Annual variation in leaf photosynthesis and leaf nutrient content of four Mediterranean seagrasses

We examine here the annual variation in leaf photosynthesis and nutrient status of four temperate seagrass species: Posidonia oceanica, Cymodocea nodosa, Zostera noltii, and Zostera marina. We also examine the relationship between leaf photosynthesis and seasonal plant growth and productivity. The four seagrass species examined co-occurred in a shallow protected cove on the NE Spanish Mediterranean coast. The results presented give evidence of large annual variation in the leaf physiological status of these four seagrasses, as well as the occurrence of significant differences in the seasonal pattern among species. We found two contrasting seasonal patterns characterized by optimal leaf photosynthesis in summer (represented by C. nodosa), and winter-autumn (represented by P. oceanica and Z. noltii). These contrasting patterns corresponded with a differential plant dependence on climate fluctuations (described by temperature and irradiance) vs. speciesspecific nutrient status (described by leaf nutrient content). As plant nutrient status results from speciesspecific differences in nutrient requirements, nutrient uptake efficiency, and internal nutrient economy, we conclude that the variation among species found in leaf nutrient content derives from species-specific responses to environmental forcing. Consequently, we can infer that an important fraction of the seasonal variability that seagrasses show may be related to species-specific responses. We also found significant differences among species in the coupling between leaf photosynthesis and plant growth and productivity. Both aspects, speciesspecific responses to environmental forcing and speciesspecific coupling between leaf photosynthesis and plant growth may result in differential seasonal patterns of growth and productivity among seagrass species.