Material and methods
Six stations positioned in the northern (Sl, S2 and S3) and the southern
(S4, S5 and S6) part of the bay were sampled from July to September
1986 with a 3 to 5 day-periodicity.
Weather conditions were recorded (Météo France) as well as seawater
temperature and salinity. Zooplankton was sampled at subsurface (1 m)
bv means of a pump. Samples were then filtered on a 200 pm mesh sieve
and stored in 4% buffered fonnalin. At the laboratory, organisms were
identified to the species level and counts were done on triplicate samples.
A correspondence analvsis (Benzécri, 1984) was applied to the whole
data set of zooplankton in order to identify major communities in the
bay. A factorial map of the stations’ scores in the plane of the first two
axis of the correspondence analvsis was then drawn to study the evolution patterns of those communities. The species abundance was finally
mapped to specifv the contribution of each species in the structuring of
zooplankton communities.
Results
Tlie ftrst plane of the correspondence analysis describes 43.5% of the total
inertia. The “horseshoe” positioning of both samples (fig. 2A), and sites
(fig. 2B) also called Guttman effect, allowed the discrimination between
the northern and the southem part of the bav.
Axis I has a spatial feature since it is made up by the southernmost stations (S4, S5 and S6). Along this axis, stations S5 and S6 as well as the
species .■ kmtia bifilosa (A.BI) and . ■ Acartia tonsa (A.T( )N) show high negative scores.
Axis II illustrates both space and time features. Samples from the
northern stations (S1, S2 and S3) essentially form this axis. It marks
the opposition between northern stations sampled at the beginning of
the study with Acartia discaudata (A.DIS) and Acartia grani (A.GRA)
which have negative scores along this axis and, northern stations sampled at the end of the study period with Penilia avirostris (P.AVI), Paracalanus parvus (PARAC) and Oncaea venusta (QNC.V) which show
high positivc scores.
Two zooplankton communities were clearly identified in Arcachon bav
during summer; i.e. an inner community characterized by thc estuarine
species Acartia bifilosa and Acartia tonsa and, an autochthonous communitv in which . ■ Acartia discaudata and. -1 cartia grani were dominant.
It appears that before and after mid-August the whole zooplankton
structure (northern and southern communities) remained quite stable
for several weeks.
336
Six stations positioned in the northern (Sl, S2 and S3) and the southern
(S4, S5 and S6) part of the bay were sampled from July to September
1986 with a 3 to 5 day-periodicity.
Weather conditions were recorded (Météo France) as well as seawater
temperature and salinity. Zooplankton was sampled at subsurface (1 m)
bv means of a pump. Samples were then filtered on a 200 pm mesh sieve
and stored in 4% buffered fonnalin. At the laboratory, organisms were
identified to the species level and counts were done on triplicate samples.
A correspondence analvsis (Benzécri, 1984) was applied to the whole
data set of zooplankton in order to identify major communities in the
bay. A factorial map of the stations’ scores in the plane of the first two
axis of the correspondence analvsis was then drawn to study the evolution patterns of those communities. The species abundance was finally
mapped to specifv the contribution of each species in the structuring of
zooplankton communities.
Results
Tlie ftrst plane of the correspondence analysis describes 43.5% of the total
inertia. The “horseshoe” positioning of both samples (fig. 2A), and sites
(fig. 2B) also called Guttman effect, allowed the discrimination between
the northern and the southem part of the bav.
Axis I has a spatial feature since it is made up by the southernmost stations (S4, S5 and S6). Along this axis, stations S5 and S6 as well as the
species .■ kmtia bifilosa (A.BI) and . ■ Acartia tonsa (A.T( )N) show high negative scores.
Axis II illustrates both space and time features. Samples from the
northern stations (S1, S2 and S3) essentially form this axis. It marks
the opposition between northern stations sampled at the beginning of
the study with Acartia discaudata (A.DIS) and Acartia grani (A.GRA)
which have negative scores along this axis and, northern stations sampled at the end of the study period with Penilia avirostris (P.AVI), Paracalanus parvus (PARAC) and Oncaea venusta (QNC.V) which show
high positivc scores.
Two zooplankton communities were clearly identified in Arcachon bav
during summer; i.e. an inner community characterized by thc estuarine
species Acartia bifilosa and Acartia tonsa and, an autochthonous communitv in which . ■ Acartia discaudata and. -1 cartia grani were dominant.
It appears that before and after mid-August the whole zooplankton
structure (northern and southern communities) remained quite stable
for several weeks.
336