falls—islands of abundance and diversity in the deep sea
In the deep bottom waters of Monterey Canyon, many food webs are sustained
by a slow drizzle of organic particles and detritus known as marine snow.
But every now and then this rain of debris includes a really big food “particle”
in the form of a dead whale. One whale fall can deliver as much organic material
as several thousand years worth of marine snow.
In February of 2002, MBARI researchers Robert Vrijenhoek and Shana
Goffredi discovered a recent whale fall while exploring the outer portion
of the Monterey Canyon with MBARI’s remotely operated vehicle, Tiburon.
They returned to the site in October 2002 with Craig Smith, a University
of Hawaii professor who has studied whale falls for nearly twenty years.
Photomontage of the whale fall in Monterey Canyon
obtained during a February 2002 dive using the ROV Tiburon. The red "fuzz"
is thousands of deep-sea worms growing on the whale bones.
Based on repeated observations of whale falls off of Southern
California, Smith has seen a consistent pattern in the development of
biological communities around whale falls. When a large whale dies, its
body often sinks directly to the sea bottom, especially if the animal is
undernourished. Within days, active scavengers, such as sleeper sharks,
rattails, hagfish, and amphipods, converge on the new food source and
voraciously remove the flesh from the bones (Smith has estimated
consumption rates of 40 to 60 kilograms of flesh per day). In many cases
the whale is stripped to the bone in a matter of months.
Within a year after the whale fall, the whalebones and nearby
organically enriched sediment typically become infested with huge
populations of polychaete worms and unusual crustaceans, as well as
molluscs and other invertebrates. Worms often carpet the seabed at
densities of up to 45,000 animals per square meter—higher densities than
in any other deep-sea environment. Animals in this “enrichment-opportunist”
community feed directly on organic material in the whalebones and
surrounding sediment. Many of these animals appear to be unique to
deep-sea whale falls, and many are new to science. Although the animals
appear in extremely large numbers, only a few different species are
present, a situation similar to that observed near other concentrated
sources of organic material in the marine environment, such as sewer
outfalls and salmon pens.
About a year or two after the whale fall, most of the easily digestible
organic material has been consumed. However, sulfur-reducing bacteria
continue to feed on fats and oils deep within the whale bones. This
process gradually releases hydrogen sulfide, which provides the basis for a
third-stage, “sulphophilic” (sulfur-loving) whale-fall community.
This sulphophilic community
is remarkable for a number of reasons. Instead of being based on
photosynthetic organisms, such as phytoplankton, it consists of a
self-contained food web based almost entirely on energy from
chemosynthetic bacteria. This food web provides many different ecological niches.
In addition to the chemosynthetic bacteria themselves, there are animals
that graze on the chemosynthetic bacteria and organisms that live off of
chemosynthetic bacteria growing within their own bodies, as well as the
more typical scavengers and predators.
Third-stage whale-fall communities are often amazingly diverse. Up to
190 different species of macroscopic bottom-dwelling animals have
been found on a single whale skeleton. Many of these species are
specifically adapted to utilize whale-falls as a food source and
substrate. These communities can also be amazingly persistent—at least
one large whale-fall community has apparently lasted more than 50 years.
In some ways, late-stage whale-fall communities resemble communities at
deep-sea hydrothermal vents and cold seeps, where chemosynthetic bacteria
also form the basis for unusual, self-contained food webs. About ten to
twenty percent of the roughly 200 sulphophilic species found at whale
falls are also found at underwater hydrothermal vents and cold seeps,
respectively. However, the majority of species found in each type of
environment are unique. Since many of these organisms are difficult to
identify based on their appearance, researchers at MBARI are using genetic
and molecular tools to understand the evolutionary patterns among
whale-fall communities, and to determine relationships between animals at
whale falls, seeps and hydrothermal vents.
Crabs and an octopus living on the skull of the dead whale
in October 2002. The spherical weight and yellow line anchor a homing
beacon that that helps researchers find the whale fall on subsequent dives.
Thirty-million-year-old assemblages of fossil clams and whale bones
suggest that whale-fall communities have been around at least as long as
whales themselves. This persistence is particularly impressive because
each whale-fall community is based on a transitory food source. Sooner or
later, planktonic larvae of invertebrates at one whale fall must somehow
find and colonize a new whale in order to survive. But whale falls might
be relatively common. Smith estimates that, based on current whale
populations and whale-fall community persistence, dead whales may occur
roughly every 5 to 16 km along the seafloor off the Pacific coast of North
America. Distances such as these are easily traversed by planktonic
The recent whale fall in Monterey Canyon is particularly interesting
because it lies in deep water (2,891 meters). At this depth mobile scavengers
are probably less active, but sulphophilic organisms may appear sooner.
Smith and the MBARI scientists are currently analyzing the results of
their latest visit, to learn more about the persistence and distribution
of these unique benthic communities.