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.
Next - Section IV. Selected Cold Seep
Community Resources
Cold Seep Communities Table of Contents
