An autonomous nutrient analyzer for oceanic long-term in situ biogeochemical monitoring.

An autonomous nutrient analyzer in situ (ANAIS) has been developed to monitor nitrate, silicate, and phosphate concentrations while deployed at sea at pressure (down to 1000 m). Detection is made by spectrophotometry. The instrument uses solenoid-driven diaphragm pumps to propel the sample, the standards, and the reagents through a microconduit, flow injection-style thermostated manifold. The analyzers are placed in an equipressure container filled with oil. The analyzers operate until a pressure of 100 bar and show a linear response up to 40 microM nitrate, 150 microM silicate, and 5 microM phosphate with a detection limit less than 0.1, 0.5, and 0.1 microM and an accuracy of 1, 1, and 3% for nitrate, silicate, and phosphate, respectively. The measurement protocol includes three steps over 13 min: rinsing with the sample stream, reagents introduction, and absorbance detection. Field tests comprise ANAIS nitrate, silicate, and phosphate testing alone in the surface ocean. Phosphate results are not yet fully satisfactory. The instrument implemented on top of a YOYO vertical eulerian profiler was then deployed successfully in the northwestern Mediterranean Sea acquiring 30 nitrate profiles between 200 and 1100 m over a 15-day period. This chemical analyzer can be a valuable observing asset adapted on any type of oceanographic platform.