Growth and Reproduction of Potamogeton Crispus in a South African Lake

(1) Potamogeton crispus L. is common in lakes ('pans') on the floodplain of the Pongolo river in South Africa. The pans are flushed by the river during summer but isolated during the winter. Growth of Potamogeton was found to be markedly seasonal, starting in autumn (April) and reaching maximum above-substrate standing crop (43 g m-2) in early spring (September). No plants were present during summer. (2) Sixty percent of the 1129 m-2 of the vegetative buds (turions) on the sediment surface in April germinated. Although seed density was high (1445 m-2), only 0.001% germinated during the study period. (3) Under experimental conditions, turion germination was significantly inhibited at a temperature of 25 'C and above. In the field, turions germinated only in autumn, when the water temperature had dropped below 25 'C. (4) Individual plants have a short life (c. 5 months), but because turion germination occurs over a period of about 3 months, and newly-produced turions do not germinate until the following year, the population persists for 8-9 months each year. (5) Leaf chlorophyll concentration and the rate of oxygen production by shoot tips declined during spring, but plants that developed late in the season, and which were present during early summer, had a high rate of shoot-tip oxygen production. It is suggested that the decline in standing crop is not caused by high summer temperature. (6) The factors initiating turion development on the mature plant are not clear. Turion production accounted for 23% of the maximum standing crop. (7) Both development and decline of the standing crop had a marked effect on the nutrient dynamics of the pan. Nitrogen appeared to be the nutrient in least supply during growth, and may have been limiting. The disappearance of P. crispus in spring was associated with a release of nutrients into the water at a rate which exceeded uptake by other components of the system. These nutrients may be flushed from the pan during the summer floods.