ALKALINE-PHOSPHATASE ACTIVITY AS A TOOL FOR ASSESSING NUTRITIONAL CONDITIONS IN THE SEAGRASS POSIDONIA-OCEANICA (L) DELILE

The effects of experimental phosphorus enrichments on alkaline phospha tase activity (APA) of the seagrass Posidonia oceanica (L.) Delile wer e tested. Short-term additions (as phosphate, 12 hours, in the laborat ory) decreased APA by 18-28%, depending on the plant part considered ( roots, young leaves or old leaves). The values of APA after this treat ment were well correlated with internal phosphorus pools (as P concent ration in plant tissues). Long-term additions (as phosphate, added to the sediment, 1 month in situ) decreased APA by 40-75%, also depending on the plant part. We conclude that alkaline phosphatase activity is a good indicator of P deficiency in this seagrass. We used this indica tor to assess the P-nutritional status in a series of meadows in the N W Mediterranean, finding a high geographical variability, but correlat ions between APA and basic features of the meadows (carbonate content of the sediment, organic content of the sediment, shoot density, etc.) were not significant. Consequently, phosphorus deficiency does not se em to be directly related to these descriptors.