There is not much information available about pelagic photosynthesis in tropical lakes in general and this is also true for African lakes although most of the work in the tropics has been in Africa. Tailing (1957) made some measurements of the primary production in a lagoon and the Jebel Aulia Reservoir in the White Nile and also in Pilkington Bay in Lake Victoria. He estimated the photosynthesis from both measurements in situ and from diurnal changes. Vollenweider (1960) made some measurements in situ in the Nile, Hydrodrome, Mariut and Quarun in Egypt and a number of measurements in vitro in different waters. Elster and Vollenweider (1961) studied various waters in Egypt within the framework of a joint FAO/Unesco project and also did some studies of the primary production in the Hydrodrome and Lakes Edku and Mariut. The primary production of the East African lakes Victoria, Albert, Edward, George, Bunyoni and Mulehe, was investigated by Tailing (1965 a). Of these only Victoria was comprehensively studied. The annual primary production was studied for a few years in Lake Mariut by Aleem and Samaan (1969). Studies are under way in Lake Chad and some results are also available (Lemoalle, 1969). There are also some data available for Lake Volta (Vinèr, 1970). For comparison, data over primary production in some Indian waters have been consulted (Ganapati - Sreenivasan, 1970), as follows:
| FISHPONDS: | RESERVOIRS: |
| Fort Moat | Amaravathi Reservoir |
| K. C. Kulam | Stanley Reservoir |
| Tammai Kulam | |
| Fort Moat, Vellore |
Samaan (1971) has also made a few measurements in Lake Nasser.
The highest production per surface area has been recorded in Indian fishponds. The fishponds and the two Indian reservoirs are all between 10° and 13° N latitude. The water temperature varies between 24° and 33° C. The highest recorded primary production was 54.78 g 02/m2/d (20.54 g C/m2/d) in Fort Moat, Vellore (Ganapati-Sreenivasan, 1970). It is not known if the ponds received fertilizers. The range of pelagic primary production is between 0.011 g C/m2/d (Mariut) and 20.54 g C/m2/d. Mariut as a subtropical lake is influenced by less radiation and lower water temperature.
Kainji, as compared with other lakes, has a very low production for a tropical lake (see Fig. 21 and Table 6). The range is also fairly narrow considering that Measurements have been made during all seasons. Higher values may be obtained in some areas but these values are not significant for the lake as a whole. The range of production for Kainji extends into the lower end of lakes Albert and Volta, Amaravathi Reservoir, Bunyoni, Jebel Aulia Reservoir, White Nile Lagoon, Stanley Reservoir and Lake Mulehe. Of these only the Indian reservoirs Amaravathi and Stanley have been studied for a longer period. For the other lakes there are insufficient measurements to draw any conclusions about the average daily production. The single measurements give some indication, however, of the magnitude of production, especially if the temperature and radiation are fairly constant throughout the year.
Of these lakes, Mariut is a subtropical lake influenced by a cool winter season. Lakes Mulehe and Bunyoni are both highland lakes with a temperature close to 20° C (Tailing, 1965). Despite the relatively low temperature, Lake Bunyoni has a higher recorded primary production than Kainji. Mulehe again is within the range of the production in Kainji. The other lakes, judging from the latitude and altitude, receive a similar amount of radiant energy and therefore also maintain a similar temperature. Temperature and radiant energy being equal the reasons for varying efficiency of utilisation of radiant energy must be found in the limnological features of the lakes. Rawson (1955) found that there was a certain correlation between mean depth and production. In Figure 22 the photo-synthetic production has been plotted against the mean depth and it can be seen that while Rawson found the turning point to be at 18 m depth it is here at about 5 m. A lake with a mean depth of 5 m is not likely to stratify unless it is completely sheltered from wind and the bottom deposits of such shallow waters may therefore be disturbed by wind action. The sedimented nutrients are therefore not lost but can be mobilized and re-utilised by the plankton organisms and thus a high production can be maintained. The curve showing the relationship between primary production and mean depth takes an unexpected turn at about 15 m mean depth. Instead of falling and levelling out it starts to rise again. The plots are too few to give the true shape of the curve and necessary data are not available for an interpretation of the results. It indicates another system of nutrient supply as compared to the lakes with re-utilisation of sedimented nutrients. Lake Nasser is one example of a lake with a high concentration of phosphates and high primary productivity and this is due to leaching of phosphorous from the flooded Nubian limestones (Samaan. 1971).
