A Study Of The Phytoplankton Of The South Western Indian Ocean

Abstract

Although the phytoplankton of the waters off the

west coast of South Africa (the Benguela Current region)

s ha .. been the subject of several detailed studies in the

past, data on that of the s.w. Indian Ocean has been almost

entirely restricted to incidental references in the reports

of expeditions which have passed through the area.

Consequently, little has been known of the species composition

and distribution of the phytoplankton, and nothing of its

seasonal fluctuations. This study was designed to provide a

broad picture of the phytoplankton of the area, the primary

objective being a critical determination of the species

composition. The material was collected by the s.A.s. Natal

on four seasonal cruises in the area as a contribution to

the International Indian Ocean Expeditions. A net-sampling

technique was used to provide the maximum amount of material

for quantitative analysis.

The phytoplankton was found to be extremely rich in

variety, 402 taxa being identified from the 98 samples collected.

Of these 233 were diatom taxa, 157 dinoflagellate taxa, and the

remainder being composed of members of the Chrysophyeeae

(coccolithophorids), Cyanophyceae and Xanthophyceae. These

are listed in the systematic section together with original

references and other references used by the author for their

identification. The local and general distributions or the

taxa are described• and many of the taxa are illustrated by

line drawings or microphotographs. 5 new species are described,

as well as 1 new variety, and it was found necessary to provide

new names for several species. Full systematic details are

given for all IW!W or rare taxa.

In order to understand the hydrography of the area

it was necessary to revise earlier concepts in the light of

data collected concurrently with the phytoplankton during the

present survey. particularly with regard to the seasonal

fluctuations of conditions in the area• and consequently the

hydrographic section is more detailed thaa is customary for

the present type of study. It is sugggested that the area be

subdivided into 5 zones, namely the Agulhas Current Zone, the

South Western Indian Oceanic Zone, the Natal Shelf Zone, the

Eastern Agulhas Shelf ''zone, and the North Edge Water.

The distribution patterns of the phytoplankton are

illustrated by discussing examples of each type, and the

relationship of the observed distributions with the proposed

hydrographic zones is discussed. It was found that there

appeared to be a local, semi-permanent population of neritic

species with centres of distribution in the Eastern Agulhas

Shelf Zone• the species probably being spread into other

zones by the action of currents. Currents were also found

to be the most likely agents in introducing the many "visitor"

species to the area. the Agulhas Current introducing tropical

and subtropical species into the Natal Shelf Zone and the

s.w. Indian Oceanic Zone, eastward-flowing local currents in

the vicinity of Cape Agulhas possibly carrying temperate,

neritic west-coast species into the Eastern Agulhas Shelf Zone,

from which some of them spread northwards into the Natal Shelf

Zone. The temperature structure and its consequent effect

on nutrient concentrations in the northern offshore waters

was the most likely factor limiting the northern spread of

temperate and subantarctlc oceanic species, as well as the

spread of many species from the shelf waters into the offshore

waters. In winter and spring, when the thermocline broke

down in the offshore waters. and virtually homogeneous water

existed in the upper 100 meters, many of the species confined

to the inshore waters in the summer were widely spread in the

offshore waters. and several temperate and subantarctic

species spread northwards from the North Edge Water. This

is thought to be a possible explanation for the isolated records

of subantarctic species found in tropical waters (remnants of

this"northern spread), and may play a part in the formation

of bipolar distributions.

Heavy concentrations of phytoplankton were found to

be present at inshore stations on most cruises. particularly

on the edge of the continental shelf in the Eastern Agulhas

Shelf Zone, this being the region where water from moderate

depths upwelled into or just below the euphotic zone on the

inner edge of the Agulhas Current.