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.