Tools for science

© Y.Chavance/Tara Expéditions

For Tara’s current mission – studying the small fragments of plastic floating on the surface of the sea – the schooner is equipped with special instruments adapted to this particular type of research. Overview of Tara’s scientific armada.

As in previous Tara expeditions, scientific research is based on sampling stations. Here in the Mediterranean these usually take place daily: a 4 to 5-hour station in the morning, followed by a 2-hour station in the evening when the plankton rises to the surface. This daily program is modified according to weather conditions and geography. The sampling points are far from random: shallow, coastal areas, eddies, zones stirred by currents, etc. Our scientific team on land is constantly juggling with meteorological and oceanographic maps to find the most relevant sampling spots. Once the schooner has arrived in the right place at the right time, sampling can finally begin.

The Manta net combs the surface

The rosette (a set of Niskin bottles collecting seawater at different depths) was the star of previous expeditions. For Tara Mediterranean it has been replaced by a Manta net – the main tool used to collect samples. This small net is lowered into the surrounding waters on an average 5 or 6 times a day.  Simple in design, the Manta has proven extremely effective for collecting floating plastic and surface microorganisms. Since the beginning of the expedition, every single cast of the net brought up plastic particles. It has a metal structure consisting of a wide-open mouth and 2 wings that allow it to remain on the surface. It was named for the Manta ray that it resembles. Attached to the structure is a long funnel-shaped net with a mesh size of 335 microns (one third of a millimeter). The net feeds into a collecting cylinder that traps all the particles passing through the Manta’s mouth. 

Bongo nets for shallow depths

Nearly 100 meters of cable are unrolled for each launch, allowing the Manta net to comb the surface far from the boat. An ingenious fastening system places the net parallel to our route to avoid getting churned up in the wake. Weather permitting, the Manta towed by Tara can swallow up the first 20 centimeters of surface water for a half-hour or an hour, according to the protocols. If the sea is rough, plastic particles escape the jaws of the Manta. In this case the Bongo nets take over – 2 large funnel-shaped nets bearing weights to keep them a meter below the water’s surface. Whichever net is used, the scientific team aboard scrupulously notes the GPS location of the launch and ascent, as well as the speed of the boat, to calculate the amount of water filtered. Once on deck, the collectors are emptied and contents are analyzed to see the results of the catch.

The CTD: physico-chemical data

The Manta and Bongo nets are not the only tools used on the back deck of Tara. Even though we are interested in plastic particles, the planktonic organisms that inhabit the water and sometimes colonize the floating debris are still at the heart of our research. To harvest these tiny crustaceans, micro-algae bacteria, etc., a Niskin bottle is regularly dipped 3-feet deep, trapping billions of microorganisms in its 5-liter capacity. To provide the broadest possible view of the sampled zone, a CTD (Conductivity-Temperature-Depth) measurement apparatus provides essential information about the water column. Sent down on a 200-meter cable, it is equipped with sensors to measure (4 times per second, throughout the descent and ascent) the essential information: pressure, temperature, conductivity (revealing salinity), fluorescence (providing information on the distribution of phytoplankton), and backscatter (to evaluate the concentration of particles). These physico-chemical data allow us to better understand the environment of the plastic particles and associated microorganisms.

The HTSRB: from ocean to satellite

Each day when the sun is directly overhead, another instrument – the HTSRB – is lowered into the water from Tara’s rear deck. Nicknamed “Star Wars” because it resembles a spacecraft straight out of a science fiction film, the HTSRB points one sensor skyward to measure the intensity of light received, and another towards the depths to record the color of the ocean – all at 150 wavelengths, in the ultraviolet range. UV rays are known to degrade plastics, so this data is of great interest to scientists studying plastic pollution. But the use of the HTSRB on Tara goes much further: data collected on ocean color will eventually serve to refine the parameters used by satellites scanning the world’s oceans. Even better, the data is involved in developing algorithms used on the next generation of satellites. In 2020, a new satellite launched by NASA will be able to make much more comprehensive measurements of ocean color, thanks to the data collected aboard Tara in recent years.

The study of micro-plastics is the focus of the current expedition in the Mediterranean, but Tara is also a windfall for other scientific fields, eager to take advantage of this research vessel criss-crossing the Mediterranean Sea for 7 months. In addition to the sampling stations focused on microplastic and the planktonic communities colonizing the plastic, Tara’s scientific team conducts other research, as diverse as counting jellyfish, and recording songs of whales. All these fields of study ultimately share the same ambition: to better understand the marine world around us.

 

Yann Chavance

 

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