DYNAMIC-RESPONSE OF THE SUBMERGED MACROPHYTE, ISOETES LACUSTRIS, TO ALTERNATING LIGHT LEVELS UNDER FIELD CONDITIONS

Using a transplant approach, we studied the response of the submerged macrophyte, Isoetes lacustris L., to natural alterations of subsurface irradiance. In 1992, potted Isoetes plants were systematically moved along a 1-6 m range of water depths and their performance compared wit h that of stationary plants during 1992-1993. We found that the Isoete s 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 le vel. 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 rat es. Plants exposed to adversely low irradiance for 1 month partly reco vered when brought to more favourable depths, but elevated mortality w as present afterwards even for plants subsequently experiencing shallo w-water irradiance. Damage was detectable for more than 1 year followi ng exposure to low light conditions. The population dynamics of this p erennial species evidently reflect critical short-term incidents of th e past, making it difficult to interpret ecological relationships base d only on the present-day situation.