NITROGEN DYNAMICS OF EELGRASS ZOSTERA-MARINA DURING A LATE SUMMER PERIOD OF HIGH GROWTH AND LOW NUTRIENT AVAILABILITY

Nitrogen acquisition and dynamics of eelgrass Zostera marina L. were assessed during a late summer period of high plant growth and low nitrogen availability in the water column. Nitrogen uptake through leaves and roots, translocation within the plant, and the role of nitrogen reclamation from older plant parts were estimated using N-15-techniques and compared to nitrogen incorporation in growing tissues. Total eelgrass biomass was 700 g (dry wt) m-2. Relative growth rate was 0.022 d-1 and eelgrass production was 5.6 g C m-2 d-1. Production was high in spite of low nitrogen availability in the water column and low internal nitrogen contents of plants. Nitrogen incorporation in growing tissues was 0.155 g N m-2 d-1, and the molar ratio of incorporated C to N (C:N) was 42 or 6-fold higher than the average ratio reported for phytoplankton. However, due to efficient translocation of nitrogen from old to young tissues, C:N in the most actively growing leaf was only 2-fold higher than that of phytoplankton. Reclamation of nitrogen from old tissues supplied 12% of total nitrogen incorporation and 88% was supplied by external media uptake. Leaf and root uptake were equally important in spite of much higher nitrogen availability in the sediment porewater compared to that of the water column. Eelgrass seems well adapted to nitrogen-poor environments because it conserves nitrogen within the plant, efficiently takes up nitrogen from both the water column and sediment, maintains high growth rates in spite of low internal nitrogen contents.