194
A. DE DECKER:
in vertical net hauls with the N 50-net (KEMP, HARDY and MACKINTOSH, 1929) in thL'
upper 50 m (Fig. 4).
II. Phytoplankton
The unpublished data on phytoplankton given below are from a personal communication by E. A. COGHLAN-NEL.
On the average, the cell counts over the Bank are about one tenth of those recorded
off the west coast. During 1961 - 67, cell counts in excess of 10 6 cells per I occurred 53
times over the Bank, as against 126 times off the west coast. The highest single cell count
recorded over the Bank was 13 . 10 6 per I at an inshore station west of Cape Agulhas;
off the west coast it was 24· 10 6 per I at an inshore station north of Columbine
Promontory.
Most of the counts exceeding 10 6 cells per lover the Bank occurred at inshore stations
west of Cape Agulhas. They were most frequent in summer (19 times), rarest in winter
(7 times).
Among the organisms responsible for the high cell counts were 25 diatom species.
Only 5 among them were cosmopolitan or warm-water forms (Chaetoceros compressus,
C. costatus, Hemiaulus hauckii, H. sinensis, Nitzschia delicatissima), the others being
temperate or cold-water forms occurring abundantly in the inshore belt off the west
coast-with the exception of two oceanic forms (Rhizosolenia alata and Chaetoceros
decipiens). The most common diatoms over the Bank are Chaetoceros socialis, Leptocylindrus danicus, Nitzschia seriata and N. delicatissima, the latter species being the
most common one throughout the whole southwestern sector of the Indian Ocean.
(NEL,1968).
The settled volumes do not necessarily follow the trend shown by the cell counts,
due to the large difference in cell size of the different species. In August 1961 the settled
volumes were low but the counts exceeded 10 6 cells per I at 6 stations, the dominant
species being Nitzschia seriata, a minute diatom. On 5 occasions only the settled volume
exceeded 100 ml at stations on the Bank during the period 1961-1967, whereas this
happened 29 times off the west coast during the same period. Nevertheless, the largest
single volumes recorded in the whole survey area were taken over the Bank west of Cape
Agulhas in January 1963: 540 ml and 580 ml respectively at two stations 20 miles apart.
Only one of the 5 high volumes occurred in autumn (April 1962) with Chaetoceros
costatus and C. socialis as dominant species. The 4 others were in summer, with the
following species in predominance:
Chaetoceros constrictus
C. curvisetus
C. debilis
C. decipiens
C. radicans
C. socialis
Nitzschia pungens var. atlantica
Skeletonema costatum
Thalassiosira rotula
Nitzschia closterium
N. delicatissima
All the above species are inhabitants of temperate waters and commonly found in
large numbers off the west coast, except the cosmopolitan N. delicatissima and the
warm-water form C. costatus. Volumes in excess of 50 ml were recorded 35 times over
the Bank during the years 1961-1967.
A study of the occurrence and composition of phytoplankton blooms is now in
A. DE DECKER:
in vertical net hauls with the N 50-net (KEMP, HARDY and MACKINTOSH, 1929) in thL'
upper 50 m (Fig. 4).
II. Phytoplankton
The unpublished data on phytoplankton given below are from a personal communication by E. A. COGHLAN-NEL.
On the average, the cell counts over the Bank are about one tenth of those recorded
off the west coast. During 1961 - 67, cell counts in excess of 10 6 cells per I occurred 53
times over the Bank, as against 126 times off the west coast. The highest single cell count
recorded over the Bank was 13 . 10 6 per I at an inshore station west of Cape Agulhas;
off the west coast it was 24· 10 6 per I at an inshore station north of Columbine
Promontory.
Most of the counts exceeding 10 6 cells per lover the Bank occurred at inshore stations
west of Cape Agulhas. They were most frequent in summer (19 times), rarest in winter
(7 times).
Among the organisms responsible for the high cell counts were 25 diatom species.
Only 5 among them were cosmopolitan or warm-water forms (Chaetoceros compressus,
C. costatus, Hemiaulus hauckii, H. sinensis, Nitzschia delicatissima), the others being
temperate or cold-water forms occurring abundantly in the inshore belt off the west
coast-with the exception of two oceanic forms (Rhizosolenia alata and Chaetoceros
decipiens). The most common diatoms over the Bank are Chaetoceros socialis, Leptocylindrus danicus, Nitzschia seriata and N. delicatissima, the latter species being the
most common one throughout the whole southwestern sector of the Indian Ocean.
(NEL,1968).
The settled volumes do not necessarily follow the trend shown by the cell counts,
due to the large difference in cell size of the different species. In August 1961 the settled
volumes were low but the counts exceeded 10 6 cells per I at 6 stations, the dominant
species being Nitzschia seriata, a minute diatom. On 5 occasions only the settled volume
exceeded 100 ml at stations on the Bank during the period 1961-1967, whereas this
happened 29 times off the west coast during the same period. Nevertheless, the largest
single volumes recorded in the whole survey area were taken over the Bank west of Cape
Agulhas in January 1963: 540 ml and 580 ml respectively at two stations 20 miles apart.
Only one of the 5 high volumes occurred in autumn (April 1962) with Chaetoceros
costatus and C. socialis as dominant species. The 4 others were in summer, with the
following species in predominance:
Chaetoceros constrictus
C. curvisetus
C. debilis
C. decipiens
C. radicans
C. socialis
Nitzschia pungens var. atlantica
Skeletonema costatum
Thalassiosira rotula
Nitzschia closterium
N. delicatissima
All the above species are inhabitants of temperate waters and commonly found in
large numbers off the west coast, except the cosmopolitan N. delicatissima and the
warm-water form C. costatus. Volumes in excess of 50 ml were recorded 35 times over
the Bank during the years 1961-1967.
A study of the occurrence and composition of phytoplankton blooms is now in