Distribution of Copepoda in the Indian Ocean
333
quite large; it is, in the main, centrally located but it abuts over the greater part of the
west coast of India. In contrast to this, the northern winter map (Fig. 3) shows the lowdensity area (3 000 to 8 999) to be smaller in extent and located more to the south and
to the west.
The lowest population density (1 to 2 999) extends over greater areas in the southern
Indian Ocean as compared to the northern. It is more noticeable over the central oceanic
areas, not extending to the western borders close to Africa or the eastern borders close
to Australia.
Copepods from day and night samples show a remarkable difference in their
abundance. In the following description of day night distribution patterns we refer to
Figs. 4 and 5 of the HOE Plankton Atlas (IOBC, 1970a). In Fig. 4 of the Atlas, depicting
day stations, the lowest density of 1 to 2 999 is fairly extensive over the southern and
central parts of the ocean but in Fig. 5 (HOE ,Atlas), depicting night station, this density
is reduced to a few small patches; there is a compensating increase in the area covered by
the next higher density of 3 000 to 8 999. In comparison to Fig. 4, Fig. 5 (night stations)
shows that the area covered by the highest density (27000 and above), is nearly doubled
and the area covered by the density, 9 000 to 26 999, is also distinctly greater. From this,
the conclusion is drawn that the night samples of the HOE, by and large, contain more
copepods than the day samples.
The copepods form the dominant element of the zooplankton. By Copepoda, in this
context, is meant the Calanoida, which are extremely numerous; the Harpacticoida and
the Cyclopoida also occur in the HOE plankton but they are quite subordinate to the
Calanoida in number and bulk.
The lowest population density (1 to 2999) extends over great areas of the southern
Indian Ocean (Figs. 1 to 3) and corresponds to the centre of the anticlockwise gyre with
the predominance of conditions of convergence. It may be pointed out that in all the
5 figures in the Atlas, a gradual increase in numerical abundance of copepods is evident
as we proceed northward from lat. 40 ° S. This has also been remarked upon in relation
to biomass by PRASAD (1969). Comparing Fig. 4 (day stations) with Fig. 5 (night stations)
of the HOE Plankton Atlas, we assume that the replacement of numerical density 1 to
2 999 by the next higher values of 3 000 to 8 999 in the night stations is related to
vertical migration in several species of copepods (CLARKE, 1933). The movement of the
copepods either upward or downward within the upper 200 m of the ocean would not
alter the population figures calculated for the standard hauls, but if the species that stay
below 200 m during the daytime migrate upwards during the night hours, there would
be an augmentation of the population density in the upper 200 m stratum. The genus
Pleuromamma is an example. It is represented by at least 6 species in the HOE collections.
It has been observed during subsorting of the Copepoda that individuals of Pleuromamma
species are abundant in the night samples only, whereas in the day samples they are very
sparse and include mostly juvenile stages.
The areas of greater density of copepod populations are the same as areas of rich
overall zooplankton production. The southern borders of the Indian Ocean are poor in
copepods but a gradual northward increase in population is observed, and the extensive
border areas of the northwestern Indian Ocean are rich in copepods. The fact that in some
areas night stations show higher numbers of ~opepods than day stations, is thought to
be due to circadian rhythms which cause the copepods to migrate to the upper 200 m of
the sea at night.
333
quite large; it is, in the main, centrally located but it abuts over the greater part of the
west coast of India. In contrast to this, the northern winter map (Fig. 3) shows the lowdensity area (3 000 to 8 999) to be smaller in extent and located more to the south and
to the west.
The lowest population density (1 to 2 999) extends over greater areas in the southern
Indian Ocean as compared to the northern. It is more noticeable over the central oceanic
areas, not extending to the western borders close to Africa or the eastern borders close
to Australia.
Copepods from day and night samples show a remarkable difference in their
abundance. In the following description of day night distribution patterns we refer to
Figs. 4 and 5 of the HOE Plankton Atlas (IOBC, 1970a). In Fig. 4 of the Atlas, depicting
day stations, the lowest density of 1 to 2 999 is fairly extensive over the southern and
central parts of the ocean but in Fig. 5 (HOE ,Atlas), depicting night station, this density
is reduced to a few small patches; there is a compensating increase in the area covered by
the next higher density of 3 000 to 8 999. In comparison to Fig. 4, Fig. 5 (night stations)
shows that the area covered by the highest density (27000 and above), is nearly doubled
and the area covered by the density, 9 000 to 26 999, is also distinctly greater. From this,
the conclusion is drawn that the night samples of the HOE, by and large, contain more
copepods than the day samples.
The copepods form the dominant element of the zooplankton. By Copepoda, in this
context, is meant the Calanoida, which are extremely numerous; the Harpacticoida and
the Cyclopoida also occur in the HOE plankton but they are quite subordinate to the
Calanoida in number and bulk.
The lowest population density (1 to 2999) extends over great areas of the southern
Indian Ocean (Figs. 1 to 3) and corresponds to the centre of the anticlockwise gyre with
the predominance of conditions of convergence. It may be pointed out that in all the
5 figures in the Atlas, a gradual increase in numerical abundance of copepods is evident
as we proceed northward from lat. 40 ° S. This has also been remarked upon in relation
to biomass by PRASAD (1969). Comparing Fig. 4 (day stations) with Fig. 5 (night stations)
of the HOE Plankton Atlas, we assume that the replacement of numerical density 1 to
2 999 by the next higher values of 3 000 to 8 999 in the night stations is related to
vertical migration in several species of copepods (CLARKE, 1933). The movement of the
copepods either upward or downward within the upper 200 m of the ocean would not
alter the population figures calculated for the standard hauls, but if the species that stay
below 200 m during the daytime migrate upwards during the night hours, there would
be an augmentation of the population density in the upper 200 m stratum. The genus
Pleuromamma is an example. It is represented by at least 6 species in the HOE collections.
It has been observed during subsorting of the Copepoda that individuals of Pleuromamma
species are abundant in the night samples only, whereas in the day samples they are very
sparse and include mostly juvenile stages.
The areas of greater density of copepod populations are the same as areas of rich
overall zooplankton production. The southern borders of the Indian Ocean are poor in
copepods but a gradual northward increase in population is observed, and the extensive
border areas of the northwestern Indian Ocean are rich in copepods. The fact that in some
areas night stations show higher numbers of ~opepods than day stations, is thought to
be due to circadian rhythms which cause the copepods to migrate to the upper 200 m of
the sea at night.