Phosphorus (P) is a potential limiting nutrient in Changjiang Estuary. Sedimentary P preservation and regeneration play an important role in indicating regional environmental changes and buffering P limitation in the water column. A series of coring experiments was implemented in the hypoxic area of Changjiang Estuary to explore sedimentary P speciation and distribution and their environmental significance. The results showed that the contents and distributions of P in the cores were largely influenced by terrestrial loading, and Detr-P was the dominant P form in the sediments, followed by Org-P, Fe-P, and Auth-P, whereas Exch-P was the minor phase of Tot-P (<5%). Auth-P was predominantly yielded by Org-P and Fe-P transformations. P preservation in Changjiang Estuary was significantly related to terrestrial inputs and environmental changes in the water column. P speciation in cores tracked the regional environmental changes effectively. Distribution of reactive P in the hypoxic area was significantly different from that in the oxic area, with fairly high C/P ratios. The benthic diffusive flux of DRP in the study area ranged from 0.90 to 1.13 μmol·(cm2·a)-1. Tot-P burial efficiency (PBE) was higher than 70% and the PBE for Detr-P was nearly 100%, whereas the PBEs for Fe-P and Org-P were 38% and 26%, respectively. Auth-P was the dominant fraction of reactive P preserved in the sediments, and about 51% of Auth-P originated from Fe-P and Org-P transformations. The PBE in the hypoxia area of Changjiang Estuary was fairly lower than that in the continental shelf of the East China Sea. Hypoxia leads to a decrease in the PBE, which would have long-term influence on ecological environmental problems, especially eutrophification. Changes in terrestrial inputs played a key role in P composition in the sediments; the P loads of Changjiang River coupled with primary production and hypoxia greatly affected the P cycling in the Estuary.
Keywords: Changjiang Estuary; eutrophification; hypoxia; phosphorus speciation; sediment.