The echo abundance of plankton was recorded during the “Dr. Fridtjof Nansen” survey. In addition to zooplankton and phytoplankton, fish larvae were also included in the scattering layer labelled ‘plankton’. During the night lantern fish migrated to the surface layer and mixed with the plankton as well.
Fig. 4.1. Distribution of plankton over the continental slope.
Usually, the maximum plankton abundance was observed over the continental slope, as seen in Fig. 4.1. This was probably caused by the hydrographic condition, as the shear or transition zone between the south-going Mozambique Current and shelf counter current coincided with the slope. During night the plankton seemed to be more or less evenly distributed in the whole water column in the shelf area, as shown in Fig. 4.2. A significant proportion of the plankton biomass apparently stayed very close to the bottom during day and spread out in the whole water column during the night.
Fig. 4.2. Typical night recording of plankton at the shelf.
During daytime the plankton was usually concentrated in one or more separate layers. Fig. 4.3 shows a typical day recording from Sofala Bank at 50-60 m depth. There is a maximum abundance layer 5-10 m above the bottom and nearly no plankton below. On several occasions this phenomenon was observed to be connected with a decrease in temperature in the near-bottom layer. Fig. 4.4 is another example of a plankton recording with several layers of concentration. The undulating character of the layers is probably due to internal waves which were occasionally observed in the vicinity of the continental slope.
Fig. 4.3. Typical day recording of plankton at Sofala Bank.
The average integrator deflection attributed to plankton was used to establish an acoustic plankton abundance index for the different areas. The results are shown in Fig. 4.5. The minimum plankton abundance occurred during Cruises I and II, i.e. during the southern summer. The maximum values were observed during Cruise I, i.e. in September - October. The minimum abundance seemed to occur later along the northern coast than further south.
Fig. 4.4. Day recording of plankton near the continental slope.
The acoustic abundance index will be a function not only of the total plankton biomass but also of the species composition. The sound reflecting properties of a plankton layer depend on the size of the organisms as well as on the frequency of the echo sounder. The abundance indices in Fig. 4.5 therefore do not necessarily give a correct measure of the relative variation of the plankton biomass. The reliability of the abundance indices can be improved by using an echo sounder of higher frequency.
Fig. 4.6 shows the average wet displacement plankton volumes for different areas and cruises. Both in the northern area and in the Bazaruto area the time variation of this parameter shows approximately the same tendency as the acoustic indices. At Sofala Bank and in Delagoa Bay, however, the pattern of variations seems to be different. Maximum displacement volume was observed during Cruise III which was the time of minimum acoustic abundance. We have no obvious explanation for this phenomenon.
In order to see if there was any difference in the average plankton volume between day and night hauls the value
was calculated for each cruise and for three areas. VD is the average volume during the day and VN during the night. Fig. 4.7 shows the result. As seen, the night hauls usually gave a higher plankton volume except on Cruise II where the day hauls contributed most. This feature, as well as the time variation of the plankton volumes at Sofala Bank and in Delagoa Bay shown in Fig. 4.6, might be explained by seasonal variation in the species composition of the plankton.
Fig. 4.5. Acoustic plankton abundance indices.
Fig. 4.6. Average wet displacement volume of plankton.
Fig. 4.7. Ratio between night and day hauls of plankton.
1) Northern, 2) Sofala, 3) Bazaruto.
