Growth patterns of western Mediterranean seagrasses: Species- specific responses to seasonal forcing

The seasonal growth pattern of the 4 seagrass species occurring in the NW Mediterranean (i.e Cymodocea nodosa, Zostera noltii, Z. marina, Posidonia oceanica) was studied in populations growing in the same locality (Cala Jonquet, Girona, NE Spain), and thus experiencing the same seasonal (i.e. temperature and light) forcing, to evaluate the contribution of species- specific responses to seagrass growth seasonality. C. nodosa, Z. noltii, and Z. marina showed comparable growth patterns as indicated by significant correlations of growth across species (cross correlation, r > 0.54, p < 0.05). This result provided evidence of a similarity in the response of these species to seasonal forcing. The seasonal pattern of P. oceanica resembled that of the other species in shoot weight, shoot elongation, and ramet recruitment, whereas it differed in internode weight and rhizome elongation. Despite some similarities in seasonal growth patterns, the patterns were lagged by 1 to 2 mo across species, and the magnitude of seasonal growth fluctuations was species-dependent. Species-specific responses of seagrasses to climate forcing should be related to differences in the capacity of the plants to store resources and to the extent of ramet integration among species, both processes being closely related to plant size. Large seagrasses [e.g. P. oceanica), with thick and long-living rhizomes, should be able to store more photoassimilates and to transport them over longer distances than small plants (e.g. C. nodosa), with thinner and shorter-living rhizomes. Large species should, therefore, be able to grow more independently of environmental conditions than small ones. Moreover, C. nodosa showed the greatest response to temperature fluctuations whereas Z. marina growth was strongly coupled to seasonal light conditions, indicating different plant sensitivity to climate fluctuations among species. This study confirms the great variability in seagrass seasonality possible under similar seasonal forcing, and demonstrates that seagrass seasonality has both an extrinsic component, dependent on seasonal forcing of light and temperature, and an intrinsic component. The intrinsic component of seagrass seasonality likely involves a differential capacity of the species to regulate the internal resource economy which may buffer, or amplify, the external seasonal forcing.