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

Eelgrass, Zostera marina, relies upon seed dispersal for colonization of ne w habitats. The seeds are not readily transported in suspension; however, t hey have low erosion thresholds and are subject to horizontal transport as bedload at relatively low bottom shear stress. Field germination patterns s uggest that seeds rarely travel far from the point of release and quickly b ecome buried in the sediment, even in habitats where boundary-layer flows e xceed those necessary to erode seeds. In many sedimentary habitats it is li kely 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 th e entrapment of Z. mal-ina seeds. In a series of flume experiments we manip ulated the densities of the subsurface deposit-feeding polychaete Clymenell a 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. topogr aphic relief) resulting from feeding and defecation activities of subsurfac e deposit feeders can act to trap seeds. Seeds were trapped in the medium a nd high density worm treatments in greater numbers than in the low density and no worm treatments. Our findings indicate that benthic invertebrates, t hrough their modification of sediments may affect the horizontal (bedload) and, hence, vertical (burial) transport of Z. marina seeds. (C) 1999 Elsevi er Science B.V. All rights reserved.