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

Efficient nutrient resorption from senescing leaves, and extended leaf life spans are important strategies in order to conserve nutrients for plants i n general. Despite the fact that seagrasses often grow in oligotrophic wate rs, these conservation strategies are not strongly developed in seagrasses. A compilation of literature data on nutrient resorption from seagrass leave s shows that the mean resorption of nitrogen is 20.4%, and that of phosphor us 21.9%, which is lower than comparable values for various groups of peren nial terrestrial plants. The actual realised resorption in seagrasses may b e even less as a result of premature losses of leaf fragments due to herbiv ory and hydrodynamic stresses, and due to leaching losses. The leaf lifespan in seagrasses on average is 88.4 days, but is highly vari able, ranging from 345 days in Posidonia oceanica to only a few days in Hal ophila ovalis. Leaf lifespan increases with increasing leaf weight, and dec reases with increasing leaf formation rate. Furthermore, leaf longevity inc reases going from tropical to temperate latitudes. We compared seagrass lea f lifespan with those of freshwater angiosperms, terrestrial herbaceous pla nts, shrubs and trees. Considerable variability in leaf lifespan was also f ound in these plant groups, but comparison among data sets shows that seagr ass leaf lifespan is significantly lower than the leaf lifespan of terrestr ial herbaceous plants, shrubs and trees. No significant difference was foun d between the leaf lifespan of seagrasses and freshwater angiosperms. Leaves are usually the major sink for nutrients in seagrasses. The combinat ion of low nutrient resorption from the leaves acid a short leaf lifespan i s, therefore, expected to result in a low nutrient residence time in the pl ants. Indeed, field experiments with N-15 labelled Thalassia hemprichii sho wed that less than 5% of the initial N-15 amount was still within the livin g 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 favour ed the development of this strategy over the development of efficient nutri ent conservation strategies. (C) 1999 Elsevier Science B.V. All rights rese rved.