MEPS 183:95-103 (1999)  -  doi:10.3354/meps183095

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; NH₄⁺, [NO₃^- + NO₂^-]) co-varied in the canopy water. Sediment NH₄⁺ concentration 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 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 did Z. 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 did Z. noltii, suggesting that clonal modules of C. nodosa were more physiologically 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.

KEY WORDS: Cymodocea · Zostera · Seagrass · Nitrogen · Glutamine synthetase