The aim of ongoing studies was to expand the non-destructive monitoring of Isoëtes lacustris (Vöge 2004) resulting in a description of the vitality and structure of the population. The influence of the environment on the development of the plant was the subject of extended studies carried out between 1995 and 2014 in more than 100 lakes throughout Europe. The environment was characterized through conductivity, pH and Secchi depth of the lake water and the accompanying submersed plant species, as well as the plant performance through the mean number of leaves of 20 sporogenous plants. The counting of the leaves was performed whilst diving. The vitality level was defined based on Vöge (2004) and related to the environmental conditions; these are referred to in the context. The inventory of the accompanying plant species may reflect a changing environment. The method is demonstrated through case examples. A highly significant correlation between the mean and the smallest number of leaves of sporogenous plants enables separating juvenile from adult plants. Finally, recent results show a relationship between the vitality level and the structure of the population.