While work on biological problems is well underway, it is not yet possible to detail these characteristics of the lake other than for the fishes. The latter is the subject for other reports. Current efforts are concentrated on the plankton of the lake while the organisms of the bottom and shore have been but little investigated. It is intended here to make only a few very general comments.
The large annual fluctuation in the water level of the lake is the most significant factor in establishing community differentiation in the lake itself. Along with the change in lake level the turbidity of the white flood (September through November) reduces the depth of the photosynthetic layer to less than 2 m from about 5 m during the remaining part of the year. At high water the photic zone (assumed to be the depth to which 1 percent of the surface illumination may reach) does not reach the depth of the shoreline at low water level. Along most of the western and northern margin of the lake erosion of the shoreline dominates owing to the predominance of southerly and easterly winds, and extensive sorting of soil components takes place. Along the eastern margin, where the gradient of the drawdown area is very low (see Fig. 1) deposition predominates and seasonal growth of rooted plants is occurring.
The above conditions are not likely to permit the development of complex littoral (shallow water) communities and, as emphasized in the previous section, will tend to enhance still further the seasonality of life in the lake.
The remaining trees in the lake form another community of high seasonality owing to the changes in lake level. While these trees provide an extensive surface upon which lower plants and other organisms may find an anchor and be provided with the nutrient resources of the open water, seasonal exposure of the uppermost 10 m leads to rapid destruction of the most favourable sites both by weathering and wood-gathering by villagers. In time, it may be expected that the submerged trees will tend to support communities that are essentially benthic (bottom living) rather than littoral.
In the shallower parts of the lake, particularly around Foge Island, extensive beds of the grass Echinochloa have developed and seem to be expanding. This plant is capable of extending its leaves four metres or more above the anchored rootstock and both mats and individual stems float readily when dislodged. It would appear at this time that the Echinochloa community may become the only major source of photosynthetic production in the lake aside from the planktonic algae.
The planktonic community, apparently in contrast to the other lacustrine communities, appearsto be well developed and diverse in Kainji Lake. Well marked blooms of bluegreen algae occur in October and November. It is not yet possible to say whether the observed succession of other forms is seasonal or related to the change from river to lake. It would appear, however, that the annual exchange of water and strong seasonality will tend to maintain planktonic conditions much as they are now. Several forms of lacustrine fauna of zooplankton, incidental in earlier studies of the river, now dominate.
The bottom community of deeper parts of Kainji has not been examined. A few attempts have been made to sample transects from shore to river bed in the region near Shagunu, but only the river bed itself was successfully sampled owing to difficulties with the Ekman dredge on the stony bottom on the landward side of the channel and on the fibrous bottom of the old swamps lakeward of the channel. The river bottom samples contained a few oligichaetes. Chaoborus larvae have been frequently collected in the plankton, particularly at night, but seem not to be abundant. Emerging mayflies, presumably Povilla, are quite evident at times but appear not to have reached the levels of abundance that are reported for the Volta Lake. These burrow in the wood of the submerged trees and feed largely on algae both attached and drifting and hence are less typically benthic.
Some preliminary determinations of primary productivity (rate of photosynthetic production) have been made for the planktonic community using the light bottle/dark bottle technique. The results suggest a gross production of 1 to 1.5 g carbon per m2 per day, which is consistent with the low concentrations of dissolved nutrients in the lake, although perhaps half the productivity expected was based on comparisons with other African lakes (e.g., Tailing, 1965), These studies are continuing. It seems reasonable to suppose that the total primary production of the plants in the lake itself may be perhaps twice but not more than three times the above, considering the poor development at present of plant growth in shallow water.
Throughout the period of investigation echosoundings at all the sampling stations have shown a variable midwater concentration of sound scatterers. Several attempts have been made to collect the organisms responsible but with no success. Unfortunately large high speed midwater sampling devices have not been available to date. Nevertheless there are now strong reasons to believe that these scatterers are small clupeid fishes of the genera Pellonula and Sierrathrissa and that these fishes may be quite abundant in the open waters of the lake. The scattering layer shows a definite nocturnal migration to the surface. During the day the depth varies according to turbidity conditions from about 20 m when the water is clear to about 3 to 5 m when the water is turbid. This depth also tends to be greater in the deeper southern parts of the lake than in the shallower main portion. At night the layer becomes rather more diffuse but extends to at least 2 m from the surface. Just before dawn the layer concentrates near the surface and begins its downward migration as a consolidated layer to the day-depth. There is evidence of air bubbles being released as the downward migration occurs, particularly as the layer reaches the day-depth. Tows with small plankton nets failed to show, on the three occasions attempted, any zooplankton which might be concentrated in the scattering layer though the depth of tow could be monitored on the echosounder. Larval fish were sometims collected but, again, there was no evidence that these were associated with the layer. As the nets available could not be towed faster than about 1 to 15 kn, it is likely that even small fishes would readily escape.
Fishery studies, using light-attractive methods, have shown the presence of the above species and the screens protecting the intakes of the spillway often become blocked by these fishes.
The preceding discussion is largely speculative as the initial biological work was largely concerned with identification of the species of lower plants and animals that appear in Kainji Lake. Quantitative work is now underway and the results will be reported when appropriate. Nevertheless there is at this stage reason to believe that a drifting planktonic community may continue to be important in the total biological production of Kainji Lake, while the shallow waters will be limited significantly to seasonal importance. This suggests that lateral migrations, particularly of the fishes, will continue to be as important as they were in the river.
