THE NITROGEN SUPPLY TO INTERTIDAL EELGRASS (ZOSTERA-MARINA)

The uptake of ammonium and nitrate by eelgrass (Zostera marina L.) was studied in two-compartment chambers. The plants were collected in 199 2 from a population growing on a tidal flat in the S.W. Netherlands. T hey were incubated under conditions which reflected field conditions; this implied the use of natural seawater and sediment porewater as inc ubation media. In all six experiments, carried out over the course of a major part of the growing period (from July to the end of September) , ammonium appeared to be much more important as a source of nitrogen than nitrate. The largest part was taken up by the leaves: uptake of a mmonium by the leaves accounted for 68 to 92% of total plant nitrogen uptake. The uptake of nitrogen compounds by the root-rhizome system re presented only 4 to 30% of total plant uptake. Thus, at least during f lood tide, the leaves play the major role in nitrogen uptake in this i ntertidal population. During ebb tide, most of the plants are submerge d in very shallow tidepools. It is suggested that during this phase of the tidal cycle, influx of porewater ammonium into the tide-pool wate r may enable the leaves to exploit local sediment resources.