The wide geographical coverage of the Continuous Plankton Recorder Survey (CPR) in the Southern Ocean is ideal for mapping biodiversity of plankton through the region. But with the survey extending over much of the year, changes in plankton patterns caused by geographical differences can be confused with those due to time. This complicates one of the objectives of the survey in identifying seasonal and long term changes in zooplankton patterns in response to environmental change. Fixed routine transects surveyed regularly would solve this problem, but a demanding Australian Antarctic shipping schedule has made this impossible.
By contrast, the Japanese icebreaker Shirase in servicing the Japanese Antarctic program’s Syowa station has a fixed time schedule and route down longitude 110°E south from Fremantle each December. Shirase also follows the same return leg in February-March from Syowa, past Casey to 150°E, then north to Sydney. Japan's National Institute of Polar Research (NIPR) had established a long-term routine zooplankton monitoring program in 1972 with daily plankton net sampling across the Southern Ocean conducted at more or less the same time (noon) and location each year.
Some cyclic patterns linked with seawater temperature could be identified in the data, but it was clear that the small size of the net and the large distances between sites, 300 nautical miles (555km), did not provide the required resolution for long-term mapping and monitoring of changes in plankton patterns in relation to the various oceanographic boundaries in the Southern Ocean. However, NIPR agreed that CPR tows from Shirase would benefit both national programs, enhancing their existing plankton monitoring program, while providing much needed fixed transects. A schedule of collaboration was agreed between Dr Graham Hosie, AAD, and Prof. Mitsuo Fukuchi, Director of the Centre of Antarctic Environment Monitoring at NIPR. Australian CPR units have been operating on Shirase since the 1999–2000 season.
The collaboration between Australia and Japan also provided the opportunity to occasionally use other Japanese vessels conducting plankton research around Antarctica — Kaiyo Maru (National Research Institute of Far Seas Fisheries), Hakuho Maru (Ocean Research Institute, Tokyo University), Umitaka Maru (Tokyo University of Fisheries) and Tangaroa (on charter to NIPR from NIWA, New Zealand). These vessels have supplied a large amount of routine data – nearly a quarter of the data has been supplied by Japanese vessels.
Use of these ships has also allowed a number of unique experiments to be conducted. The first of these was a set of almost simultaneous tows across the Antarctic Circumpolar Current in November-December 1999, along three widely spaced transects south of Africa (Kaiyo Maru), Fremantle (Shirase) and Macquarie Island (Aurora Australis) to test for similarities in zooplankton patterns across the frontal zones of the Antarctic Circumpolar Current. In theory, because the ACC flows uninterrupted around Antarctica, the species composition of zooplankton should be the same within any part of the current. The three-ship survey found the hypothesis to be true.
The complementary experiment of looking at change within a season along a single transect, conducted in November and March of the 2001–02 summer, involved CPRs towed repeatedly by four ships, Aurora Australis, Hakuho Maru, Tangaroa and Shirase, along longitude 140°E. A number of plankton assemblages were identified with strong north-south zonation in association with the various oceanographic fronts. While these fronts varied in position through the season, the composition and integrity of the plankton assemblages remained consistent relative to the fronts, and strongly correlated with temperature.
The third experiment produced perhaps the most interesting results so far. Later in the 1999–2000 summer Kaiyo Maru closely followed Sir Alister Hardy’s April 1927 CPR transect across Drake Passage. We are still looking for Sir Alister’s original raw data in order to make a full comparison, but an initial comparison with his published descriptions of the 1927 data suggests a major change in plankton patterns has occurred between 1927 and 2000.
There is no doubt that the collaboration between Australia and Japan has greatly improved the CPR survey with the additional tows. The survey would benefit further with additional fixed transects. Germany is now assisting the survey with tows from their research vessel Polarstern south of Cape Town in March–April 2004, and it is hoped that CPR tows will then become routine on all future Polarstern voyages. Other nations have expressed interest in joining the survey.
Graham Hosie, Southern Ocean Continuous Plankton Recorder Survey, Biology Program, AAD