High-frequency measurement of seawater chemistry: Flow-injection analysis of macronutrients

Burke Hales, Lex van Geen, Taro Takahashi

Limnol. Oceanogr. Methods 2:91-101 (2004) | DOI: 10.4319/lom.2004.2.91

ABSTRACT: We adapted a commercially available flow-injection autoanalyzer (Lachat Quik-Chem 8000) to measure seawater nitrate concentrations at a rate of nearly 0.1 Hz and phosphate and silicate concentrations at a rate half that. Several minor improvements, including reduced sample-loop size, high sample flushing rate, modified carrier chemistry, and use of peak height rather than peak area as a proxy for nutrient concentration aided in the increase in sampling rate. The most significant improvement, however, was the construction of a copperized cadmium NO3– reduction column that had a high surface area to volume ratio and a stable packing geometry. Preliminary results from a cruise in the Ross Sea in austral spring of 1997 are shown. Precision of all three analyses is better than 1%. Comparison of the nutrient concentrations determined by the rapid analysis method described here with traditional discrete analyses shows that nitrate and silicate determined by the two approaches are within a few percent of each other, but that the phosphate concentrations determined by the rapid analysis are as much as 10% lower than those determined by the discrete analyses.