This study examines experimentally how water
movement may alter epiphyte–grazer systems in intertidal
seagrass beds. Field observations in the Sylt–Rømø
Bay (German Wadden Sea, SE North Sea) showed that
the biomass of seagrass epiphytes was highest on seagrasses
exposed to water movement, whereas at sheltered
sites the epiphyte cover was negligible. In contrast, the
seagrass shoot density and aboveground biomass was
comparably sparse and the abundance of Hydrobia ulvae
was extremely low at exposed areas, but showed maximum
values at sheltered seagrass beds. Cross transplantation
experiments and enclosure experiments
between sheltered and exposed seagrass beds showed
that adhering snails were washed off from seagrasses
soon after transplantation into an exposed seagrass bed,
and epiphytes started to grow. After 4 weeks the epiphyte
biomass was similar to the that of the adjacent
exposed seagrass bed. When heavily epiphytised seagrasses
were transplanted from exposed into sheltered
areas, the epiphytes were completely grazed down by
immigrating snails within a week. Experiments carried
out by means of an in situ ‘‘three-current-flume’’, modifying
the entire current velocity, showed that snail
density was significantly negatively correlated with increasing
current velocity, whereas epiphyte biomass
showed a significant positive correlation with current
speed. These results suggest a cascading impact of hydrodynamics
on an epiphyte–grazer system in intertidal
seagrass beds, by directly affecting the density of grazers
and indirectly leading to enhanced epiphyte growth,
thereby inhibiting seagrass development. Additionally it
shows that cascading effects within the trophic web
cannot only be triggered by biotic interdependencies, but
can also be caused by physical factors.