Isoetids are small evergreen submerged macrophytes commonly inhabiting the littoral zone of pristine softwater lakes. In NW Europe, this vegetation community has suffered a substantial reduction as a consequence of habitat deterioration due to acidification, eutrophication, alkalinisation and/or water table regulation. The objective of this thesis was to determine the main factors affecting isoetid vegetation, with emphasis on not only surface water but also porewater and sediment characteristics, and to investigate how these factors affect isoetid eco‐physiology.
The identification of the main factors affecting isoetid vegetation was accomplished by an extensive data collection in NW European softwater lakes, while in‐situ experiments and lab experiments were used to test how these factors (mainly oxygen availability) affected isoetid eco‐physiology (i.e. photosynthesis, germination, growth, survival, biomass).
The combined studies showed that not only surface water but also porewater and sediment characteristics play a crucial role on isoetid vegetation. Low sediment oxygen availability, in particular, was one of the main factors adversely affecting isoetid vegetation. Oxygen availability in the sediment depended on both sediment mineralisation rates (oxygen demand) and isoetid performance (oxygen production). Healthy dense isoetid populations flourish on sediments with refractory organic matter, while scattered populations have difficulties to survive on sediments with highly labile organic matter.
The adverse impacts of high sediment oxygen demand on isoetids have substantial management implication. As a first step, the high sediment oxygen demand must brought down and this can be done by applying existing management tools such as removal of the accumulated organic top layer. Then, to preserve the low sediment oxygen demand, the next step must involve a successful recolonization of isoetids because the extensive radial oxygen loss from the roots maintains the oxic conditions of the sediment. This study has shown that the process of recolonization may be stimulated in two ways: i) by increasing seedling survival from an existing seed bank by lowering the water table or ii) by transplanting adult plants if there is no viable seed bank.