Effects of a deposit-feeding invertebrate on the entrapment of Zostera manna L. seeds

Eelgrass, Zostera manna, relies upon seed dispersal for colonization of new habitats. The seeds are not readily transported in suspension; however, they have low erosion thresholds and are subject to horizontal transport as bedload at relatively low bottom shear stress. Field germination patterns suggest that seeds rarely travel far from the point of release and quickly become buried in the sediment, even in habitats where boundary-layer flows exceed those necessary to erode seeds. In many sedimentary habitats it is likely that the activities of benthic and demersal organisms will affect the horizontal movement and burial of seeds, thus providing an explanation for the patterns of seedling establishment in previously reported experiments. We investigated the effects of a common animal in estuarine sediments on the entrapment of Z. marina seeds. In a series of flume experiments we manipulated the densities of the subsurface deposit-feeding polychaete Clymenella torquata (Low: 96 worms m-2 Medium: 192 worms m-2 High: 288 worms m -2) and related trapping of seeds to worm density and bioturbation rates. The results suggest that modifications to the sediment surface (i.e. topographic relief) resulting from feeding and defecation activities of subsurface deposit feeders can act to trap seeds. Seeds were trapped in the medium and high density worm treatments in greater numbers than in the low density and no worm treatments. Our findings indicate that benthic invertebrates, through their modification of sediments may affect the horizontal (bedload) and, hence, vertical (burial) transport ofZ. marina seeds.