Effects of herbivorous birds on intertidal seagrass beds in the northern Wadden Sea

During autumn migration (September to December), brent geese (Branta b. ber nicla) and wigeon (Anas penelope) feed on the seagrass Zostera noltii in th e nearshore, upper tidal zone leeward of the island of Sylt (eastern North Sea). To graze on leaves and shoots above the sediment and on rhizomes and roots below, these birds reworked the entire upper 1 cm layer of sediment e ight times within this 3-month period. In addition, brent geese excavated p its 3-10 cm deep by trampling in order to feed on below-ground phytomass. A bout 12% of the seagrass beds became pitted to an average depth of 3.5 cm. Using net exclosures, it was estimated that birds removed 34 g dry weight m (-2) of above-ground and 28 g of below-ground phytomass. This corresponds t o 45% of the phytomass in September. Of the overall loss of phytomass from September to December, 63% was caused by birds. Roughly half of the leaves fell off anyway until December and the other half were taken by the birds. Below the ground, phytomass remained almost constant where birds were exclu ded, while with birds phytomass of rhizomes and roots was halved. In spite of this strong effect, in the next vegetation period the blade density was lower at former exclosure sites compared to the ambient seagrass bed. The u nderlying process seems to be a self-inhibition of dense overwintering seag rass by mud accretion. Assuming our experimental results can be scaled up t o the entire seagrass bed, we hypothesize that in the sheltered upper inter tidal zone, seasonal erosion caused by herbivorous geese and ducks is neces sary for the persistence of Z. noltii.