Submerged vegetation colonizes behind artificial wave shelter after a 10-year time-lag and persists under high grazing pressure by waterbirds

Construction of artificial wave shelters is a promising measure to stimulate submerged vegetation in large wind exposed lakes. Here we tested whether the construction of shelter results in the colonization of submerged vegetation and whether grazing by waterbirds hampers vegetation development under those sheltered conditions. We studied the effect of breakwaters that were constructed between 1992 and 1996 in the large (695 km2), wind exposed and turbid Lake Markermeer, The Netherlands. We used monitoring data to evaluate the development of submerged vegetation and its relation with the abundance of herbivorous birds and conducted a field grazer exclosure experiment to determine the effect of grazers on the newly established vegetation. We found that in the sheltered area, a dense charophyte dominated vegetation developed over 16 years, while submerged vegetation remained very sparse outside the breakwaters. The area also attracted many herbivorous birds, especially molting Mute swans during summer. After the breakwaters had been completed, 10 years passed before the development of the charophyte vegetation started. This delay was probably not caused by bird grazing, but more likely due to unfavorable light conditions in the first decade. The exclosure experiment confirmed that while grazing reduced macrophyte biomass by 50% and plant height by 45%, it did not affect vegetation cover, which remained high (90–95%) throughout summer. The water depth in most of the study area exceeded the depth range at which Mute swans prefer to forage and this probably prevented overgrazing. We conclude that building artificial wave shelters is an effective measure to stimulate submerged vegetation in large wind exposed lakes.