Phytoplankton production and respiration in the Blue Nile and White Nile at Khartoum were measured during the period November 1969-January 1971 using ...
Primary production and respiration of the phytoplankton in the Blue and White Niles at Khartoum Faisal Sinada & A. G. Abdel Karim Department of Botany, Faculty of Science, University of Khartoum, Sudan
Keywords: Sudan, algae, Blue Nile, primary productivity, tropical rivers, White Nile
Abstract Phytoplankton production and respiration in the Blue Nile and White Nile at Khartoum were measured during the period November 1969-January 1971 using the light and dark bottle technique. Maximum rates of production coincided with periods of maximum phytoplankton densities. In the Blue Nile gross production varied between 0.00 gCm- 3 d-l during the flood season and 2.19 gCm- 3 d-' (0.49 mgO 21-lh-') during November 1969. In the White Nile the range was from 0.41 gCm- 3d- l (0.09 mgO 21-lh - ') in May to 3.74 gCm- 3 d-' (0.83 mgO2l-lh-') in November. The maximum rates of respiration in the Blue Nile and White Nile were 0.10 and 0.63 mgO21- h-i respectively. The ratios net:gross production were generally higher in the White Nile than in the Blue Nile.
Introduction
Methods
Very little has been published about phytoplankton production in tropical rivers in general and in the Nile in particular, while much had been done in temperature regions. Sinada & Abdel Karim (this volume) studied the variations in physical and chemical factors as well as changes in densities and species composition of the phytoplankton of the Blue and White Nile during August 1968 December 1970. They showed that nutrient concentrations were, in general, greater in the White Nile than in the Blue Nile, yet the algal densities in the latter river were far greater. This paper reports on the primary productivity and respiration by the phytoplankton measured at two sites in the Blue and White Nile during November 1969-January 1971. The data given in the present paper are the first to be published for these two African rivers within the Sudan.
Primary productivity and respiration were measured by estimating the changes in dissolved oxygen concentration by the dark-and-light-bottle technique. Water samples were obtained from 0.3 m depths at stations chosen in mid river. Three sets of transparent and opaque 250 ml glass-stoppered reagent bottles were exposed in situ at a depth of 0.3 m for 2 and 4h (09.00-11.00 and 0.8.0012.00 hours) in the White Nile and Blue Nile, respectively. Initial oxygen in each river was determined in similar bottles. Dissolved oxygen was fixed in the field after the exposure period and determinations were resumed in the laboratory following Winkler's technique as described by Mackereth (1963). Productivity and repiration were expressed as mgO21 lh-'. To facilitate comparison with other investigations, oxygen production values obtained as mgO 21 t were converted into the equivalent carbon values, assuming that photosynthetic quotient is unity, and hence: g Oxygen X 0.375 g Carbon
Hydrobiologia 110, 57-59 (1984). © Dr W. Junk Publishers, The Hague. Printed in the Netherlands.
58 (Westlake, 1963). Then productivity is expressed as gCm- 3 d-' assuming that the period during which photosynthesis occurs is 12 h per day.
Results and discussion The seasonal variation of gross and net oxygen production and respiration in the Blue and White Nile are shown in Fig. 1. Gross primary productivity in the Blue Nile varied between 0.00 and 2.19 gCm- 3 d- ] (0.00-0.49 mgO21-lh-l). Throughout the flood period, primary productivity in this river was undetectable. This may be correlated with the fact that the Blue Nile during the flood period is very poor in phytoplankton (Sinada & Abdel Karim, in prep.), and that photosynthesis is very much impaired by the great quantity of silt which reduces Secchi disc transparency to < 5 cm. The maximum primary productivi-
ty value (2.19 gCm-3 d-') in the Blue Nile occurred in November 1969 during maximum development of Melosiragranulata(Ehrenb.) Ralfs. A sub-maximum of 1.22 gCm- 3d-l (0.27 mgO21-h -) occurred in January 1970 concomitant with maximum growth of Melosira distans (Ehrenb.) Ktitz. In the White Nile, gross primary productivity ranged from a minimum of 0.41 gCm- 3 d- (0.009 mgO21 lh-l) in May to a maximum of 3.74 gCm 3d- (0.80 mgO 2 1- lh 1) in November 1969. A similar maximum of 3.56 gCm- 3 d- I recurred in November 1970. Peaks of production rates in the White Nile coincided with peaks of Anabaenaflosaquae Born. & Flah var. intermedia Woron. f. spiroides Woron. The maximum rates of respiration in the Blue Nile and the White Nile were 0.10 and 0.63 mgO21 -l h- ' respectively and these were recorded during maximum development of Melosira and Anabaena.
