Variation in size and chemical composition of seeds from the seagrass Zostera marina—Ecological implications

• We evaluate the range, origin and implication of eelgrass seed size variations.
• Variation originated mostly from differences within rather than between populations.
• Seed dry weight was correlated with seed reserves.
• Seed dry weight decreased with seed dispersal potential.
• Intraspecific size variations assure patch maintenance and colonisation of new areas.

Seed size correlates with several plant traits spanning from seedling emergence depth or survival to seed dispersal potential. The ecological importance of seed size has been widely recognised for terrestrial plants, but few studies address this for marine angiosperms. With focus on the seagrass Zostera marina, we (1) relate seed size (length, diameter, fresh and dry weights) to its main reserves: starch, proteins and minerals, (2) quantify inter- and intra-specific population differences in seed size and composition, and (3) determine the effect of size on seed dispersal. We found that the seed size variation originated mostly from differences within rather than between populations. Seed dry weight (DW) was the most variable morphological characteristic across populations (0.64–6.03 mg DW). DW was positively correlated with the amount of reserves (p < 0.001). An eelgrass seed contained on average 52% DW starch, 13% DW proteins and 4% DW minerals. Interestingly, the relative contents of minerals and proteins were higher in smaller seeds, whereas starch content of seeds >2 mg DW remained constant. Seed settling velocity, a proxy for seed dispersal, increased with DW from 1.4 to 8.6 cm s−1. Seeds of different quality may disperse differently: heavy seeds stay close to the parent plant and contain relatively more carbon to compensate for low light regime in the adults’ shadow, whereas smaller seeds spread further and contain relatively high nutrient content to establish rapidly in more open areas. Intraspecific seed size variations in the seagrass Z. marina may therefore assure both patch maintenance and colonisation of new areas.