Factors Affecting Nutrient Concentration and Stable Carbon and Nitrogen Isotope Ratio of Particulate Organic Matter in the Ishikari River System, Japan

Abstract

Water and particulate organic matter samples were analyzed for dissolved nutrients and stable carbon and nitrogen isotope ratio, respectively, in the Ishikari River system, from August 2009 to July 2010. Dissolved nutrient concentration showed significant spatial and seasonal variations within the river system. Mean concentration and standard deviation of ammonium, nitrite, nitrate, phosphate, silica, and dissolved organic carbon were 0.17 ± 0.17, 0.01 ± 0.01, 0.93 ± 0.59, 0.03 ± 0.01, 9.07 ± 2.40, and 3.7 ± 0.93 (mg l−1), respectively. The δ 13C, δ 15N, and molar C/N ratio of particulate organic matter samples ranged from −34.6 to −24.6‰, −4.8 to 15.8‰, and 5.3 to 17.8, respectively, in the Ishikari River system. Nutrient concentration showed increasing trend during the past 50 years owing to increased urban growth and development taking place in the river basin. The molar ratio of dissolved inorganic nitrogen (DIN)/dissolved inorganic phosphate (DIP) was higher than the Redfield ratio (16:1), which implies phosphorus limitation of phytoplankton growth in the river water. The negative value of the indicator of coastal eutrophication potential for nitrogen for the Ishikari River system indicated the absence of eutrophication problem in its coastal areas. Annual nutrient fluxes from Ishikari River for dissolved inorganic nitrogen (DIN-N), phosphate (PO4-P), dissolved silica (SiO2-Si), and dissolved organic carbon (DOC-C) were 1.6 × 104, 379, 13.2 × 104, and 5.4 × 104 t year−1, respectively.