The ecology and production of copepods, particularly Thermocyclops hyalinus, in the tropical Lake George, Uganda

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Abstract
Summary: (1) Total copepod numbers per litre in Lake George, Uganda, vary slightly during the course of a year. Numbers increase during the dry seasons June‐August and January‐February, and decrease during the rains. This variation is more marked in the mid‐lake area, where copepod numbers are higher, but the change in numbers is not more than two‐fold. By comparison with populations in temperate regions the population in Lake George is stable in size and shows little seasonal variation.(2) The carnivorous cyclopoid Mesocyclops leuckarti forms only 4–21 %, by num‐bers, of the total copepod population and does not occur in numbers large enough to be analysed profitably.(3) The bulk of the population is the much smaller Thermoeyclops hyalinus and analysis of this population into the main development stages of its life history shows an almost constant age structure over the lake as a whole, throughout the year.(4) The development time of the eggs, as determined previously (Burgis, 1970), is 1–5 days. This, plus mean development‐times of 6 and 11 days for nauplii and copepodites respectively, has been used to determine the daily rate of recruitment from each stage of the life history to the next. The recruitment from nauplii to copepodites and from copepodites to adults are very similar and almost constant. Recruitment from eggs to nauplii is. however, mueh greater, and it thus appears that the major loss to the population is at this eariy stage in the life history. This may well be due to predation by the larvae of Chaohorus.(5) Using this recruitment rate from eggs to nauplii, the potential turnover time of population numbers was calculated to have a mean value of 4–2 days for the lake as a whole. This is similar to maximum summer turnover times found by authors such as Hall (1964) for Daphnia populations in temperate waters.(6) The biomass of individuals at various stages in their life history was deter‐mined as carbon, and application of these determinations to the numerical counts was used to calculate the standing‐crop biomass in the lake. With a constant population of stable age‐structure it is possible to estimate the daily biomass production from the turnover time of numbers. For the population of Thermocyelops hyalinus in Lake George this gives a mean figure of 211 μg C/I/day during 1969–70.(7) Instantaneous birth rates and instantaneous rates of change in population numbers (b and r) have also been calculated for the population of Thermocyclops. The values of r show some variation at one centre site, fluctuating around zero, but when calculated for lake mean figures r is hardly different from zero throughout the year. The values of b also show fluctuations when calculated for one site but when calculated for lake mean figures show no more than two‐fold variations, being higher at the beginning of the two rainy seasons. The values of b are not as high as those attained during the summer in some temperate Daplmia populations such as those described by Wright (1965). The values of ZJ tend to be higher inshore than in mid‐lake areas of the lake.(8) The instantaneous mortality rate r/has been calculated from b‐r. With such very small values of r, d is always very similar to b. Most, if not all, recruitment to the population is counterbalanced by loss, probably predation.(9) At one inshore site the population was analysed and counted in detailed cate‐gories, over a period of 10 days. This allowed a more accurate estimation of the standing‐crop biomass and thus of production. A mean value of 103 μg C/l/day was obtained for these samples.(10) The mean daily level of production, c. 211 μg C/l/day, o(Thermocyclops hyalinus in Lake George, is among the higher values of zooplankton production available for comparison most of which are from temperate regions of the world. The fact that it is applicable throughout the year gives an annual value of 77g C/m3/year which is probably high.