Mesoscale distribution of zooplankton : relationships with hydrological structures and chlorophyll distribution in the northeast Atlantic ocean. Use of an Optical Plankton Counter.
Labat J. Ph., P. Mayzaud, S. Gasparini, P.-E. Lapernat, S. Sabini, L. Mousseau and M. Boutoute
Laboratoire d'Océanographie de Villefranche sur mer (LOV), UPMC-INSU-CNRS, BP. 28, 06234 Villefranche sur mer, France.
Distribution of the zooplankton populations at mesoscale has been studied using a continuous recording system: the optical plankton counter (OPC) fitted on a Seasoar vehicle. This work was part of a description of a North Atlantic system characterized by subduction phenomenon and mesoscale eddies (French JGOFS program POMME). At two periods (April and September), OPC transects were surveyed continuously over depths ranging between 10 and 250 m in an area located between 38-45° N and 16 –21° W.
Transects were organized to cross different hydrological structures, i.e. anticyclonic and cylonic gyres. In April, a 3D type grid was used to obtain detailed spatial structure within and on the edge of the gyres. In September, transects went across the different gyres in a 2D type description. Chlorophyll distribution was monitored using a fluorimeter sensor fitted on the same Seasoar. In April, phytoplankton was located within the top 50 m layer with relatively high values (3 times the values found in October) mainly within and on the edge of the gyres. In October, chlorophyll was observed in a subsurface maximum around 50m within the strong thermocline both within and on the edge of the gyres.
Distribution of zooplankton biovolumes by size was recorded between 250µm and 1 cm of equivalent spherical diameter. Calibration tows were made with 200 µm mesh WP II nets to identify the main taxa. Size distributions of biovolume appeared to be similar in spring and summer, but the mean abundance was higher in spring (80 mm3.m-3) than in summer (50 mm3.m-3). Copepods were the dominant taxa. The OPC appears to give an interesting answer on the distribution of the zooplankton at the mesoscale (10- 50 nautical miles): biovolume 2D and 3 D maps showed a maximum of biovolume in the most dynamic parts of the frontal structures: the external parts of the eddies. The snapshots presented by the view of the OPC, showed different patterns: high zooplankton biovolumes linked some time with high fluorescence signal or some time with low fluorescence signal, could be interpreted as different ages of a dynamic system.
Continuous description using oscillating vehicles provided a dynamic of the zooplankton population structure, which seemed related to both hydrological features, availability of food supply and age of the system.