『Abstract
　The phosphorus fractions in three tropical mangrove systems of Cochin region were analysed by sequential extraction method. Iron-bound phosphorus was the major fraction in the first two stations, while station 3 was exclusively dominated by calcium-bound phosphorus. Compared to other stations, about tenfold increase in total phosphorus content was observed at station 3. This station is a congregation of communally breeding birds, and there is accumulation of bird guano. Mineralogical analysis showed the presence of monetite, a thermodynamically metastable calcium phosphate mineral, in this unique system. The excreta and carcass of the birds in this sanctuary seems to be the reason for the formation of monetite, which is favoured by periodic fluctuations in redox potential. The high mass percentages of calcium and phosphorus by XRF and SEM-EDS analysis confirm the existence of calcium phosphate mineral at station 3. First two stations did not show any noticeable difference in phosphorous fractions and inorganic fractions constituted to about 65％ of total phosphorus. But at station 3, inorganic fractions were about 92％. Low C:P ratios and low organic phosphorus content indicated active mineralisation of phosphorus at station 3. Bioavailable fractions of phosphorus at stations 1 and 2 were about 75％, whereas 98％ of the total phosphorus was bioavailable at station 3. Since the bulk of the total phosphorus is available, these mangrove sediments have the potential to act as source of phosphorus to the overlying waters.

Keywords: Mangrove sediments; Phosphorus fractions; Sequential extraction; Geochemistry; Guano; Monetite』

Introduction
Materials and methods
　Study area
　Sampling and analytical methodology
Results
Discussion
Conclusion
Acknowledgments
References