Impact of the introduced Japanese seaweed, Sargassum muticum, in native kelp forests of the San Juan Archipelago (yr. 1)

J. Timothy Wootton, University of Chicago
$29,219

The introduction of species into habitats outside their normal geographic range is accellerating as human mobility increases. Introduced species have the potential to radically alter the structure and function of native ecosystems and are a leading threat to biodiversity. The Japanese alga Sargassum muticum has been invading kelp forests in the San Juan Islands of Washington since its introduction nearly fifty years ago (Scagel 1956). Despite its widespread distribution as an exotic in North America and Europe very little is known about how Sargassum impacts native communities. Subtidal kelp communities in the San Juan Islands are important habitat for a wide variety of animals including invertebrates, fish, mammals and birds. Furthermore, the kelp species that dominate these plant assemblages add three-dimensional structure to the habitat and are important sources of carbon in coastal food webs. Therefore, any widespread effects of Sargassum on these algal communities are likely to have broader consequences for the nearshore ecosystem in this region. We are using an experimental approach to explore the dynamics and consequences of the Sargassum invasion in the San Juan Islands.

 In 2001, we will focus on experimentally uncovering the effects of Sargassum on the native communities of the San Juan Islands. We will carry out surveys of subtidal transects at several sites to identify patterns of association between native species and Sargassum over a range of Sargassum densities. These surveys will indentify interactions that may be explored more completely in subsequent experiments. We will also implement an experiment evaluating directly whether Sargassum is responsible for differences seen in the surveys. At two sites with established Sargassum populations, we will experimentally remove Sargassum from randomly selected plots, and examine the responses of native algal and animal species relative to unmanipulated control plots. We will also compare key physical characteristics of the environment in experimental and control plots to identify potential mechanisms underlying any effects on the native biota that we uncover. These factors will include light levels, nutrient concentrations and water motion. Finally, we will experimentally test the importance of two factors that may be influencing the spread of Sargassum. There are two likely reasons why an organism is not found in areas where the abiotic environment is suitable: either it cannot disperse to that locale (dispersal limitation) or interactions with existing species prevent it from becoming established once it arrives. A field experiment will allow us to assess the role of these two factors in natural communities. Our results will provide insight into the impacts of this ongoing invasion on the native plant and animal biota of the San Juan Islands and the mechanisms by which these impacts arise. By increasing our understanding of the dynamics and consequences of this invasion, we may also gain insight into possible mitigation strategies. This study will provide critical information regarding how important a threat this species is to the integrity of the coastal ecosystem in this region.