Nitrogen acquisition, storage, and use by the co-occurring Mediterranean seagrasses Cymodocea nodosa and Zostera noltii

Shoot density, shoot, root, and rhizome tissue biomass and N content, and N assimilation by leaf and root tissues of the 2 co-occurring seagrasses Cym odocea nodosa and Zostera noltii were measured over the course of a year at a central Mediterranean site. Concentrations of dissolved inorganic nitrog en (DIN; NH4+, [NO3- + NO2-]) co-varied in the canopy water. Sediment NH4concentration varied within the top 10 cm of the sediments, while those of [NO3- + NO2-] were virtually invariant. C. nodosa and Z. noltii appeared to contribute to the replenishment of the sediment DIN reservoir as growth de clined and senescence ensued in the fall; the peak in plant-based N precede d the peak of sediment DIN by 1 to 2 mo. C. nodosa had a June peak in gluta mine synthetase (GS) activity in led tissue, and showed greater variation o ver the course of the study than did Z. noltii, for which there was a Febru ary peak in shoot GS activity. The leaves, rhizomes, and roots within each species exhibited different patterns of tissue N content over the course of the year, indicating different strategies of N storage and subsequent use. Within a species, assimilated N was first allocated to leaves, then rhizom es and roots. Leaves, in addition to rhizomes, appear to have a N storage f unction. C. nodosa maintained high tissue N levels for longer periods than did Z. noltii, suggesting that clonal modules of C. nodosa were more physio logically integrated than modules of Z. noltii. The average potential for N assimilation through Z. noltii tissues was estimated to be roughly 43 % of the total for C. nodosa and Z. noltii.