by Eric Karsenti, co-director of Tara Oceans and chief scientist currently on board
We call ‘zooplankton’ certain organisms present in the ocean that drift with the currents. They include larva, tiny crustaceans, copepodes, etc. and feed by devouring other zooplankton, or by eating unicellular photosynthetic organisms (micro-algae).
Zooplankton can not perform photosynthesis. During the day, most of these organisms are dispersed in the water column at depths varying from 60 meters to several hundred meters. They avoid the zone where sunlight reaches in order to hide from predators. At nightfall, they come to the surface and occupy a zone called the “Deep Chlorophyll Maximum? (DCM). This zone contains a huge quantity of photosynthetic microorganisms, including unicellular species such as diatomes. The DCM is generally found between the ocean’s surface and a depth of 60 meters.
We can locate this layer of photosynthetic life by using detection devices that measure the presence of photosynthetic pigments. Aboard Tara, these devices are attached to the “Rosette?, the apparatus used for taking samples of sea water at different depths (Figure 1).
Also attached to the Rosette is a system for taking images – the UVP or Underwater Vision Profiler. This device photographs organisms of different sizes, including zooplankton present in the water column (Figure 1).
During our sampling station in the Malouine Current, detection devices revealed that the DCM was located at a depth of about 60 meters, just above an abrupt change in the temperature of the seawater (Figure 1). At the surface in the Malouine Current, the temperature of the water is 7°C. At 80 meters, the temperature drops suddenly to 3°. This change, found at varying depths in different oceans, is called the “thermocline?. In our case, the DCM is located just above this temperature change. Aboard Tara we can also detect the DCM and zooplankton with a very sophisticated ultrasound device – the same kind used by fishermen to locate schools of fish.
During the daytime, the ultrasound screen often shows a clear line at the level of the DCM, and a more diffuse signal below. At nightfall, around 21:00 a second solid line appears at the thermocline level (80 meters) just below the DCM (Figure 2, left). Between 21:00 and 23:00 this line gradually decreases until it disappears completely, blending with the line corresponding to the DCM, which appears much thicker (Figure 2, right).We know that zooplankton moves upwards towards the surface at precisely this time. From these images, it seems that zooplankton is first concentrated at the thermocline level, before gradually invading the DCM zone where it can feed on photosynthetic micro-algae.
During our sampling stations, we always throw the nets to capture zooplankton at night, between 21:00 and midnight, precisely because of this vertical migration when the zooplankton becomes concentrated.
It seems that these organisms propell themselves, swimming towards the surface, which represents a great distance considering their size (several hundred meters to travel for organisms smaller than one centimeter). Also fascinating is their collective behavior. A great number of different species take part in this migration, yet they all move together, at the same time. How does this happen? So many fascinating questions to answer.
Figure 1: See photo below
Figure 2: Ultrasound images taken aboard TARA.The surface is represented by the red line at the very top.The DCM at 60 meters is represented by the red & yellow zone. The denser the substance detected, the thicker & redder is the line. The second line at 80 meters seems to correspond to the zooplankton which accumulates in the evening, starting around 21:00. Before that time, this line is not visible. Afterwards, the line becomes concentrated and blends with the DCM. Depth is indicated in meters.