Impact of light climate history on seasonal dynamics of a field population of Potamogeton pectinatus L. during a three year period (1986–1988)

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The impact of light climate during the growing season and light climate history on the growth and survival of a Potamogeton pectinatus L. population was studied in a eutrophic shallow lake, Lake Veluwe (Netherlands), during the period 1986–1988. Four different light conditions were created in an experimental setup by manipulating the photon flux density using artificial shading (three levels of artificial shading and one control situation without artificial shading), during the growing season of 1986. During the growing season of 1987, part of the P. pectinatus vegetation in the experimental setup was artificially shaded in the same way as in 1986, while another part was not shaded. No artificial shading was applied at all during the growing season of 1988. Growth conditions for P. pectinatus were less favourable in 1987 than in 1986. On the whole, above- and below-ground biomass values were lower, the growing season was shorter, maximum above-ground biomass was lower and was reached earlier in the season, and net tuber production was lower in 1987 than in 1986 in the control situations. In both years, the above- and below-ground biomass values were lower, the growing season was shorter and maximum above-ground biomass was lower with higher levels of shading. The impact of shading on relative above-ground biomass development was similar in 1986 and 1987, whereas the impact of shading on relative tuber bank size at maximum above-ground biomass was more pronounced in 1987 than in 1986 in the experimental areas which were shaded both seasons. Mean individual tuber ash-free dry weight was lowest at the highest level of shading in both years. It is concluded that light climate is a dominant factor in controlling the biomass and tuber bank dynamics of a P. pectinatus population in Lake Veluwe. Additionally, water quality and meteorological characteristics are involved. At the highest level of shading, the photosynthetic tissue was able to sustain tuber growth and maintenance of the vegetation during a relatively short period only. As a consequence, the vegetation sloughed early in the growing season and the tuber bank size was smallest. The maximum above-ground biomass was positively correlated with the size of the hibernated tuber bank. Therefore, the biomass development in one season is at least partly determined by the light conditions of a previous growing season(s) through the size of the hibernated tuber bank. The Lake Veluwe vegetation has the potential to recuperate from negative shading effects on above-ground biomass and tuber bank size under field conditions. This may require more than one growing season, depending on actual growth conditions and size of the hibernated tuber bank.