Death of submerged macrophytes—Actual field observations and some implications

Abstract

Using a photographic sampling method, the spatial distribution of dead submerged macrophytes was investigated in lakes Tyrifjord and Steinsfjord, in South Norway. Death incidents were observed in less than 5% of the samples ( N = 3257). Distributed sites with unstable sediments had dead plants occuring at all depths, whereas at normal sites, the incidence of dead plants was higher towards deeper waters. In this case, the estimated mortality rose sharply, close to the lower limits of depth extension for the annual Najas flexilis (Willd.) R.&S. and the perennial Isoëtes lacustris L. The statistical analysis of death incidence for the predominant macrophyte Isoëtes lacustris showed that neither depth nor light intensity accounted adequately for the observed deep-water mortality increase. However, probit-transformed mortality correlated significantly ( P < 0.001) to a hazard function, derived from light threshold statistics. Mortality magnitude was sufficient to explain the observed steep deep-water decline of I. lacustris. It is concluded that the commonly-observed double-exponential depth decline of submerged macrophytes could well be invoked by light-induced mortality.