Respirotion Gross productivit Net productivity
4
3 Blue
a
Nile
't
0
1
2
'0
6
0 1969
9 7 0
1969
1 9 70
Fig. 1. Seasonal changes of phytoplankton primary production and respiration in the Blue and White Nile at Khartoum.
59 It is evident from Fig. I that the White Nile showed higher production rates than the Blue Nile. During January-May 1970 the gross production rates in the White Nile varied between 0.41 and 3.04 gCm- 3 d-l with an average value of 1.32 gCm 3d-' whereas during the same period the production rates in the Blue Nile were in the lower range of 0.29-1.22 gCm 3 d-l and with a low average value of 0.68 gCm 3d-l. This is surprising since the observed algal densities in the Blue Nile during the same period were far greater than in the White Nile (Sinada & Abdel Karim, in prep.). This discrepancy can be interpreted in terms of nutrient availability and decreased photosynthetic efficiency in the Blue Nile. During this period (JanuaryMay 1970) iron and phosphate-phosphorus in the Blue ile were below or near the lowest limit of detection and nitrate-nitrogen was in the lower range of 47 75 g 1 , compared to 85 123 ugNO 3N 1', 25-108 PgPO 4-PI I and0.12 0.23 mgFel in the White Nile. Lewis (1974) reported a increase in production efficiencies in response to an increase in nutrients, particularly nitrate, and vice versa. Another explanation for the lower rates of production in the Blue Nile can relate to differences in turbidity of the two rivers. Apart from the flood season, the Blue Nile is more transparent than the 'milky' White Nile. Therefore, the lower rates of production in the Blue Nile may be attributed to photoinhibition of photosynthesis in the light bottles suspended 30cm below the surface of the water. Numerous workers have reported the phenomenon of surface inhibition of primary productivity in tropical waters (Talling, 1957, 1965; Lewis, 1974; Ganf & Horne, 1975). A further explanation may be sought in the fact that p-algae which were not estimated in this study, were probably at greater
density in the open water of the White Nile than the Blue Nile. Of course these p-algae, if present, would contribute appreciably to total production. Hendrey & Welch (1974) found that nannoplankton in Findley Lake accounted for 58 to 94% of productivity. However, owing to the high surface illumination to which the rivers are subjected and owing to the differences in transparencies of the two rivers more work is necessary for determining the optimum depth of efficient photosynthesis within the trophogenic zone of each river before concluding that the White Nile is more fertile and productive than the Blue Nile.
References Ganf, G. G. &Horne, A. J., 1975. Diurnal stratification, photosynthesis and nitrogen fixation is shallow, equatorial lake (Lake George, Uganda). Freswat. Biol. 5: 13 39. Hendrey, G. R. &Welch, E. B., 1974. Phytoplankton productivity in Findley Lake. Hydrobiologia 45: 45 63. Lewis, W. M., 1974. Primary production in the plankton community of a tropical lake. Ecol. Monogr. 44: 377 409. Mackereth, F. J. H., 1963. Some methods of water analysis for limnologists. Scientific Publications, Freshwater Biological Association 21. Sinada, F. & Abdel Karim, A. G., 1984. Physical and chemical characteristics of two African rivers: The Blue Nile and White Nile at Khartoum. Hydrobiologia (this volume). Sinada, F. & Abdel Karim, A. G., 1984. A quantitative study of the phytoplankton in two African rivers: The Blue Nile and White Nile at Khartoum. Hydrobiologia (this volume). Talling, J. F., 1957. Diurnal changes of stratification and photosynthesis in some tropical African waters. Proc. r. Soc., B. 147: 57 83. Talling, J. F., 1965. The photosynthetic activity of phytoplankton in East African lakes. Int. Revue ges. Hydrobiol. 50: 1 32.
