Temporal and spatial variations of surface pCO2, phosphate, nitrate, and silicate in the western equatorial Pacific

Abstract

Nutrients and CO2 are important oceanographic variables, as they provide information which can be used to understand phytoplankton abundance and processes such as the oceanic carbon cycle. Therefore, as climate change impacts ocean systems, it is increasingly important to measure how nutrient and CO2 concentrations in the ocean change over time and space. This study measured pCO2 (which takes into account temperature, total CO2, salinity, and alkalinity of the water), nitrate, phosphate and silicate concentrations in the western equatorial Pacific (5S-5N along 167W) in January 2024. Over space, pCO2 and nutrients were analyzed for correlation with physical processes, primarily upwelling, using sea surface temperature (SST) and mixed layer depth. To determine the relationship of pCO2 and nutrient concentrations to the biomass of microorganisms, correlations with fluorescence and beam transmission were also analyzed over space. Over time, pCO2 was compared to atmospheric CO2 and El Nino Southern Oscillation (ENSO) state to determine correlations between temporal pCO2 trends and atmospheric phenomena. pCO2 surface concentrations in the western equatorial pacific were found to have increased from 1983 to 2024 at an average rate of 2.02 +/- 0.034 ppm/yr and had a positive correlation with increasing average atmospheric CO2 (R = 0.71, p-value < 0001). Spatially, surface pCO2 and the macronutrients nitrate, phosphate, and silicate in the upper 200 m showed similar patterns from 5S to 5N along 167W. The concentrations of nitrate and phosphate had a significant negative correlation to mixed layer depth (R = -0.4, p-value < 0.001) and nutrients and pCO2 had a significant negative correlation to sea surface temperature (p-value < 0.001). They peaked from 0-2N due to upwelling and exhibited smaller secondary peaks around 3S and 3N, likely due to mixing caused by north and south subsurface countercurrents. These results reinforce the importance of physical oceanic and atmospheric processes as a control for nutrient and inorganic carbon cycles in the western equatorial Pacific.