An ecological study on nutrients behavior of a pristine river

Abstract

River water and seawater NH₄⁺ and NO₃^- concentrations were measured during summer and autumn, 2007, to examine temporal and spatial changes in a pristine river and coastal bay located in the northern part of Okinawa Island, Japan. The river water inorganic nitrogen concentrations ranged from 0.43 to 0.70 μmol NH₄⁺L^-1 and 15.06 to 35.38 μmol NO₃^-L^-1 during the summer and from 0.07 to 0.31 μmol NH₄⁺L^-1 and 18.48 to 28.13 μmol NO₃^-L^-1 during the autumn. The seawater inorganic nitrogen concentrations ranged from 0.00 to 1.04 μmol NH₄⁺L^-1 and 0.00 to 23.35 μmol NO₃^-L^-1 during the summer and from 0.00 to 0.29 μmol NH₄⁺L^-1 and 0.00 to 1.15 μmol NO₃^-L^-1 during the autumn. At the same time, the seawater suspended solid (SS) concentrations were up to 10 mgL^-1 during the summer and 3.5 mg L^-1 during the autumn. The highest NH₄⁺ and NO₃^- concentrations in the bay were found near the river mouth, which indicates that these materials are probably discharged from the river. The river water NH₄⁺ and NO₃^- concentrations were higher than the seawater concentrations during both seasons, but the seawater NH₄⁺ and NO₃^- concentrations were not relatively higher than in other coastal areas of Okinawa Island. NH₄⁺ and NO₃^- can negatively influence coral growth in this bay, and the corals were absent at the high NH₄⁺ and NO₃^- concentration sites in the bay. Inorganic nitrogen from seawater may have flown into the bay during rainfall events.