To Mow or Not to Mow: An ecological and societal perspective on submerged aquatic plant growth

Abstract

To mow or not to mow is the question managers of shallow freshwater ecosystems often have to ask themselves these days. On the one hand, they want submerged vegetation in the ecosystem because these plants provide many important functions and services. On the other hand, massive growth of these submerged plants is a major problem for people using the ecosystems worldwide. Furthermore, very dense stands of just a few species may also impair the beneficial ecosystem functions submerged plants generally have. Therefore, submerged plants are often managed when growing to nuisance proportions, with mechanical cutting and harvesting being one of the most used management methods. However, this type of management of the aquatic vegetation is often not successful or is short-lived. Because mowing is expensive and time consuming, some might wonder whether the plants should be mown at all. In this thesis, I gathered scientific knowledge on which problems are caused by nuisance submerged aquatic plants, which plant characteristics can be used to separate nuisance from non-nuisance vegetation, which environmental factors affects plant tissue elemental composition, plant growth, and the plant’s response to cutting. I discuss how this knowledge can further our understanding of the nuisance problems and how it can aid managers in tackling problems caused by rapid growth of submerged plants, while maintaining the important ecosystem functions these plants provide. Last, I investigated whether the nuisance submerged plant species can be used for beneficial applications, such as nutrient recovery. I conclude that the best method to tackle nuisance growth of submerged aquatic plants is to tackle the bottom-up processes enabling fast submerged plant growth, in particular the abiotic factors enabling high productivity of the ecosystem. However, as this may not always be possible, top-down management such as mowing, can alleviate the problems caused by these plants if tailored to the ecosystem, plants and location at hand. Harvesting parts of the plants directly reduces standing biomass and vegetation height, removes nutrients, creates a valuable resource and if applied well, maintains the plants important functions in shallow freshwater ecosystems.