Abstract
Using a transplant approach, we studied the response of the submerged macrophyte, Isoëtes lacustris L., to natural alterations of subsurface irradiance. In 1992, potted Isoëtes plants were systematically moved along a 1–6 m range of water depths and their performance compared with that of stationary plants during 1992–1993. We found that the Isoëtes plants, subject to low levels of daily insolation, declined rapidly. Massive shoot deaths occurred within 2 months under the insufficient irradiance regimes which prevailed beyond 3 m depth at median water level. Hazard rates correlated to low insolation according to a Weibull probability law, and the hazards rose sharply below a daily insolation of approximately 2 mol m −2. Low temperature caused lower hazard rates. Plants exposed to adversely low irradiance for 1 month partly recovered when brought to more favourable depths, but elevated mortality was present afterwards even for plants subsequently experiencing shallow-water irradiance. Damage was detectable for more than 1 year following exposure to low light conditions. The population dynamics of this perennial species evidently reflect critical short-term incidents of the past, making it difficult to interpret ecological relationships based only on the present-day situation.
Published Version
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