Morphometric variations as acclimation mechanisms in Zostera noltii beds

The use of morphometric variations as acclimation mechanisms was tested in natural populations of the intertidal seagrass Zostera noltii in Ria Formosa (Algarve, southern Portugal). To achieve the objective, two populations of Z. noltii were studied; a bed exposed and physically impacted by the artificial opening of a new inlet, and a second bed protected from dynamic impacts. At each meadow, morphometry and other plant-key variables such as tissue composition, photosynthetic parameters and dynamic properties (i.e. modules and whole plant growth rates) were monitored. After severe physical impacts in winter 1998, the newly appeared plants in the exposed meadow showed permanent morphometric changes. This new morphotype (small-leaved morphotype, SLM) had shorter (65%) and narrower leaves (25%) with longer internodes (4 times), than the large-leaved morphotype (LLM) found in the protected meadow or in the exposed meadow prior to the large disturbance. The SLM also had a larger non-photosynthetic biomass fraction, but increased photosynthetic capacity compared with the LLM. The morphometric and physiological changes were related to the fast recovery (<6 months) exhibited in the exposed meadow. In the protected meadow, morphometric changes (minor and short term) were associated with seasonal patterns. After the recovery, exposed and protected meadows gave similar production estimates supporting the hypothesis that a new stable state had been established in the exposed meadow. From these results, it can be concluded that Z. noltii has a remarkable phenotypic plasticity supported by coordinated changes in dynamic variables (i.e. leaf and internodal elongation rates and plastochrone index). Phenotypic plasticity seems to be the main mechanism by which this species is able to cope with short and long term environmental changes.