NUTRIENT RESORPTION FROM SEAGRASS LEAVES

The resorption of nutrients (C, N and P) from senescent leaves of six seagrass species from nine different locations in tropical (Indonesia and Kenya), Mediterranean (Spain) and temperate (The Netherlands) regi ons has been investigated. Resorption was quantitatively assessed by c alculating the difference in nutrient content between the leaves with the highest content, and the oldest leaves. In order to do so, the lea ves were classified according to their age. The nutrient contents of l eaves of a given age category were calculated by multiplying the measu red nutrient concentration in this age category with its corresponding modelled leaf biomass. N- and P-concentrations declined during ageing and senescence of the leaves in all of the investigated situations bu t two. The decline in concentration varied up to 58% for N and up to 6 6% for P. The C-concentration declined on three of the investigated oc casions and varied up to 24%. Despite a decline in concentration, the leaf C-content did not change, indicating no resorption of carbon. The efficiency of N-resorption from intact seagrass leaves varied between 3.8 and 29% (average: 15%), while the efficiency of phosphorus resorp tion varied between 0 and 51% (average: 21%). The resorption efficienc y was not significantly different in seagrasses with a relatively high and a relatively low nutrient concentration, although within-species comparisons showed that in some cases resorption efficiency was positi vely related to the nutrient concentration of the leaves. Premature lo ss of leaves and leaf fragments (by e.g. herbivory) may substantially interfere with the resorption process. In Indonesian seagrasses we est imated that as a result of fragmentation and premature detachment only between 56 and 77% of the physiological resorption potential actually was realised. It is concluded that internal resorption may play a rol e in the nutrient dynamics of seagrass plants, but that its quantitati ve importance probably is limited. Nutrient resorption from senescent seagrass leaves may reduce the nutrient requirements for seagrass leaf production by approximately 10% for nitrogen and 15% for phosphorus.