Structure and growth dynamics of Cymodocea nodosa meadows
Keywords:seagrasses, primary production, phenotypical plasticity, environmental variability, water temperature
The seasonal changes in the structure and growth dynamics of a Cymodocea nodosa meadow off the island of Ischia (Tyrrhenian Sea) were studied from July 1988 to August 1989 using leaf and rhizome marking methods. High levels of leaf production (3.1 g dw m-2 d-1) significantly related to water temperature regimes, were observed. The number of new leaves per year (16 leaves y-1), the leaf Plastochrone Interval (23 days) and the life span of the single leaves (from 2 to 6 months) were also calculated. Relevant yearly fluctuations of the leaf canopy, representing about 20% of the total meadow biomass, testify the strong seasonal variability of leaf phenological parameters and shoot density (the latter, between 925 ± 323 and 1925 ± 267 shoots • m-2). On the other hand, a constant and well developed layer of rhizomes and roots is present throughout the year (80% of total biomass), with an annual rhizome elongation of about 30 cm. In spite of the temporal variability of the above-ground compartment (CV=55%), the below-ground portion represents the conservative compartment of the meadow (CV=7%). Nevertheless, the remarkable number of seeds present in this meadow (up to 2112 m-2), does not seem to effect the stability of the system through the sexual reproduction. Although similar growth trends have been reported for C. nodosa meadows from different geographical areas and habitats (eutrophic zones, confined shallow waters, estuaries), remarkable differences may be found in the highest and lowest values of growth rate. This comparison highlights the ability of this species to grow in different habitats and that growth process seems to be amplified by a high influence of environmental constraints. Moreover, differences from P. oceanica, both in the growth rate and reproductive patterns, are identified in order to explain the dynamics of these vegetated systems and their role in the Mediterranean basin.
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