Scale-dependent and indirect effects of filter feeders on eelgrass: Understanding complex ecological interactions to improve environmental impacts of aquaculture

Oysters and Eelgrass
Funding Sources: USDA Western Regional Aquaculture Center; Taylor Shellfish; Seattle Shellfish
Co-PI’s: Jennifer Ruesink (UW), Sally Hacker (WSU to OSU), Brett Dumbauld (USDA ARS),
Shahid Naeem (Columbia University), Washington Department of Fish and Wildlife
Graduate Students: Lorena Wisehart (WSU to OSU), Heather Tallis (UW), Kirsten Rowell (Arizona)
Image Purpose
We are examining ways that shellfish and aquaculture practices influence water clarity, sediment characteristics, and benthic habitat structure in Washington estuaries. As a rooted, photosynthetic plant, eelgrass responds to the nutrient and light conditions of its environment. Consequently, aquaculture may have indirect, large-scale impacts on eelgrass, in addition to the local disturbance that can occur during harvest operations. To develop best management practices, growers need scientific information on these larger-scale impacts, as well as direct small-scale interactions. This information will allow growers to select among culture techniques and distribute impacts in ways that sustain ecologically-important habitats. Applied on a broader estuarine scale, the study will assist managers in understanding the role of filter feeders and designating areas for aquaculture use.
Eelgrass Primer
One species of eelgrass, Zostera marina, is native to Washington state… at least, it is currently believed to be one species. However, plant morphology (and probably demography) varies dramatically across our study sites. In Willapa Bay, Z. marina occurs at relatively low densities (100 m-2), and plant size is large (up to 2 m). In southern Puget Sound, densities are much higher (800 m-2) and plant size smaller (30 cm). Z. marina occurs worldwide in temperate latitudes. Much of its growth is vegetative. New leaves are produced in the center of the plant, and leaves grow from their base, not at the tip. Leaves on the outside eventually stop growing, die, and fall off. The underground rhizome contains internodes showing where each leaf was attached. New shoots are sometimes produced inside a new leaf, and as the plant continues to grow, these new shoots become branches in the rhizome. In addition to this asexual reproduction (which makes identical genetic copies of the parent shoot), sexual reproduction also occurs through flowering. Flowers are produced on shoots that change their shape dramatically, becoming round-stemmed and elongate. Flowering shoots are terminal; that is, after flowering, a shoot dies. Male flowers release pollen in strings of mucus, and female flowers are pollinated when they encounter this pollen. Seed maturation requires several weeks, during which time the seeds grow, darken in color, and develop a hard seed coat. Most seeds are believed to fall near the parent plant, but they may be transported longer distance if the plant is dislodged before the seeds are ready to fall.
We use a hole-punching technique to measure the growth rate of eelgrass. This technique takes advantage of the fact that eelgrass leaves grow from the base, and outer leaves stop growing. We use a needle to make two holes just above the leaf sheath. After 3-5 days, we collect these plants and look for the holes. The outer leaves show where the mark was originally placed. On the inner leaves, the holes have moved up as the leaves grew. The “new biomass” is the amount in between the place where the shoot was originally marked and where the holes are now found. In Willapa Bay during the summer, fast-growing leaves extend by 3 cm (1 inch) each day.
Zostera japonica was introduced to Washington state in the 1950s and now occurs widely in north Puget Sound, Willapa Bay, coastal bays of Vancouver Island and mainland British Columbia, and many Oregon estuaries. Its eradication was attempted recently from Humboldt Bay, California. A parallel study to look at the morphology, production, and demography of this species is on-going in collaboration with Jae-Sang Hong, a visiting professor from Inchon University, Korea. Zostera japonica is native to Korea (and elsewhere in Asia) but is in serious decline due to habitat loss and pollution.
1. Lorena Wisehart is in charge of a project to measure seasonal changes in eelgrass in Willapa Bay, as a context for our studies of how shellfish may change conditions for eelgrass. About every two months, the following morphological and demographic attributes are measured at seven locations: Shoot length and width, number of leaves, internode lengths, branching, growth rate, flowering, and recruitment. Lorena is also looking experimentally at whether eelgrass recruitment is affected by oysters, given widespread reports of eelgrass appearing after shellfish are planted.
2. Heather Tallis has coordinated sampling of eelgrass biomass and growth rate across aquaculture beds in Willapa Bay. This study will provide information on the consequences of different aquaculture techniques (dredged ground culture, hand-picked ground culture, and longlines) and frequencies of harvest (some beds are harvested several times a year, whereas others have oysters for up to 3 years between harvests).
3. Undergraduate Nehemiah Richardson has counted thousands of bacteria to understand how microbes in the sediment respond to oysters. These microbes are essential in the nutrient cycling of estuaries, and along with macrobiota determine the conditions for eelgrass.

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Created by Lee McCoy, Updated by Jerome Tichenor, March 19, 2013