Ocean observations are critical in developing our understanding of the interaction between the ocean and climate. Detailed information about the nutrient composition of the ocean is achieved using in situ sensing and the collection and analysis of physical samples from research vessels. The analysis of nutrients underpins work in understanding both the carbon cycle and biological productivity in oceans. Although in situ sensors are becoming more common for some nutrients, the best and most reliable method for making these measurements is to conduct analysis of physical samples at sea. This article discusses the analysis and data processing methods developed by the hydrochemistry team at Commonwealth Scientific and Industrial Research Organisation (CSIRO) to provide repeatable and accurate analysis of nutrient samples at sea. Nutrients are measured by segmented flow analysis and data are processed using in-house software (Hydrology Processor [HyPro]). These methods were used during the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) P15S voyage in 2016. Accuracy and precision of the analyses during the voyage was determined from measurement of the certified reference material for nutrients in seawater (RMNS) for silicate, phosphate, nitrate, and nitrite and is presented and discussed here. The accuracy of the silicate and phosphate RMNS measurements was within 1-3% of the certified values, and nitrate + nitrite within 1-2%. The precision for silicate, nitrate + nitrite, and phosphate was greater than 0.2%. The hydrochemistry team has established a standard operating procedure which assures the quality of nutrient data obtained is accurate and precise.
Ammonium is a fundamental nutrient for phytoplankton growth in seawater and is a key component of the microbial loop. Ammonium measured in parallel with other nutrients is crucial in understanding the small temporal scale changes in oceanographic ecology. Despite the importance of measuring ammonium at sea, owing to its lability, there is no consensus on the best method. The lack of availability of certified reference materials for ammonium in seawater also makes it difficult to assess the accuracy and reproducibility of ammonium measurements. In this study we present a modified segmented flow analysis method using ortho-phthaldialdehyde (OPA) with fluorescence detection to measure ammonium at sea together with four other macro-nutrients (nitrate, nitrite, silicate and phosphate) in near real time. An in-house ammonium quality control (QC) material was produced to improve the accuracy and repeatability of the measurement at sea. The QC was prepared following two different methods and stored in two types of containers. The suitability of the in-house QC’s as a reference material were assessed onboard the RV Investigator in 2018 during two oceanographic voyages, including one on the repeat SR03 CLIVAR transect. This paper describes the production and assessment of the in-house QC for ammonium in seawater, providing groundwork for creating a short-term stable ammonium reference material for sea going voyages. The uncertainty of this method of ammonium measurement was found to be 0.10 μmol/L at ammonium concentration of 1.0 μmol/L. Results show that preparation of the QC inside a laminar flow cabinet and directly into 10 mL polypropylene sample tubes just prior to the commencement of the voyage improved its stability.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.