Sediment phosphorus forms and levels in two tropical floodplain wetlands

Abstract

Inorganic phosphorus is one of the critical nutrients determining trophic state and freshwater productivity. Sediment may act as a sink or source of phosphorus to the overlying water depending on its pH, redox state, various forms of phosphorus present, etc. To examine potential sorption or mobilization of sediment phosphorus in floodplain wetlands, the amount and distribution of phosphorus fractions were evaluated using a sequential chemical extraction procedure. Exceedingly high levels of total phosphorus (mean: 6040 ± 344, 5470 ± 363 mg kg−1), consisting largely of organic and refractory fraction (70 – 98%), followed by calcium-phosphorus (mean: 584 ± 31.3, 143 ± 8.42 mg kg−1) and iron-phosphorus (mean: 108 ± 10.1, 91.0 ± 7.68 mg kg−1) were recorded respectively in Bhomra and Akaipur wetlands of West Bengal, India. The inorganic phosphorus, comprising the loosely sorbed phosphorus and all the mineral bound forms contributed only about 6–14% to the total phosphorus indicating their less significance in phosphorus sorption or desorption in these tropical wetlands. Although the loosely sorbed phosphorus was in moderate level (2.69 ± 0.69, 1.54 ± 0.53 mg kg−1), water dissolved phosphorus was recorded at higher concentrations (mean: 0.16 ± 0.02 mg l−1 in Akaipur and 1.08 ± 0.12 mg l−1 in Bhomra). However, the higher level of water available phosphorus was not reflected in plankton production since the dominance of weeds suppresses their growth. This study recorded large accumulation of organic matter and nutrients in the form of detritus in these wetlands which may be channelized for fish production through stocking of suitable detritivorous fishes and/or reducing macrophyte coverage that would give space and nutrients for phytoplankton growth.