Enregistrement de l'activité dans les conditions naturelles sur les pistes deFormica polyctena
Résumé1oOn donne une technique simple d'enregistrement de l'activité des fourrageuses deF. polyctena sur leurs pistes.2oL'activité dépend non seulement de la distance par rapport au nid, mais encore de la piste considérée.3oL'activité n'est nullement comparable d'une piste à l'autre; l'activité nocturne apparaît très souvent dans les pistes populeuses et peut manquer dans les moins populeuses.4oLes pistes différent donc non seulement quant à leur capacité d'apprentissage et leur agressivité (Chauvin, 1962, 1964), mais aussi quant à leur activité; le comportement est également différent suivant l'âge de la piste, tout au moins dans les pistes qui relient les fourmilières filles à la fourmilière mère.5oL'aller et le retour prédominent à des périodes du jour très irrégulières et qui diffèrent pour chaque piste.6oLa température mesurée près du sol n'a qu'une faible corrélation avec l'activité des fourmis.Summary1oA technique is described which permits an easy recording of forager ants activity ofFormica polyctena on their trails.2oAnts activity is regulated by the distance from the nest, but this regulation is not the same in all the trails.3oAnts activity is not the same in all the trails; nocturnal activity appears very often on roads upon which the rate of flow is very high and may lack entirely on neighbouring, less crowded, roads.4oThe ants on, the differend roads are very heterogeneous regarding not only their activity but also their agressivity, their learning facilities (Chauvin, 1963–1964); their behaviour differ also according to road age; i. e. time clapsed from the road foundation, at least in the trails between a mother-colony and a daughter-colony.5oOutgoing and ongoing are very irregular and don't occur at the same time in different roads.6oTemperature close to the ground shows only a weak correlation with and activity.
Observations sur les pistes deFormica polyctena
RésuméLes pistes deFormica polyctena présentent des hétérogénéités quant à la longuer, à l'agressivité et à l'action de la température sur l'activité. De plus, elles restent en grande partie stables d'une année sur l'autre, ce qui ne s'explique que difficilement par l'hypothèse d'un marquage odorant, ou par celle d'une rétention mnémonique.SummaryF. polyctena roads are very heterogenous regarding length, workers agressivity and temperature influence on activity. Almost all of them remian the same for years, what is explainable only with difficulty by scent-marks hypothesis or mnemonic traces.
Lake Muzahi, Rwanda: limnological features and phytoplankton production
Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NHinf4sup+-N can be high.Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.