An attempt was also made to relate the primary production to the morphoedaphic index (Ryder, 1965) (see Fig. 23). There is a fairly good positive correlation between the two factors. Although the morphoedaphic index originally was used for estimating the fish catch when the total dissolved solids and mean depth were known, it may also be used for estimating the magnitude of primary production. Caution is however necessary when utilising the data. The figure according to the index shows a primary production for Kainji twice as high as the measured average production and higher than the maximum value obtained* However, the discrepancy of the value for Kainji from the assumed regression seems of the same order as others of the series. A low observed value for Kainji might result from low average transparency or a disproportionately low concentration of major nutrients. such as nitrogen and phosphorous relative to the total dissolved solids.
As a function of latitude, the net radiation received on the surface is the source of energy both for the photosynthetic process and for the maintenance of the temperature of the environment. The intensity of the biological processes is also dependent on the temperature according to Van't Hoff's rule. Although the intensity of the processes increases with temperature, the efficiency of utilisation of the available energy and nutrient resources does not necessarily increase with temperature. Gulland (1970) states that the efficiency of transfer of energy from one trophic level to another may even be greater in the discontinuous production of high latitudes than in the continuous, tropical, system.
With a total radiation of about 162 kcal/cm2/a for Kainji, the radiant energy available for photosynthesis is not likely to be a limiting factor. Tailing (1965 b) quotes for Lake Windermere a total radiation of about 60 kcal/cm2/a and for Lake Victoria 154 kcal/cm2/a. He found that the production per unit area is therefore five times higher in Lake Victoria than in Windermere although the average population densities in the euphotic zone appear to be similar.
4.3.1 Nutrients
Kainji Lake is poor in nutrients and this partly explains it's low production for a tropical lake. The photosynthetic rates at light saturation level, however, are considerably higher than in Lake Victoria. The production per unit area is nevertheless much higher in Victoria than in Kainji owing to an euphotic layer that reaches to 15–20 m depth in Victoria as compared to 1 m and only occasionally over 10 m depth in Kainji. The effect of introducing phosphorus into the lake water was demonstrated in June and August 1972. The incubating bottles were accidentally cleaned with a detergent containing phosphate. The phosphate remnants in bottles even after rinsing were large enough to boost the production to over 6 g O2/m2/d.
The replacement of nutrients is mainly dependent on the supply from the Niger and the lateral inflow. The annually flooded areas also contribute part of the nutrients and may, especially in farmed and fertilized areas, be of some importance. Organic matter in general is not available for photosynthesis. In natural waters, iron is present as ferri-iron component as long as the water contains oxygen. The ferri-iron is not soluble in water and therefore precipitates absorbing phosphates from the water, thus enriching the bottom with phosphates. The nutrients incorporated in settled plankton organisms and organic matter in general are not available for photosynthesis because the bottom sediments are not disturbed by wind action because of depth and the ferri-phosphate complex is stable as long as the bottom sediments are covered with oxygenated water. During stratification the oxygen content decreases and when the redox potential of the bottom sediment is low enough the ferri-iron is reduced to ferro-iron with the result that the phosphates bound to the ferri complex are released. The simultaneous gas exchanges between the sediments and water also carry dissolved salts into the water and as a result the bottom water (hypolimnion) becomes enriched with dissolved salts. This can be noted as a considerable increase in the conductivity. Because the thermocline presents a barrier between the surface and bottom water the released nutrients are not available for plankton production. The levels of both the turbine intakes and the spillways are below the thermocline and therefore the nutrient-rich bottom water is drained out. The disappearance of the thermocline is believed to be partly due to the drainage through the outlets (FAO report in press) indicating that most of the bottom water and the nutrients disappear from the lake. The main nutrient supply is therefore the Niger inflow. The stratification and drainage depth therefore make the lake act as a temporary nutrient trap releasing the nutrient downstream once a year during a few months duration. When more turbines are installed the water will be drawn entirely from the lower part of the bottom layer and it may be expected that the drainage of the nutrient enriched bottom water will be more complete than during present conditions.
