Effect of Vallisneria americana (L.) on community structure and ecosystem function in lake mesocosms

Submerged aquatic vegetation is known as a key structural component and reg ulator in ecosystems. In this mesocosm study, we examine community- and sys tem-level responses to the presence of Vallisneria americana (L), a deep-ro oted macrophyte. Phytoplankton, bacteria and filamentous algal biomasses we re significantly lowered in the presence of V. americana. In addition, meso cosms with macrophytes had significantly reduced porewater phosphate and ir on, water column dissolved organic carbon and total suspended solids, but e levated sediment redox. All mesocosms were net autotrophic (gross primary p roduction/respiration > 1). Compared to the macrophyte treatments, the cont rol mesocosms had lower diel net primary production (NPP) midway through th e experiment (d 16), but at the end of the experiment (d 36), the controls had the higher values, presumably due to increased filamentous algae. NPP a nd NPP/R were constant in the macrophyte treatments, whereas NPP/R increase d significantly from middle to end of the experiment in the controls. We sh ow that community and system-level responses to the presence of V. american a have significant consequences on system structure and function.