estuarine and marine communities. As with other seagrasses, the globally distributed eelgrass (Zostera marina L.) has been declining throughout its range. On the US west coast, shellfish aquaculture co-occurs with protected eelgrass. Many aquaculture practices constitute a pulse perturbation, and a key question concerns the ability of eelgrass to recover. We studied seed production, germination, and seedling growth and survival of eelgrass under different oyster aquaculture practices: dredging and off-bottom longline culture. To study germination, we added seeds to two different aquaculture types, as well as eelgrass reference areas, in paired control and eelgrass removal plots. Germination of experimentally added seeds was highest in dredged areas, where adult shoot densities were lowest. Seed germination was greater and seedlings were bigger in plots where adult plants had been removed. We also found natural seedling recruitment and seed production to be highest in dredged beds and lowest in longlines. We propose that greater recruitment in dredged beds is due to both enhanced seed densities as well as removal of neighboring adult plants. Low success in longlines may be due to a combination of physical factors including increases in sediment accretion and significantly lower redox values. Harvest dredging can enhance or at least maintain seed density and seed germination, but longline aquaculture appears to significantly reduce eelgrass recruitment.