The influence of hydrodynamics and ecosystem engineers on eelgrass seed trapping

Propagule dispersal is an integral part of the life cycle of seagrasses; important for colonisingunvegetated areas and increasing their spatial distribution. However, to understandrecruitment success, seed dispersal and survival in habitats of different complexity remainsto be quantified. We tested the single and synergistic effects of three commonly distributedecosystem engineers—eelgrass (Zostera marina), oysters (Magellana gigas) and blue mussels(Mytilus edulis)—on trapping of Z. marina seeds in a hydraulic flume under currents.Our results suggest that seed retention increases with habitat complexity and further revealinsights into the underlying mechanisms. In eelgrass canopy, trapping occurred mostlythrough direct blocking of a seed’s pathway, while trapping in bivalve patches was mainlyrelated to altered hydrodynamics in the lee side, i.e. behind each specimen. With increasingflow velocity (24–30 cm s-1 in eelgrass canopy, 18–30 cm s-1 in bivalve patches), modificationsof the sediment surface through increased turbulence and erosive processes becamemore important and resulted in high seed trapping rates. Furthermore, we show that whilemonospecific patches of seagrass and bivalves had different trapping optima depending onflow velocities, intermixing resulted in consistently high trapping rates throughout the investigatedhydrodynamic gradient. Our results highlight the importance of positive interactionsamong ecosystem engineers for seed retention and patch emergence in eelgrass.