No. 15 - Survey Generalizations
We finished the survey a few days ago. We did some other
round-the-clock experiments but they only involved a few people,
which allowed everyone else to start getting back into their
normal biorhythms. We stopped at Cape Shirreff and picked up a
couple of seal biologists going home, steamed a full day and then
dropped four off at Seal Island to weigh fledglings and dismantle
another building. They'll stay there for ten days while the ship
returns to Punta Arenas for fuel and groceries, and where a few
people will leave and some new ones will join. I'll be among
those who leave but I'll be back in a month's time for the final
cruise leg of the season.
People are organizing and backing up their data, drawing maps,
forming impressions and furiously generating preliminary reports
so that they can enjoy the two days in port. It will be several
months before this job is complete, but some generalizations are
obvious:
The ever present ocean front, where coastal rich water from
Bransfield Strait drains the continental shelf and merges with
the bluer circumpolar water as flows past the Antarctic
Peninsula, is still there but further offshore than usual. The
front runs parallel to the long axis of the archipelago and the
islands act as a string of speed bumps right at the merger of the
two flows, generating eddies and counter currents and creating
places where phytoplankton can bloom and krill swarms can
accumulate.
But this is a salp year. Phytoplankton are producing at a high
rate, but apparently grazed down as fast as the algae grow.
There are few copepods (small crustaceans that usually number in
the 10's of thousands per catch) and only 1/3 of the plankton
species common in other years. Seabirds are fewer in number and
variety. Krill are in small tight swarms or intertwining thin
dense layers throughout survey area. As we have come to expect,
one- and two-year old krill are more common in the Bransfield
Strait and close to the islands and 3-6 year old krill are
further offshore. But we found no areas where swarms
accumulated, and there was little evidence of active
reproduction. And the overwhelming presence of a massive,
rapidly growing population of salps dominated our impression.
Although this doesn't bode well for the short term future of
krill, we'll have to see what next month's survey turns up.
Everything seems to be late this year (salp growth has yet to
peak, sexual maturation of krill is occurring but slowly, penguin
chicks at Seal Island appear to be as much as two weeks behind
schedule) and a second survey may cause us to modulate our
impressions. Without a doubt, however, this is an extreme year
and the second survey will tell us how extreme.
The conditions that prevail this year are not a surprise,
however. Each winter large areas of the open sea around
Antarctica freeze, but in the Peninsula area the extent and
duration of sea ice is more variable from year to year than
anywhere else. We have come to expect salp blooms and poor krill
reproduction after winters with relatively little sea ice -- and
this was such a winter.
But what about those icebergs? Everywhere -- many more than
we've ever seen here. From the vantage point of a skua's nest
high on the headland, one of the ornithologists counted 185 in
the small bay next to Cape Shirreff. We don't know what's going
on with the bergs. Could be a period of strong easterly winds
blew them in from the Larsen Ice Shelf in the Weddell Sea which
happens from time to time. Or it could be the ice shelves to the
southwest of us breaking apart in response to a long-term warming
trend in the region. Ironically, air and sea temperatures are
below norm this year.
-Roger