Comparative assessment of preservation methods for major nutrients in polar seawater

Major nutrients—nitrate, phosphate, and silicate—are fundamental building blocks of marine biomass. To understand the flow of material and energy in the ecosystem, it is essential to accurately quantify nutrients concentrations. When shipboard analysis is not possible, seawater samples must be preserved without altering their contents. In this study, we investigated a range of commonly used preservation methods, including filtration, chemical poisoning with HgCl₂, and freezing at −20 °C or − 80 °C without pre-treatment, all aimed at minimizing biological activities. We also evaluated the effects of sample storage periods and thawing periods required before analysis in freezing treatments to determine their impact on nutrients contents. Using four different statistical methods, we assessed a total of 19 experiments to identify the most effective preservation method. Comparison between filtered and untreated seawater samples showed no detectable effect on the preservation by filtration. Deviations from shipboard measurement were detected in samples stored in a freezer, even at −80 °C. On the other hand, the nutrient content in the poisoned samples remained virtually intact. The deficiency in nutrient content observed during prolonged freezing and thawing or storage in a refrigerator at 4 °C, compared to shipboard measurements, aligns with the Redfield relationship reported in the Arctic Ocean, suggesting that biological activities occurred either within brine channels during freezing, during refrigerated storage, or both, likely due to viable cold-adapted microbes. Although our findings are based on polar seawater samples, potential biological activities during storage and post-treatment should be carefully examined in the other oceanic regions.