Levin Seeps

Researchers from Scripps paid a return visit in April 2001 to cold seeps on the continental margin off Northern California's Eel River. (The project is sponsored by the West Coast & Polar Regions NURP office). Lisa Levin and collaborators are conducting an interdisciplinary biogeochemical study of the oasis communities centered around methane-rich seeps at 500m water depth. This site is at the shallow edge of known methane hydrate deposits. Scripps scientist Kevin Brown reports that the geological setting of the cold seeps along the crest of a plunging anticline suggests updip migration of fluids and gases from the subsurface hydrates.

The discovery of low-oxygen, sulfide-rich reduced environments in the deep sea, especially hydrothermal vents and cold seeps, has had a dramatic influence on perception of life on the seafloor. These chemically "hostile" environments nevertheless support unusually high concentrations of organisms, forming oases of life on the seafloor. Most studies of cold seeps have focused on the larger fauna such as clams and tubeworms, or the chemosynthetic bacteria that are unique to these environments. In a way, those are the "charismatic fauna" of the cold seeps. However, the biological community at seeps is much more extensive than this; it includes a myriad of small, sediment-dwelling invertebrates. An understanding of the ecosystem as a whole will not be possible without understanding the role of the smaller animals.

In October 2000, Levin's group used the ROV Jason to deploy a series of colonization experiments and fluid flux meters and to core sediments containing the smaller organisms such as foraminifera, annelids, crustaceans, bivalves and other vermiform taxa. In April 2001, the commercial ROV Ocean Explorer was deployed to collect these experiments and to further sample the natural community. Samples were collected from three different environments: seep areas covered by clambeds, by bacterial mats, and non-seep areas. Although animal densities were similar across environments, each supported a different suite of organisms. Levin's group found that the flux of methane and concentration of sulfides in the bacterial mat sites was much higher than in the clambed sites, providing a strong basis for understanding their different ecologies. A complex of dorvilleid polychaete species (segmented worms) exhibited clear preference for bacterial mat sediments having exceptionally high sulfide concentrations. Other worms (e.g., oligochaetes) were associated only with the clambed sediments. Studies of the carbon and nitrogen isotopes of these animals indicate that the fauna in these seep sediments rely on a range of nutritional sources including chemosynthetically derived organic matter, marine plankton and terrestrial flood deposits. Food-chain linkages among bacteria, clams, snails, and starfish appear to transfer seep-generated organic matter to the surrounding non-seep ecosystem. Comparison of the Eel River seep communities with those observed during previous cold seep surveys off the coasts of Alaska and Oregon reveal geographical diversity in the ecosystem's structure and function, including highly variable reliance on methane.