Distribution of inorganic nitrogenous species and nitrification in the mangrove environment of the Indian Sundarbans

Nitrification is a vital biogeochemical process during which ammonium (NH₄⁺) is oxidized to nitrite (NO₂⁻) and nitrate (NO₃⁻) using oxygen. Nitrification is particularly important in estuaries, where nitrification rates have significant implications for ecosystem functioning. Our study investigated seasonal and spatial variations of nitrification rates in both water and sediment phases of mangrove ecosystems in the Indian Sundarbans, integrating the three major estuaries (Saptamukhi, Thakuran, and Matla). We used on board incubation to measure nitrifying activity [i.e., ammonium oxidation rate (Ra) and nitrite oxidation rate (Rn)] in the water column. We found that average Ra and Rn rates were similar, ranging from 31.1 to 31.8 nmol N L⁻¹ h⁻¹, respectively. Water column nitrification rates showed an inverse relationship with salinity (r = − 0.51, p < 0.001), while substrate concentrations of ammonium had a direct impact on nitrification rates. Annual dissolved inorganic nitrogen (DIN) concentrations were 11.7 ± 7.65 µM, with an average nitrate (NO₃⁻) concentration of 4.5–16.3 µM and average ammonium (NH₄⁺) concentrations from 2.1 to 5 µM. Suspended particulate matter and chlorophyll showed positive (r = 0.38, p = 0.009) and negative (r = − 0.31, p = 0.034) correlations, respectively, with water column nitrification rates. For water, both Ra and Rn occurred at similar rates in our study area. In sediment, nitrifying activity (SEDn) rates ranged from 0.34 to 8.69 nmol N g wet wt⁻¹ h⁻¹. SEDn showed positive and negative correlations with sediment temperature (temp) and pH, respectively. Based on our observations, the average water column nitrification potential ranged from 43 to 420 nmol N L⁻¹ h⁻¹ considering three estuaries, while the sediment nitrification potential estimated at Saptamukhi estuary showed a value of 69 nmol N g wet wt⁻¹ h⁻¹.