Whale falls, wood, or kelp: A bonanza
for life in the deep sea
This story entered on 14th Jan, 2003 09:00:00 AM PST
Most of the deep sea has a very slow supply of nutrients
raining down from the surface waters. Whale carcasses that fall
to the seafloor provide a sudden, concentrated food source, a bonanza
for organisms in the deep sea. Different stages in the decomposition
of the whale carcass support a succession of marine biological communities.
Scavengers consume the soft tissue in a matter of months. Organic
detritus from this stage also enriches the sediments nearby, for
a year or so. The skeleton, however, supports rich communities for
years to decades, both as a hard substrate for invertebrate colonization
and as a source of sulfides from anaerobic decay of bone lipids.
Microbes live off of the energy released from catalyzing chemical
reactions, particularly oxidation of sulfide, and form the basis
of chemoautotrophic ecosystems for as long as the food source lasts.
Wood and kelp falls appear to play a similar role, as
islands of concentrated organic nutrients on the seafloor. In each
case, locally intense decomposition of organic matter creates reducing
and high-sulfide conditions.
The West Coast & Polar Regions Undersea Research
Center (NURP) is supporting research by Craig Smith (University
of Hawaii) and Amy Baco-Taylor (Woods Hole Oceanographic Institution)
to track the progression of biological communities supported by
decomposing whale falls and to compare them with wood and kelp falls.
During October 2002 these researchers used the ROV Tiburon, operated
by the Monterey Bay Aquarium Research Institute, to investigate
3 whale falls, 4 wood parcels, and 6 kelp parcels off the coast
of southern California.
Because one of the purposes of the study is to determine
the pattern and duration of colonization on these falls, the experiment
includes a range of ages for each type of fall material. One of
the whale carcasses (a 35 ton grey whale) was towed into place by
these researchers in 1998. They have dated the bones of the other
two (natural) whale falls at 50 yrs and 80 yrs. The ROV dives showed
that colonization of enriched sediment around the 1998 whale fall,
including newly discovered species, is still active and is expected
to last for several more years. This stage ended long ago at both
of the older skeletons, but they still support sulfophilic communites
and have changed little since they were discovered 15 years ago.
This suggests that whale falls can act as a food source in the deep
sea for a century or so, which is much longer than expected. Among
other things, this raises questions about the impact of 18th and
19th century whaling on the biology of the deep sea.
Wood parcels were implanted on the seafloor in two groups,
3 years prior and 6 months prior to the ROV dives. These were cubes
of untreated douglas fir (appx. 200 kg total) held in a nylon net
bag and weighed down with steel ballast. The ROV dives showed abundant
biological activity on the wood and in surrounding sediment demonstrating
that ecological succession at wood falls occurs over years to decades.
Kelp falls were deployed 6 months and 3 months prior to the dives.
The appx. 100 kg kelp parcels consisted of 4-5 kelp plants with
their holdfasts, also in nylon net bags with steel ballast. In contrast
to the whale and wood falls, the kelp were nearly gone after 6 months,
by which time the main inhabitants may have been suspension feeders
using the holdfasts as an anchor point.
One of the questions raised with initial discoveries
of whale falls was whether they might serve as reservoirs for biological
colonization of the more geographically isolated hydrothermal vents
and cold seeps, another type of isolated low oxygen, high sulfide
environment. Despite the common environmental characteristics of
isolation, low oxygen, and high sulfide, Smith's research group
found relatively little overlap between the biological communities
at whale falls, wood falls, and kelp falls. Furthermore, they saw
only modest species overlap between these communities and those
at cold-seeps. This suggests that there may be specialized communities
living on each type of organic enrichment with modest exchange between
them. Nonetheless, DNA-based studies on bathymodiolin mussels suggest,
for example, that a small subset of the whale-kelp-wood-fall biota
may colonize vents and seeps, and may contribute fundamentally to
the vent-seep biota over evolutionary time.
Name: Jennifer Reynolds
Tel: (907) 474-5871