Cold Seep Communities
III. Cold Seeps in Monterey Bay

A. Overview of current research

In the Monterey Bay region, cold seep communities were first discovered in the axial valley of the Monterey Submarine Canyon during dives in the research submersible ALVIN (Embley et al. 1990). The presence of seep communities was suspected in Monterey Bay from samples of vesicomyid clams (bivalves associated with seeps) dredged from the bay during research cruises (J. Nybakken, pers. obs.). Seeps have recently been discovered at three more sites under distinctly differing geologic settings (Figure 1) using the remotely operated vehicle (ROV) Ventana, operated by the Monterey Bay Aquarium Research Institute (MBARI). These sites are presently being investigated by scientists from MBARI, Monterey Bay Aquarium, U.S. Geological Survey, Hopkins Marine Station, and Moss Landing Marine Laboratories. Research topics include the distribution and extent of cold seeps in Monterey Bay, geologic settings under which they exist, and characteristics of biological communities found at seeps.

B. Chemistry

Studies of the pore water chemistry of seeps indicate that considerable variation exists in the concentrations of sulfide, methane, and other chemical constituents at seeps in the Bay (Barry et al. in press), and that the mechanisms regulating fluid flow are also vary among sites (Barry et al. in review). Fluid seepage at one seep region (Mt. Crushmore) is suspected to result from artesian flow of rainwater which enters a sandstone aquifer (Purisima Formation) in the Santa Cruz mountains and is released along the walls of Monterey Canyon after percolating downslope 1000 m or more. Analysis of pore fluids from Mt. Crushmore seeps show these fluids to be low in sulfide (~100 µM) and lacking methane. In contrast, fluids at the Clam Flat seep, where fluid flow is thought to derive from tectonic-based sediment compression, had nearly 100 times more sulfide (11,000 µM) and very high methane concentrations (300 µM). Pore fluids at a third site (Clam Field) had high sulfide (6,000 µM), and low methane (10 µM).

C. Biology

Biological surveys of these sites show that seep communities are composed of various faunal groups, including 1) obligate species [bacterial mats, bivalve taxa (Vesicomyidae, Solemyidae, Thyasaridae), and vestimentiferan worms], 2) potentially obligate species (columbellid gastropods, pyropeltid limpets, and an unknown galatheid crab), and 3) non-obligate species that utilize seep-derived production (anemones, brachyuran and galatheid crabs, gastropods, and soft corals), but are cosmopolitan in distribution (Barry et al. in review). Obligate species base all or most of their nutrition on chemosynthetic production by endosymbiotic bacteria. In vesicomyid clams, these endosymbionts are thiotrophic bacteria held in gill tissues.

Finer scale studies of the distribution of vesicomyid clams show that the relative abundances of particular species, as well as the local spatial distribution of species is related closely to the fluid chemistry of individual seeps (Barry et al. in press). At least 2 new species of vesicomyid clams occur at seeps in Monterey Bay, including a species (Calyptogena packardana) recently described by Barry et al. (in review).

Investigations of the species composition of cold seeps in the bay show that 5 to 6 species of vesicomyid bivalves dominate the fauna at seeps (Barry et al. in review). Moreover, the relative abundance and distribution of these species varies considerably among seeps, in relation to the fluid chemistry of seeping fluids. Cold seep sites with high sulfide levels (Clam Flat, Clam Field) are dominated by Calyptogena kilmeri, while low sulfide seeps are dominated by C. pacifica (Barry et al. in press b). In addition, the distribution of clam species within individual seeps varies according to localized gradients in sulfide concentration, similar to the zonation of flora and fauna along a desiccation gradient in the rocky intertidal zone.

Zonation of vesicomyid species along a sulfide gradient at cold seeps is very likely related to species-specific patterns of sulfide physiology. Studies of the sulfide-binding ability of the hemoglobin-containing blood of vesicomyid clams indicate that sulfide binding in Calyptogena pacifica is 10 times greater than that for C. kilmeri (Kochevar and Barry 1993, 1994). Consequently, C. pacifica is apparently able to maintain sulfide levels in its gill tissues at suitably high levels for endosymbiotic bacteria, at environmental concentrations 10 times lower than that required by C. kilmeri. Thus, C. pacifica may occupy the peripheral zones of seeps which are likely intolerable to C. kilmeri.

Some research on the reproductive patterns of vesicomyid clams from cold seeps has been completed, though these data are somewhat inconclusive. Studies of gonadal development through the year indicate that reproduction may be seasonal, with a peak in reproductive output during winter for Calyptogena kilmeri (Lisin et al. in press).

Little is understood of the microbiology of cold seep habitats. Sulfur-oxidizing bacteria, especially the genus Beggiatoa, dominate the free-living bacteria of seeps in Monterey Bay. We have recently found that the central vacuole of Beggiatoa can have extremely high concentrations of nitrate (McHatton et al. in press), which may be related to the gliding motility of these organisms.

D. Ecology and future work

Aside from the work decribed above, aspects of the ecology and energetics of these communities are poorly understood. For example, the extent to which chemosynthetic production at these underwater oases fuels secondary productivity by the local non-seep biological assemblage is unknown. Our observations indicate that seep communities may act as seafloor oases in a relatively energy-poor seafloor landscape. A variety of species of cosmopolitan benthic fauna appear to benefit from foraging at cold seeps.

Little or no information is available concerning ecological processes that influence demographic rates of biological populations at cold seeps. Predation, competition, and disturbance likely play a major role, but few hypotheses regarding these population processes have been addressed. Thus, there is a constellation of questions concerning the geologic and biologic structure and function of cold seeps. The presence of four geologically distinct seep sites in Monterey Bay provides a unique opportunity for these investigations.