Effect of aquatic vegetation on nutrient retention in ditches, with a main focus on denitrification.
02 / 2007 - 12 / 2011
The main objective of this study is to analyze the effect of aquatic vegetation on permanent and temporary removal (retention) of nutrients in ditches, with a special emphasis on denitrification. Since the Dutch ditches undergo regular maintenance, like mowing (removing all aquatic vegetation) and dredging (removing sludge), the effects of maintenance will also be taken into account. The research will focus on three research questions: 1) How do different aquatic vegetation types (rooted submerged, non- rooted submerged, floating, or emergent vegetation) affect the removal of nutrients (N and P) in ditches? 2) Which specific mechanisms regulate the differences in denitrification rates observed in the field? 3) How do maintenance events affect nutrient removal in ditches? For each of those research questions I have specific hypotheses to test: For research question 1: N removal by denitrification will be highest when rooted submerged vegetation is dominant, since it provides most habitat for nitrifying and denitrifying bacteria and enhances denitrification in the sediment. For research question 2: Available substrate for biofilms (sediment and aquatic vegetation), oxygen, nutrient levels and temperature regulate denitrification rates. For research question 3: Nutrients are removed from the system directly by harvesting plant biomass. Uptake of nutrients from water and sediment by macrophytes is reduced. Biofilms are disturbed and (partly) removed by ditch maintenance, causing a decrease in denitrifying organisms. Disturbing of ditch sludge causes anoxic conditions and resuspension of organic matter, favouring the denitrification process. Effects of maintenance on nutrient removal processes are acute but diminish with time after maintenance until the macrophyte community has matured. A second objective of the project is to develop a method for measuring denitrification in the field, using Membrane Inlet Mass Spectrometry.