Vertebrate herbivore exclosure study of aquatic macrophytes in the lake Lauwersmeer

The exclosure study preventing bird herbivory on macrophyte biomass in summer and belowground tuber biomass in winter by Bewick's swans was established in September 2003 and terminated in September 2007. Four different treatment plots (6 x 6 m) were randomly assigned within each of 8 blocks of 12 x 12 m. The treatments were: a control plot, a winter exclosure consisting of mesh wire at the sediment surface (September to March), a summer exclosure from March until September (mesh wire fences with bird netting), and a combination of both treatments. Data were collected on Potamogeton pectinatus belowground tuber biomass in March and September from September 2003 until September 2007. At sampling events twelve 7 cm diameter cores were probed in the sediment evenly spaced within plots. Tubers were enumerated, dried (72 hrs at 60 degrees Celsius) and weighed. Aboveground biomass of P. pectinatus was sampled each summer three times starting mid June with five week intervals. On each occasion three out of 36 within plot squares (75 x 75 cm) were sampled by placing a core with diameter 50 cm over the submersed vegetation and harvesting all aboveground material by hand. Some samples were sorted to species level, but all aboveground samples consisting almost entirely of Potamogeton pectinatus were dried for 72 hrs. at 60 degrees Celsius and weighed.