Quantifying the nutrient import from roosting waterbirds to eutrophicated lakes

By Tony Fox, Department of Ecoscience, Aarhus University, Denmark

A major strength of the Water Framework Directive (WFD) is its requirement for European Union Member States to reduce external nutrient loading in lakes in moderate/poor/bad ecological condition and restore them to good status. This is crucial for lakes degraded by nutrient runoff from agriculture or industry, where potentially reversible eutrophication has severely impacted their ecological functions and services.

Photo by Martin Birchall / WWT

The Directive allows exemptions where other sources affect a lake’s nutrient status, such as day- or night-time roosting waterbirds that deposit organic material from food sourced outside the lake. This is common for species like swans, geese, and gulls, which feed on farmland by day and roost on water at night, adding significant nitrogen and phosphorus to the waterbody.

Several poor-quality Danish lakes had previously been exempted (and more were proposed) from WFD criteria in the past due to suspected nutrient contributions from waterbirds, but often without supporting evidence. In our recent analysis (Clausen et al. 2024), we estimated the nutrient contributions from roosting waterbirds compared to total nutrient loadings in four brackish lakes (each 43-491 hectares in extent) at Vejlerne in northern Jutland, an important Ramsar site and EU Birds Directive Special Protection Area. Despite their conservation importance, increasing nutrient levels in three of these lakes have reduced their water quality. To address this, we investigated the nutrient contributions from over 30,000 roosting waterbirds using these areas.

We combined morning and evening counts of different species of roosting waterbirds with intake and faecal output models to estimate their nutrient contributions to the waterbodies. In three lakes, roosting birds (mostly geese) contributed 0.2–0.4% of nitrogen (0.3–1.2 kg N/ha/year) and 0.6–2.0% of phosphorus (0.04–0.12 kg P/ha/year). However, at a fourth clearwater lake, in excellent ecological condition, birds contributed up to an estimated 14% of nitrogen and 76% of phosphorus, though overall nutrient levels there remained low. These results show that waterbirds were not the cause of increasing nutrient levels in the three lakes with poor water quality, which likely stem from the upstream agricultural catchment. The fourth lake, despite receiving higher nutrient proportions from birds, maintains a natural balance. Herbivorous swans, like Whooper Swans Cygnus cygnus, could potentially add nutrients when they feed on fields and return to lakes at night as is common in Denmark, although this was not the case in our Vejlerne study.

Overall, these results showed that waterbirds were not the cause of increasing nutrient levels in the three lakes with poor water quality, which likely originated from other sources. Despite the proportionately high contributions at the fourth lake, nutrient levels were generally low and had no effects on good water quality there. These findings confirm the need for case-by-case assessments of avian nutrient contributions before exempting lakes from WFD conditions due to supposed natural loading from waterbirds.

The full article can be found here

Reference

Clausen, P., Lauridsen, T.L., Pedersen, C.L., Nielsen, H.H., Jeppesen, E., Søndergaard, M., Schreven, K., Nolet, B.A., Madsen, J. & Fox, A.D. (2024) Are increasing roosting waterbird numbers responsible for eutrophication of shallow lakes? Examples from a Danish Ramsar site. Hydrobiologia doi: 10.1007/s10750-024-05475-9