A rare intercomparison of nutrient analysis at sea: lessons learned and recommendations to enhance comparability of open-ocean nutrient data

Triona Mcgrath,Elizabeth Kerrigan,Claire Normandeau,Douglas Wallace,Evin Mcgovern,Margot Cronin,Clynton Gregory

doi:10.5194/essd-11-355-2019

Abstract

Abstract. An intercomparison study has been carried out on the analysis of inorganic nutrients at sea following the operation of two nutrient analysers simultaneously on the GO-SHIP A02 trans-Atlantic survey in May 2017. Both instruments were Skalar San++ Continuous Flow Analyzers, one from the Marine Institute, Ireland and the other from Dalhousie University, Canada, each operated by their own laboratory analysts following GO-SHIP guidelines while adopting their existing laboratory methods. There was high comparability between the two data sets and vertical profiles of nutrients also compared well with those collected in 1997 along the same A02 transect by the World Ocean Circulation Experiment. The largest differences between data sets were observed in the low-nutrient surface waters and results highlight the value of using three reference materials (low, middle and high concentration) to cover the full range of expected nutrients and identify bias and non-linearity in the calibrations. The intercomparison also raised some interesting questions on the comparison of nutrients analysed by different systems and a number of recommendations have been suggested that we feel will enhance the existing GO-SHIP guidelines to improve the comparability of global nutrient data sets. A key recommendation is for the specification of clearly defined data quality objectives for oceanic nutrient measurements and a flagging method for reported data that do not meet these criteria. The A02 nutrient data set is currently available at the National Oceanographic Data Centre of Ireland: https://doi.org/10.20393/CE49BC4C-91CC-41B9-A07F-D4E36B18B26F and https://doi.org/10.20393/EAD02A1F-AAB3-4F4E-AD60-6289B9585531.

Highlights

Dissolved nutrients such as nitrate, nitrite, silicate and phosphate can be a critical limiting factor constraining the growth of phytoplankton, which in turn form the base of the marine food web
It is important to note that the Marine Institute (MI) data used in this comparison is calculated using split calibration curves: any total oxidised nitrogen (TOxN) and silicate data < 5 μmol L−1 were calculated from a calibration range of 0–10 μmol L−1, while all other data were calculated using the 0–50 μmol L−1 calibration range
Without any adjustments based on certified reference materials (CRMs) analysis results, there was relatively good agreement between vertical profiles of nutrients measured with the two systems, as can be seen from vertical profiles presented in Fig. 2 and Supplement (Fig. S1)

Summary

Introduction

Dissolved nutrients such as nitrate, nitrite, silicate and phosphate can be a critical limiting factor constraining the growth of phytoplankton, which in turn form the base of the marine food web. There is growing evidence for significant variability, including long-term trends in nutrient levels in both coastal (Kim et al, 2011) and open-ocean surface (Yasunaka et al, 2014), and deepwater (Kim et al, 2014) These changes reflect direct human intervention in the global environment, especially the effects of the massive ongoing perturbation of the nitrogen cycle (Yang and Gruber, 2016), as well as changes in ocean circulation and biogeochemical cycling that may or Published by Copernicus Publications. McGrath et al.: A rare intercomparison of nutrient analysis at sea may not be anthropogenically influenced (e.g. Di Lorenzo et al, 2008)

Methods

Results

Discussion

Conclusion