Maintenance of estuarine water quality by mangroves occurs during flood periods: a case study of a subtropical mangrove wetland.

Seasonal changes in water quality were measured in samples taken at various distances from shallow water across mudflat to mangroves during flood period and from mangroves across mudflat to shallow water during ebb period in a subtropical mangrove estuary (Zhangjiang Estuary, Fujian, China). The TN (total dissolved nitrogen), TP (total dissolved phosphorus), COD (chemical oxygen demand), and DOC (dissolved organic carbon) contents during the flood period were significantly higher than those during the ebb period. In contrast, the opposite was true for the POC (particulate organic carbon) content and transparency. The mangroves at Zhangjiang Estuary may trap nutrients at rates of 90.5 g N/m(2)/yr, 2.2 g TP/m(2)/yr, and 13.7 g C/m(2)/yr in the form of DOC, and export POC at a rate of 81.8 g/m(2)/yr. Our results support the hypothesis that the maintenance of estuarine water quality by mangroves occurs during flood periods.