The present invention discloses a seawater nutrient on-line monitoring system and seawater nutrient detection method, the monitoring system includes a sealed cabin, a main control module, a detection module, a reagent holding device, a liquid inlet interface and an air interface; The detection module includes a first detection module for detecting nitrate and nitrite, a second detection module for detecting phosphate, a third detection module for detecting ammonium salts, and a fourth detection module for detecting silicates independently of each other; During the detection, the first detection module, the second detection module, the third detection module and the fourth detection module work independently of each other to detect different nutrients in seawater, and introduce air masses into the detection for defoaming. The invention has high detection accuracy, small drift, stable and reliable operation, small test dose, less waste liquid, and can simultaneously or independently complete the in-situ online analysis and monitoring of five kinds of nutrients : nitrate, nitrite, ammonia nitrogen, phosphate and silicate.
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