Plankton survey uses old technology to monitor the future

In the mid 1920s, British scientist Sir Alister Hardy developed a radically new method for sampling plankton continuously, rather than taking spot samples using conventional plankton nets. Net sampling can often be inaccurate because of the unique patchy distribution and behaviour of plankton. Sir Alister designed the continuous plankton recorder to help study and to map these patches. He conducted the first trials of his 'Type I CPR' in Antarctic waters during the 1925-1927 voyages of RRS Discovery and RRS William Scoresby. Initial tows across the southern Atlantic Ocean brought mixed results, but a series of tows across Drake Passage, over nearly 300 nautical miles, produced the first continuous trace of Antarctic plankton patterns.

Hardy later designed the more compact Type II CPR and established the North Sea and North Atlantic CPR survey, which is now the longest running marine biological survey, providing detailed synoptic plankton data for more than 70 years. Over that time the CPR has proven to be the most cost-effective method of repeatedly surveying large ocean areas quickly. The survey has been successful in detecting gradual changes in zooplankton and phytoplankton composition and distribution patterns, and the introduction and spread of non-indigenous species in the North Sea. In the mid-1980s the survey detected a significant regime shift in the plankton that may lead to catastrophic effects on the region's ecosystems and subsequently its fisheries. This shift happened in just one or two years rather than as a linear response to climatic or environmental change over several years.

Sixty-five years after the CPR was first trialled, the instrument returned to Antarctic waters when the Australian Antarctic Division initiated a CPR survey of the Southern Ocean. The purpose was first to map the patterns of biodiversity of plankton through the region, and then to use the sensitivity of plankton to environmental change as early warning indicators of the health of the Southern Ocean. That survey will also serve as reference on the general status of the Southern Ocean for comparison with other monitoring programs. Understanding patterns of variation in biological systems, both natural and those caused by climate change, has been an integral part of Australia's Antarctic research strategic plan. Plankton form the foundation of the Antarctic marine ecosystem and are thus the logical place to start such research.

CPR units were obtained from the Sir Alister Hardy Foundation, and trialled on RSV Aurora Australis during the ship's maiden voyage to Heard Island in July and August 1990. Trials continued in the sea ice zone during subsequent summer seasons. The units performed well enough but were easily damaged by ice, and during deployment or retrieval. In 1995, the Australian Antarctic Division instrument workshop and drafting team used a computer to designed a new 'Type II Mark V' CPR. The device was constructed from marine grade stainless steel rather than phosphor bronze as used previously. It was more streamlined then previous models, and internal recording cassettes were redesigned for easier loading and unloading of plankton mesh and preservative. The precision provided by computer control of machining meant that all new cassettes were interchangeable between external CPR bodies and had fully-interchangeable parts. But otherwise, the Mk V CPR differs little in overall design and performance from Hardy's original Type II.

The CPR looks like a Heath Robinson device but actually functions simply. It is a self-contained automatic sampler towed behind the ship at normal ship speed and can operate in nearly all sea conditions. As the CPR is towed along, water and zooplankton enter a small 1.25 x 1.25 cm aperture in the nose cone, which then expands into a wider collecting tunnel, slowing down the water flow. The plankton are then trapped between two bands of 270 µm mesh silk (6m long x 15 cm wide), loaded in a removable cassette. The silk and plankton 'sandwich' is wound on to a take-up spool inside a formalin preserving chamber, all driven by passing water turning an external propeller. Regardless of the speed of the vessel, the sheets of silk are advanced at a fixed rate of 1cm per nautical miles travelled. Each tow represents a 450 nautical mile track of continuous sampling. Back in the laboratory, each set of silks is unrolled and cut into sections representing five nautical mile samples. Plankton are then identified by microscope and counted.

The northern hemisphere survey has always relied on ships of opportunity such as cargo vessels and ferries, but the SO-CPR Survey uses research vessels which gives us access to a suite of oceanographic, meteorological and navigational data recorded continuously onboard. These data can be spliced with the CPR data, giving for each five-nautical-mile plankton sample the position and time of sampling, plus averaged environmental data such as water temperature, salinity, fluorometry (indicating chlorophyll concentration), light levels, and ultra-violet light levels.

The principal survey area extends from 60°E to 160°E and south of about 46°S to the Antarctic coast – an area of more 14 million square kilometres or just under 30 percent of the Southern Ocean. CPRs are towed on all voyages of Aurora Australis when travelling to and from Antarctic stations and on dedicated marine science voyages, from early spring to autumn and occasionally in winter. Sampling has been done in all months except June. This sampling forms the core of the data, both geographically and temporally.

Since 1999, CPRs have also been towed from the Japanese icebreaker Shirase during its annual resupply of Syowa station. This takes advantage of the ship's fixed route and time schedule as a temporal reference for measuring long-term annual variability and to help interpret the Australian data. Tows have also been conducted on other Japanese research vessels opportunistically which has allowed hypotheses on spatially and seasonally variation to be tested (see 'Australia and Japan: two decades of collaboration in Antarctic marine science', p. 15).

Sampling was less intensive prior to 1997, during the development of the survey, and the redesigning, trialling and commissioning of the AAD machines. However, to date, the survey has already completed over 200 tows, providing more than 60,000 nautical miles (110,000 km) of records equivalent to 12,000 samples for over 200 zooplankton species, and all coupled with environmental data. This number of samples would be impossible to collect using conventional net sampling.

Typical CPR tows show very high abundance of zooplankton in the surface waters (top 20 m) of the permanent open ocean zone between the sea-ice zone and the Subantarctic Front, located not far south of Australia, Africa and South America. This is an area previously thought to have low plankton abundance, because it is low in nutrients (oligotrophic). By comparison, the surface waters of the sea ice zone have considerably lower species diversity and abundance. The demarcation between these areas of high and low abundances is consistently observed and quite abrupt, just south of 60°S latitude. This indicates the existence of a regular and perhaps new oceanographic event, as it cannot as yet be easily related to other known oceanographic features in the area.

Other north-south differences in abundance and species composition in the CPR tows are also regularly observed in relation to other frontal zones, such as the Polar Fronts and Subantarctic Fronts further north. Even subtle variation in zooplankton patterns have shown greater sensitivity in identifying oceanographic features that would normally only be readily identified through detailed profiling with oceanographic equipment.

The survey so far has identified new community and species distribution patterns, and we now have sufficient data to start commenting on patterns of biodiversity in relation to the various oceanographic zones in the region, as well as seasonal and inter-annual patterns. We cannot yet identify longer term patterns or trends, but the northern CPR survey has shown that changes in an ecosystem can occur very abruptly when changes in environmental conditions reach a point that causes a species or group of species to suddenly flourish or decline, altering the ecosystem balance. Both northern and southern surveys have also proven the value of plankton as sensitive indicators of environmental patterns and the CPR as a useful tool for mapping the consequences of change in the marine environment.

Graham Hosie, Southern Ocean Continuous Plankton Recorder Survey, Biology Program, AAD