Plant replacement trend in soft-water lakes with isoetids

A number of small, shallow inland lakes in the Northern Hemisphere are acidic, soft-water water bodies, mostly fed by precipitation. Some of these lakes are home to Lobelia dortmanna and other aquatic macrophytes from the isoetid group. The present study identifies a plant species replacement trend along the main environmental gradient in such lakes, i.e. pH of water.

In 70 lakes covered by the study and ordered along the gradient of increasing alkalinity, bryophytes gradually decline. Their decreasing frequency is 86.8% (pH 4.0), 32.5% (pH 6.5) and <10% (pH 6.6–9.5). At the same time, the frequency of vascular plants increases from 13.2% in strongly acidic lakes up to 92.8% in alkaline ones. The majority of isoetids usually occur at pH 5.5–6.7, but one of them (Littorella uniflora) often (frequency 40.5%) occurs also in eutrophic, alkaline lakes (pH 9.0–9.5), where it forms large populations. In lakes ranked according to the decreasing alkalinity gradient, bryophyte populations have been found to be smaller, while vascular plants become more abundant. The decreasing water acidity leads to partial or total elimination of bryophytes, which are replaced by eutrophilic vascular plants, frequently accompanied by some stonewort species.