4.3.2 Transparency
In Kainji the transparency is determined by the allochtonous silt material and other matter carried by the Niger and to a lesser extent by the phytoplankton and decomposition products of various biological processes. Although the plankton community has a certain ability of adaptation to varying light conditions, the turbid water of the White Flood restricts the euphotic layer to such an extent that the loss in depth of production cannot be compensated by a corresponding increase in the rate of production at light saturation level. During turbid conditions the light saturation level is at the surface and no light inhibition can be measured. The production per unit of surface area therefore follows the transparency in general, although the adaptive capability of the phytoplankton affects the correlation. A transparency of 1 m measured with a secchi disc may be taken as a dividing line of production. Most of the measurements then give a lower daily output that 2 g O2/m2 when transparency is less than 1 m and during higher transparency most of the values are above. Similarly the highest values have been obtained when the transparency has been higher than 1.5 m and the lowest with a transparency of 0.1 – 0.2 m. The varying spectral composition of the subsurface light is not believed to have any measureable effect on the production. Vollenweider (1960) found that the energy flux controls the photosynthetic rates below the level of optimum light regardless of its specific spectral composition*
4.3.3 Effect of Silt on Photosynthetic Production
Assuming that the nutrient content of the Niger water is fairly constant and considering that both radiation and consequently water temperature do not limit the photosynthesis, the hypothesis can be made that the silt content in the water may be a factor triggering the plankton bloom annually observed shortly after the White Flood. Because the silt presents surfaces for attachment of bacterial growth, the increased bacterial activity may cause a more rapid turnover of nutrients and thus promote plankton production. Microcystis spp is the dominating plankton and generally blue-green algae outgrow competitors under favourable conditions. Kuzmyenko (1972) has shown that introducing silt into a culture of Microcystis aeruginosa increases the number of algal cells as much as 1.5 times. He believes that this effect is caused by an improved exchange of components due to the two-phase system of silt and water. The fact that the bloom is restricted in area and moving from the upper part of the lake downwards indicates that there must be also another growth promoting factor. Probably the flood brings some nutrients washed out from the surrounding soil.
4.3.4 Effect of Stratification on Benthos
The effect of stratification on the nutrient economy in the lake has been discussed earlier* The benthos has not been studied in Kainji mainly due to lack of equipment. The stomach contents of fishes caught show that there is a benthic community that ought to be studied and the biomass and production estimated. Because the lake is stratified during about four months and the bottom water is deoxygenated and hydrogen sulphide present during about three months, this must have a negative effect on the benthos. There are not likely to be any bottom organisms, except microbes, in the deeper water where the bottom water contains hydrogen sulphide for any longer period. There may be an immigration into these areas when the stratification disappears.
The effect of the submerged trees and the insect population they support on fish production has not been studied. This source of energy is, however, of temporary character and tree trunks extending above the low water level are already disappearing as a result of insect activities.
The production of macrophytes and periphyton has not been studied in Kainji. The low transparency of the water restricts the green plants to a rather narrow layer along the shore and it is assumed that neither macrophytes nor epiphyton play any significant role as a good source for the fish population.
4.3.5 Niger Inflow
An attempt was made to estimate the amount of phytoplankton brought into the lake by the Niger river. The pre-impoundment studies (White, 1965) indicate that the plankton abundance in the river was of the same magnitude as in the lake at present. The amount of phytoplankton was therefore calculated from average plankton density and inflow and it was found that the amount of phytoplankton carried into the lake was only 5% of the amount produced in the lake itself.
Daget and Bayagbona (1961) made a pre-impoundment estimate of the fish catch in Kainji Lake based on catch figures from waters upstream from Kainji. The estimate was 45–65 kg/ha/a, yielding about 5 400 t a year for the whole lake with a possible increase to double the quantity. Henderson (1971) estimated the fish catch using Ryder's morpho-edaphic index to be about 35 kg/ha/a with a range of 10–100 kg/ha/a depending on the effort. Lelek and El-Zarka (1971) estimated the catch from the average number of days the fisherman spent and the average catch per canoe. This indicated a total yield of 8 000 t/a corresponding to a catch of 65 kg/ha/a. Another estimate based on the number of boats indicated a yield of 4 600 t/a corresponding to 36.9 kg/ha/a.
Bazigos (1972) estimated the catch from a catch assessment survey made between October 1970 and June 1971 at a total of 28 638 t/a. It may be noted that more recent catch estimates suggest that this figure represents the peak that seems to follow impoundment of new reservoirs.
According to the same estimate the average catch per canoe per day was 23.0 kg (365 days). If the average number of days of 240 spent fishing by a fisherman estimated by Lelek and El-Zarka (1971) is used, the catch per canoe is 35.1 kg/d. These figures seem high compared to 11.3 kg average catch per canoe and day as estimated by the authors above.
Daget (1972) estimated the annual catch for the Nigerian part of Niger River, excluding Kainji Lake, to be 21.9 kg/ha/a.
The regression of catch figures relative to Ryder's index, primary production and primary production through mean depth shows that the catch estimate for Kainji is above the expected level. These relations indicate a catch rather lower than the earlier estimates, namely about 20 kg/ha/a. The Indian fishponds have to be excluded in these extrapolations as the catch is the total removal of fish from the ponds and therefore equal to total production of fish flesh. The catch in natural lakes is, however, only part of the production and is dependent on the effort. An absolute comparison of catch from various lakes is therefore not possible as long as the effort is unknown.
