Initially four sampling stations were chosen on the basis of limnological features of different parts of the lake. Three of than, numbers 9, 14 and 15, were situated in the main basin (see Fig. 1 ) and one, station D, was close to the damsite 1. Later, stations 9 and 15 were abolished because of similarity in results with station 14, and stations 11 and 6 were chosen as more representative of the conditions in the lake. With these four stations the lake was sampled along a longitudinal section. Preliminary measurements started in November 197° and were made at all four stations during cruises made at intervals of 4–6 weeks. The field work continued until September 1972.
Station 11 lies in the channel on the northwestern side of Foge Island. It is characterised by a year-round current that gives the station a riverine character. The maximum depth is about 20 m at high water. The water is stratified for a short period annually during the harmattan. The station is characteristic of the northern arm of the lake.
Station 14 was initially located as shown on the map but was later moved to the channel across Foge Island to get sufficient depth for the research vessel during low water. Maximum depth is about 12 m. The station represents the lacustrine conditions of the middle part of the lake. Bottom deposits are stirred up during storms, reducing the transparency. Stratification occurs for a short period annually at 7–10 m depth.
Station 6 lies in the river channel outside Old Bussa where the two river channels merge and the lake narrows. The maximum depth is about 26 m. It represents lacustrine conditions although sometimes a surface current can be observed on both sides of the Old Bussa Island. The water is stratified at the station from about February to April.
Station D is situated in the western channel about 1 km from the dam. There are two river channels close to the damsite, the western channel being the deepest with a maximum depth of about 60 m. It represents lacustrine conditions and this area is least affected by the drawdown. The water is stratified annually between February and April–May. A permanent stratification has also been detected in some deep pockets close to the station.
1 The numbered stations are the limnological sampling stations used by the project.
Transparency was visually determined using a white-painted secchi disc of 25 cm diameter. The depth of transparency was determined to the nearest 10 cm.
The hydrogen ion concentration was estimated with a Lovibond comparator with an accuracy of one tenth of a pH unit.
A mercury thermometer enclosed in the water sampler was used for temperature measurements. The thermometer could be read to 0.05°C. Temperature profiles were obtained with a bathythermograph for use in shallow water. The reading was always calibrated against a mercury thermometer.
Weekly recordings of solar radiation were made from March 1972 with a SLAP bi-metallic actinograph. The actinograph was temporarily installed at the project site on the east side of the dam. The radiation was recorded as cal/cm2 week (gross radiation). Monthly values for 1971 were approximated from mean values of Gunn-Bellani readings in Bida (southeast of New Bussa) and Yelwa (north of New Bussa) by comparison with the actinograph (Table 4 ).
The utilisation of radiant energy by phytoplankton was also calculated for measurements made after installation of the actinograph (Table 5 ). The energy required to produce 1 mg O2 was taken as 3.63 cal (Ganapati and Sreenivasan, 1970). The part of the energy spectrum available for photosynthesis, 400–700 m wave-length, was considered to be 46% of the total radiant energy (Tailing, 1957). A surface loss of 10% due to reflection was also considered (Sauberer, 1962). The efficiency of utilisation was calculated as the relation between the radiant energy utilised by the phytoplankton during photosynthesis and the part of the energy available.
The vertical light transmission was measured with a submarine photometer (Model No 268 WA 310) with two photocells permitting simultaneous readings of both deck and sea photocells. The spectral composition of the sub-surface light was determined using three filters, the blue filter having a peak transmission of 440 m , the green filter at 530 m and the red filter at 700 m (Fig. 4). The seasonal variations in transmission of light of various wavelengths were observed and the extinction coefficients were also calculated for the most penetrating component of the spectrum according to Sverdrup, Johnson and Fleming (1942).
The recommendations of Sauberer (1962) were followed as far as possible.
The theoretical photosynthetic integrals were calculated using Rodhe's (1965) formula based on Tailing's (1957) model:
Σ a = Z0.1Jmpc × a max
| where | Σ a | is the total photsynthesis per unit area |
| Z0.1Jmpc | is the depth in metres of the 10% light transmission level for the most penetrating component | |
| a max | is the maximum photosynthetic rate measured in the water column. |
The factor for converting the short time exposures to daily total estimates was obtained from cumulative measurements in situ. Surface water samples were placed in light and dark bottles and the bottles were incubated at the same depth. Every hour one pair of light and dark bottles was removed and the oxygen concentration determined. The increase of the oxygen content over the initial value was plotted for each hour until no further increase was detected* The time for the maximum production level was taken as the point of optimum daily production and through graphical integration the total daily amount was calculated. The output between two hours before noon and two hours after (the exposure time used in the measurements) was then calculated and the corresponding factor used for the daily estimates*
The phytoplankton primary production was measured in situ using the oxygen light and dark bottle technique. The water samples were obtained from different depths with a non-metallic van Born water sampler equipped with a mercury thermometer. The whole procedure was always carried out in the shade to avoid light shocks (Steemann-Nielsen and Hansen. 1959).
The incubation bottles 250 ml Kimax bottles, were filled with water and suspended horizontally at the depth from which the water sample had been taken. The horizontal position gives, according to Ohle (1958), better results than having the bottles hanging vertically. One light and one dark bottle covered with black plastic were tied to a steel hanger attached to a line hanging from a wooden bar. Five to seven pairs of light and dark bottles were generally used. The bar was supported by one small buoy at each end to minimise shading. The incubation time was four hours, starting from two hours before noon; this was long enough to obtain a significant increase in oxygen concentration but short enough to avoid unwanted side effects (Vollenweider. 1969). Immediately after incubation the precipitation agents were added to the bottles to stop the photo-synthetic process and the oxygen titrations were carried out without delay.
The oxygen concentration was determined according to Winkler's method with a 0.01 N thiosulphate solution using starch as indicator. The titrations were carried out in the field with a precision of ± 0.04 mg O2/1. The oxygen production was measured as the difference in oxygen concentration between the light and dark bottles (gross production). The gross production has been used here because more reliance can be placed on estimates of gross than on net production (Tailing, 1969).
The phytoplankton production potential was estimated for different depths by exposing samples obtained at intervals from surface to bottom at the depth of light saturation. Plankton samples were also collected and counted to relate the potential photosynthetic activity of plankton from deep water to the number of plankton organisms counted.
The respiration of the total enclosed community was measured as the difference in oxygen concentration between the start value and the dark bottles after incubation. For calculation of the respiration the mean values for the first 0.5 m were considered.
The turnover time for the plankton population was calculated from the photo-synthetic rates at various depths and the total biomass of phytoplankton counted at the same depth. Approximative values were used for determining the volume of the plankton organisms as the exact volumes had not been determined. The turnover rates for the plankton biomass both at optimum light level and for the whole water column were calculated. The specific weight for the algae was taken as 1 and the dryweight as 10% of the freshweight 1. The photosynthetic rates expressed in mg oxygen produced per m3 and day were converted to carbon using a photosynthetic quotient of 1 giving a coefficient of 0,375,
