Leaf nutrient resorption, leaf lifespan and the retention of nutrients in seagrass systems

Abstract

Efficient nutrient resorption from senescing leaves, and extended leaf life spans are important strategies in order to conserve nutrients for plants in general. Despite the fact that seagrasses often grow in oligotrophic waters, these conservation strategies are not strongly developed in seagrasses. A compilation of literature data on nutrient resorption from seagrass leaves shows that the mean resorption of nitrogen is 20.4%, and that of phosphorus 21.9%, which is lower than comparable values for various groups of perennial terrestrial plants. The actual realised resorption in seagrasses may be even less as a result of premature losses of leaf fragments due to herbivory and hydrodynamic stresses, and due to leaching losses. The leaf lifespan in seagrasses on average is 88.4 days, but is highly variable, ranging from 345 days in Posidonia oceanica to only a few days in Halophila ovalis. Leaf lifespan increases with increasing leaf weight, and decreases with increasing leaf formation rate. Furthermore, leaf longevity increases going from tropical to temperate latitudes. We compared seagrass leaf lifespan with those of freshwater angiosperms, terrestrial herbaceous plants, shrubs and trees. Considerable variability in leaf lifespan was also found in these plant groups, but comparison among data sets shows that seagrass leaf lifespan is significantly lower than the leaf lifespan of terrestrial herbaceous plants, shrubs and trees. No significant difference was found between the leaf lifespan of seagrasses and freshwater angiosperms. Leaves are usually the major sink for nutrients in seagrasses. The combination of low nutrient resorption from the leaves and a short leaf lifespan is, therefore, expected to result in a low nutrient residence time in the plants. Indeed, field experiments with 15N labelled Thalassia hemprichii showed that less than 5% of the initial 15N amount was still within the living plant biomass 240 days after labelling. Limited nutrient retention in the plant biomass necessitates the capture of new nutrients for persistent growth. We speculate that effective nutrient uptake by seagrass leaves is an important strategy to maintain an adequate nutrient balance in seagrasses, particularly in thin vegetation or in small patches. The constraints imposed by the marine environment may have favoured the development of this strategy over the development of efficient nutrient conservation strategies.