Estimation of the potential fish production through ecological energetics is another method. The rates and efficiencies of transfer of energy from one trophic level to another have, however, not been subject to any extensive studies in tropical conditions. The food objects of many fishes may change with age, directing the fish to different trophic levels during its developmental stages. The food web is partly unknown and the role of the benthos and “aufwuchs” in Kainji has not been studied.
With these shortcomings in mind a simple model can be worked out indicating the magnitude of the fish production potential for the lake when stabilised. Assuming an energy transfer efficiency of 10% from one trophic level to another, and disregarding the energy sedimented from the system, the following estimate can be made* The annual plankton production per square metre for Kainji is 300 g C. The respiration is taken as 20% and thus the annual net production is 240 g C/m2; 1 g C equals about 10 kcal making the net annual photosynthetic production 2400 kcal/m2. With the assumed energy transfer of 10% the primary production of consumers will be about 240 kcal/m2/a, secondary consumers 24 kcal/m2/a and tertiary consumers or top carnivores 2.4 kcal/m2/a. Assuming that the fishes are mainly either secondary or tertiary consumers the amount of production will be dependent on the relation between the two groups. If the production were made up of only secondary consumers the corresponding fish production would be 24 kcal/m2/a, or taking 1 g wet weight as 1 kcal, 24 g /m2/a fish flesh equal to 240 kg/ha/a. If on the other hand the fish production is entirely made up of top carnivores, the corresponding production would be 24 kg/ha/a. The actual production of the lake lies somewhere between these figures.
According to FAO (1972) the high proportion of carnivorous fish would be of long duration in the lake. It was also found that the carnivores amounted to nearly 50% of the catch by weight. Assuming that the herbivores and carnivores are equally exposed to gill nets, the catch should reflect the relation also of the production in the lake. Assuming therefore an equal production of herbi-and carnivores in weight, a total production of catchable fish of about 75 kg/ha/a is derived. With a fishing efficiency of 20% a potential catch of 15 kg/ha/a may be expected. Considering the potential variation in the energy transfer rate, such an estimate can only be considered as being very approximate. It is noteworthy that the differing kinds of estimates are consistent within the assumed range of error. This is however the lowest of the estimates yet given.
Measurement of the photosynthetic activity of the phytoplankton is one of the most common methods of estimating the trophic status of a natural water. It is also a sensitive way to detect any changes in the ecosystem that affect production. Changes like eutrophication and pollution affecting the aquatic environment can thus be observed at an early stage when counter measures may still be effective. Although there is a wide gap between the plankton production and fish catch in a lake, changes in production at the primary production level are sooner or later reflected in the catches*
Tropical lakes cannot easily be inserted in the classification systems created mainly from experience of temperate lakes. The uninterrupted production, high water temperature and high solar radiation all affect them and call for an entirely new classification system. Compared to temperate lakes, Kainji Lake is, with its relatively high production both per surface area and per volume of water, a eutrophic lake. The annual oxygen deficiency in hypolimnion confirms the picture. Comparing Kainji with other tropical lakes, however, the production rates seem low, regardless of high temperature and high insolation but the nutrient concentration is low with some chance of nutrient limitation. No attempt is made here to create a classification system but, using the classical parameters, the lake may be classified as eutrophic but with comparatively low production.
It seems evident from the present study that the lake conditions stabilized shortly after impoundment; this was to be expected because of the high replenishment rate of the lake water. The present stability of the limnological factors as reflected in the primary production is expected to last until major changes take place in the drainage area or in the hydrological conditions. Because of the relatively long lifetime of the individual fishes, present stable conditions cannot be expected to be immediately reflected in the fish population; the adjustment to the new conditions requires time. The competition between the species forces them to find their own ecological niches and before the different species have found suitable locales and have adjusted to the new hydrological regime, a period of at least five years is required.
Considering the future of the lake on the basis of the present development it is evident that the intensified agriculture in the drainage area, with increasing use of synthetic fertilizers, is the major factor threatening to change the ecosystem in Kainji Lake through introduction of plant nutrients. The recent development in navigation on the Niger River also gives reason for concern. It is planned that the river will be used for oil transport from Nigeria to neighbouring Niger. The risk of oil pollution cannot therefore be excluded, with unknown consequences for the lake fisheries. The distance to the nearest big population centre, Niamey, is too great for any impact on the lake from river pollution.
