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

Abstract

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 Cymodocea nodosa and Zostera noltii were measured over the course of a year at a central Mediterranean site. Concentrations of dissolved inorganic nitrogen (DIN; NH,', [NO< + NOT]) co-varied in the canopy water. Sediment NH,+ concentration varied within the top 10 cm of the sedunents, 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 declined and senescence ensued in the fall; the peak in plant-based N preceded the peak of sediment DIN by 1 to 2 mo. C. nodosa had a June peak in glutamine synthetase (GS) activity in leaf tissue, and showed greater variation over the course of the study than &d 2. noltii, for which there was a February 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 rhizomes and roots. Leaves, in addition to rhizomes, appear to have a N storage function. C. nodosa maintained high tissue N levels for longer periods than &d 2. noltii, suggesting that clonal modules of C. nodosa were more physiologically integrated than modules of Z. noltii. The average potential for N assirmlation through Z. noltii tissues was estimated to be roughly 43% of the total for C. nodosa and 2. noltii.