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Cascading effects of hydrodynamics on an epiphyte-grazer system in intertidal seagrass beds of the Wadden Sea
Schanz, A.; Polte, P.; Asmus, H. (2002). Cascading effects of hydrodynamics on an epiphyte-grazer system in intertidal seagrass beds of the Wadden Sea. Mar. Biol. (Berl.) 141: 287-297
In: Marine Biology. Springer: Heidelberg; Berlin. ISSN 0025-3162; e-ISSN 1432-1793, more
Peer reviewed article  

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    This study examines experimentally how watermovement may alter epiphyte-grazer systems in intertidalseagrass beds. Field observations in the Sylt-RømøBay (German Wadden Sea, SE North Sea) showed thatthe biomass of seagrass epiphytes was highest on seagrassesexposed to water movement, whereas at shelteredsites the epiphyte cover was negligible. In contrast, theseagrass shoot density and aboveground biomass wascomparably sparse and the abundance of Hydrobia ulvaewas extremely low at exposed areas, but showed maximumvalues at sheltered seagrass beds. Cross transplantationexperiments and enclosure experimentsbetween sheltered and exposed seagrass beds showedthat adhering snails were washed off from seagrassessoon after transplantation into an exposed seagrass bed,and epiphytes started to grow. After 4 weeks the epiphytebiomass was similar to the that of the adjacentexposed seagrass bed. When heavily epiphytised seagrasseswere transplanted from exposed into shelteredareas, the epiphytes were completely grazed down byimmigrating snails within a week. Experiments carriedout by means of an in situ ‘‘three-current-flume’’, modifyingthe entire current velocity, showed that snaildensity was significantly negatively correlated with increasingcurrent velocity, whereas epiphyte biomassshowed a significant positive correlation with currentspeed. These results suggest a cascading impact of hydrodynamicson an epiphyte-grazer system in intertidalseagrass beds, by directly affecting the density of grazersand indirectly leading to enhanced epiphyte growth,thereby inhibiting seagrass development. Additionally itshows that cascading effects within the trophic webcannot only be triggered by biotic interdependencies, butcan also be caused by physical factors.

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