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

Abstract

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 numbers, 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 determined 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 21! /.igC/1/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 mi...

Durations of development and production estimates by two crustacean zooplankton species Thermocyclops oblongatus Sars (Copepoda) and Diaphanosoma excisum Sars (Cladocera), in Lake Naivasha, Kenya

“…The turnover time of biomass for T. oblongatus was slightly less than but not significantly different from that estimated for T. hyalinus (12.5 days) from Lake George, Uganda where the mean temperature was found to be slightly higher at 26 C (Burgis, 1971(Burgis, , 1974. The shortest turnover times of biomass of T. oblongatus in Lake Naivasha was recorded in January/February 1979 when biomass was low.…”

“…But the small differences ob- unlikely to be because of a difference in their egg sizes, as they are similar in the two species. Egg volumes of T. oblongatus varied between 2.14 x 10 3 and 2.891 x 10 3 Pm 3 as compared to T. hyalinus egg volumes of between 2.016 x 103 and 2.958 x 103 m 3 given by Burgis (1971). The retardation of egg development rates recorded by Burgis ( , 1971) at 30 C has been explained by Bottrell et al (1976) to be a common occurrence at experimental temperatures in excess of 30 C. Some tropical shallow lakes especially Lakes Chad and George do experience, from time to time, temperatures up to and above 30 C. The highest temperature recorded in Lake Naivasha (1890 m above sea level) have rarely been above 26 C. Therefore, experimental temperatures above 28 C for the durations of development were not considered ecologically important to the zooplankton of this lake since they are never experienced in the Lake, and further, there is no evidence of retardation in development within the range of temperatures used in these experiments.…”

Errors in the estimation of recruitment of early stages of mesocyclops leuckarti (claus) caused by the diurnal periodicity of egg-production

“…The population of the copepoda is characterized by a relatively high rate of mortality in the early stages as stated by Odum (1959) and described by Burgis (1971) . This can be illustrated by the higher values of recruitment of eggs to the nauplii in comparison with the values of nauplii to copepodites, as recalculated from the data presented by Ravera (1954), according to the procedure utilized by Elster (1954 ;1955), Burgis (197,), found similar differences in the population of Thermocyclops hyalinus (Rehberg) in Lake George .…”

The productivity of mesocyclops leuckrarti (Claus) in Lake Kinneret (Israel)

“…I) . Assuming that 44% of dw is carbon (Burgis, 1971) these values correspond to 2 .2 g C/m 2 /month . The average monthly P/ B ratio of M. leuckarti populations in Lake Kinneret is 3 .1, ranging between 1 .4 and 4 .8 (Fig .…”

“…The physiological characteristics relating to seasonal distribution and productivity of cyclopoids have been studied by many authors (Hilbricht-Ilkowska & Patalas, 1967 ;Shushkina, 1968 ;Hilbricht-Ilkowska & Weglenska, 1970 ;Burgis, 1971 ;Smyly, 1973 ;Ivanova, 1973) . Very little, however, is known on the physiology of Mesocyclops leuckarti (Kukyna, Klekowsky & Shushkina, 1968) .…”