Development of the Southern Ocean Continuous Plankton Recorder survey

Abstract

The Continuous Plankton Recorder (CPR) Type I was first used in Antarctic waters during the 1925–1927 Discovery Expedition, and has been used successfully for 70 years to monitor plankton in the North Sea and North Atlantic Ocean. Sixty-five years later the CPR as a Type II version returned to Antarctic waters when the Australian Antarctic Division initiated a survey of the Southern Ocean on RSV Aurora Australis south of Australia and west to Mawson. The objectives are to study regional, seasonal, interannual and long-term variability in zooplankton abundance, species composition and community patterns, as well as the annual abundance and distribution of krill larvae. The survey covers a large area from 60°E to 160°E, and south from about 48°S to the Antarctic coast—an area of more than 14 million km². Tows are conducted throughout the shipping season, normally September to April, but occasionally as early as July (midwinter). The large areal and temporal scale means that it is difficult to separate temporal and geographical variation in the data. Hence, CPRs are now also towed on the Japanese icebreaker Shirase in collaboration with the Japanese Antarctic programme. Shirase has a fixed route and time schedule, travelling south on 110°E in early December and north on 150°E in mid-March each year, and will serve as an important temporal reference for measuring long-term interannual variability and to help interpret the Australian data. Since 1991, over 90 tows have been made, providing over 36,000 nautical miles of records. The most successful seasons to date have been the 1997/1998, 1999/2000 and 2000/2001 austral summers with 20, 31 and 26 tows, respectively. The 1999/2000 season included a unique, nearly simultaneous three-ship crossing of the Southern Ocean along 25° 30’E, 110°E and 157°E. Typical CPR tows show very high abundance of zooplankton in the uppermost 20 m of the permanently open ocean zone between the sea-ice zone and the Sub-Antarctic Front; this is an area thought to be oligotrophic. Appendicularians and small calanoid and cyclopoid copepods dominate the plankton. By comparison the surface waters of the sea-ice zone have low species diversity and abundances. Zooplankton data, and hence distribution patterns, can be time- and geo-coded to GPS data and environmental data collected by the ships’ underway monitoring system (e.g. fluorescence, water temperature, salinity, and meteorological data).

Introduction

Alister Hardy first conducted trials of his Type I Continuous Plankton Recorder (CPR) in Antarctic waters during the 1925–1927 voyages of the RRS Discovery and RRS William Scoresby (Hardy, 1936). Initial tows across the southern Atlantic Ocean were not always successful, but satisfactory samples were collected over 2300 nautical miles, including a successful series of tows across Drake Passage producing a continuous trace of Antarctic plankton abundance for nearly 300 nautical miles. There have been numerous surveys of Antarctic zooplankton over the years, some looking at taxonomy, life histories or distribution of individual species, and others studying large-scale distribution patterns of communities (e.g. Mackintosh, 1934, Hardy and Gunther, 1936, Baker, 1954, Foxton, 1956, Voronina, 1968, Voronina, 1972, Hopkins, 1971). These surveys have relied on traditional methods using zooplankton nets or midwater trawls, and have generally been sporadic in both space and time. More concerted sampling of zooplankton was conducted in the early 1980s during the international BIOMASS (Biological Investigations of Marine Antarctic System and Stocks) experiments in the region of the Antarctic Peninsula and Scotia Sea and the Prydz Bay region of eastern Antarctica (see Pakhomov, 1989, Smith and Schnack-Schiel, 1990, Hosie, 1994, Schnack-Schiel and Mujica, 1994). Australia continued surveying eastern Antarctic waters after BIOMASS finished and by 1996 had surveyed the area south of 60°S between 50 and 150°E (Hosie and Stolp, 1989, Hosie, 1994, Hosie and Cochran, 1994, Hosie, Cochran, Pauly, Beaumont, Wright and Kitchener, 1997, Hosie, Schultz, Kitchener, Cochran and Richards, 2000). All the previous surveys were useful in defining species composition, community structure and distribution patterns; these are ideal foundations for future monitoring, but they lack the spatial and temporal resolution for assessing long-term variation in the system. The CPR has been re-deployed in the waters around eastern Antarctica, six decades after its first trials, as a more effective rapid method for surveying and monitoring zooplankton patterns in the region, in order to assess the effects of environmental change. This paper describes the development of the Southern Ocean CPR survey and provides an overview of initial results. Several acronyms are used in this paper and have been listed for convenience in Table 1.