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.