Waterfowl Herbivory on Potamogeton pectinatus in the Baltic Sea

Abstract

The impact of waterfowl herbivory on the macrophyte Potamogeton pectinatus was tested at six shallow sites adjacent to Asko island, in the northern Baltic proper. Protection by means of floating nets affected P. pectinatus at the three most sheltered sites but had no influence on the more wave-exposed sites. Ramet density in July of the first study year was 10-23 times higher in protected plots than in unprotected plots, at the sheltered sites. The density at two of the sites was constantly low in unprotected plots and remained much higher (10-80 times) in the protected plots throughout the 3-year study. At the third very sheltered site the impact of waterfowl was more variable. Additional studies of permanent plots and pot experiments revealed that waterfowl mainly suppressed P. pectinatus, not by grazing on shoots, but by eating subterranean tubers between late April and early June. This study suggests that waterfowl herbivory can constrain submerged macrophyte populations in shallow sheltered areas.