Aarhus Universitets segl

No. 513: Monitoring of wetlands 2018 – 2019 – 2020 - 2021

Hoffmann, C.C., Audet, J., Ovesen, N.B. og Kjeldgaard, A. 2021. Overvågning af vådområder 2018 - 2019 – 2020 - 2021. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 80 s. - Videnskabelig rapport nr. 513.


In the years 2018-2019 and 2020-2021, twelve re-established wetlands and shallow lakes were monitored with the aim of documenting the environmental effects following restoration. The monitoring included eight shallow lakes: Lake Torsted (small lake surrounded by large wet meadows) and the lakes Tissing Vig, Slivsø, Mollerup, Hundsø Nord, Grynderup (also surrounded by large wet meadows), Mjels and Bundsø, two river valley projects: Tolstrup Meadows and Rævs River, and two meadow–fen areas: Feldsted Kog and Gundesbøl River.

The measurements included stream discharge at the inlet and outlet of the lakes and wetlands as well as water sampling. The water samples were analysed for nitrate-N, total nitrogen, phosphate-P and total P. For each site, the monthly transport of water and nutrients was calculated for inlets and outlets, and also yearly water and nutrient balances were calculated.

The results of the mass balances revealed that nitrate removal varied from 34 to 320 kg NO3- -N ha-1 project area year-1. Removal of total nitrogen ranged between 22 and 324 kg TN ha-1 project area year-1. For Rævs River, two sets of calculations exist, of which one showed nitrogen loss.

The mass balances for phosphorus in the monitored areas varied between net retention and net loss. Thus, retention of phosphate varied between -2.16 and 5.36 kg PO43- -P ha-1 project area year-1 (- means net loss of phosphate). Retention of total phosphorus varied from -3.00 to 3.35 kg TP ha-1 project area year-1.

The results presented in earlier monitoring reports are included in the current report in order to conduct an overall analysis of all results from the entire monitoring program (2015-2021).


Nitrogen removal in the twelve monitored areas was highly variable. Seven areas removed between 7 and 45 kg TN ha-1 year-1, three areas removed 185 - 346 kg TN ha-1 year-1, one area removed 100 kg TN ha-1 year-1 and one area was discarded due to uncertainty in the calculations.

There was retention of TP in eight of the monitored areas and loss of TP in three areas. Only two areas showed loss of PO43- -P. Including all monitored areas since 2015 revealed that lakes and river valleys had the highest nitrogen removal with a mean of 132 - 167 kg TN ha-1 year-1, respectively, while fens only removed 51 kg TN ha-1 year-1. The river valleys projects showed the highest retention of TP with a mean of 5.67 kg TP ha-1 year-1, while the lakes retained 0.68 kg TP ha-1 year-1 and the fens had a net loss of  0.13 kg TP ha-1 year-1.