Diurnal stratification, photosynthesis and nitrogenfixation in a shallow, equatorial Lake (Lake George, Uganda)

Abstract

From a series of experiments and measurements covering one 24-h period in LakeGeorge the diurnal patterns of photosynthesis, nitrogen-fixation and stratification are described. The thermal cycle shows three distinct phases of isothermy, intense stratification and mixing. During stratification the surface water temperature reached 36°C, whereas the bottom temperature remained at 25°C. The phytoplankton are evenly distributed at dawn, sink out during the day and tend towards an even redistribution as thermal stratification breaks down. Depth-time profiles of oxygen concentration and pH values showed that intense photosynthetic and respiratory activity occurred. The shortness of the predominant limnological cycle is thought to be instrumental in restricting species fluctuations.Changes of total CO2 concentration in situ within the euphotic zone indicated that 2-25 g C/m2.12 h were photosynthetically fixed during daytime. From changes of in situ oxygen concentration, gross photosynthesis was estimated to be 12 g Oa/m-. 12 h and net particulate production over 24 h as zero.Photosynthetic activity was also measured with the i^C and oxygen techniques by enclosing algal samples in light and dark bottles. The patterns of activity recorded by the two methods were similar, although quantitatively the oxygen method gave rather higher values than did the i*C method. Daily gross photosynthesis was estimated as 15-56 g O2/m2.12 h and 4-5 gC/m2.12 h. These figures are compatible with one another and with the production estimates calculated from Tailing's model of integral daily photosynthesis.The balance between respiration and photosynthesis is described as a function of the underwater light climate and the relative rate of respiration. Both the experimental and theoretical approaches suggest that the column 24-h net particulate production is an extremely small percentage of gross photosynthesis.Nitrogen-fixation was measured using both the i^N2 and the acetylene reduction techniques. The day-long incubation of the ^^^2 experiment showed that U mg N/m^.day were fixed. The series of shorter acetylene reduction experiments gave a value of 58 mg N/m^.day. The acetylene technique showed relatively high nitrogenase activity at very low light intensities. The ^^Ng results indicated little N2fixation at low light intensities. The differences between the two techniques are discussed.Gross photosynthesis estimates calculated from seven different methods run simultaneously are compared. The figures show reasonable agreement, but since the gaseous environment in the euphotic zone is particularly suitable for photo respiration the values are not considered reliable estimates of phytoplankton photosynthesis.

Conditions stimulating planktonic N2‐fixation in Lake Rotongaio

“…For instance, recalculating from the data of , Clear Lake (California) produced 121 mg N m 2 day 1 on one occasion, but the annual mean was apparently about 5 mgN nr 2 day'. Ashton (1981) found up to 47.94 mgNm -day ', depending on conditions, in a South African impoundment, and Ganf & Home (1975) found 58 mgNm ~2day~', in comparable experimental conditions to ours, for Lake George (Uganda). These results were for plankton in eutrophic lakes.…”

“…Of all these studies only that of Ganf & Home (1975) includes any details of light through the water column, and even in this instance they were analysed only with respect to photosynthesis, although the N 2 -fixation was related to photosynthesis. The predictive power of the light field has therefore been given scant attention by most workers, though the use of heterocyst numbers, and to some extent nutrients, in this regard has not been missed, e.g., , and in most other studies where heterocysts have been counted.…”

Control of dissolved oxygen in northern temperate lakes over scales ranging from minutes to days

“…5). While lakes in the present study had little DO variation at scales ranging from about 8 to 15 h, others have found significant variability in tropical, shallow lakes (Ganf & Horne 1975). It would be interesting to conduct a similar study in tropical lakes where different controls of O 2 dynamics may operate.…”

Nitrogen fixation in an oligotrophic, saline desert lake: Pyramid Lake, Nevada

“…The higher daily rates recorded for Pyramid Lake in 1979 were 3-5 mmol CzH4 m-2 d-l during the bloom period, similar to values of l-l 4 mmol C2H4 mm2 d-l found for shallow, warm eutrophic lakes (Horne and Viner 197 1;Ganf and Horne 1975). Some values for the Baltic Sea are much lower: 6-80 bmol m-2 d-l (Rinne et al 198 1).…”