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.

Marine heterotrophic bacteria play a pivotal role in microbial community dynamics. This study aims to understand interactions within the microbial community of the oligotrophic (nutrient-poor) equatorial Pacific, specifically investigating how heterotrophic bacteria respond to the growth of autotrophic picophytoplankton. This experiment attempts to provide a more faithful representation of in-situ conditions, overcoming previous difficulties in capturing the dynamic behavior of microbial communities in the field. A novel methodology utilizing continuous chemostats with natural communities in the field accomplishes this objective. Three growth chambers were employed, two as chemostat systems and one in batch culture mode. All three growth chambers contained the natural microbial community that passed through a 3 µm pore-size filter. Chemostat systems continuously received 0.2 µm-filtered seawater (without microbes) while an equivalent volume was removed from the growth chamber simultaneously. Dissolved inorganic nutrients required by autotrophs – silicate, phosphate, and nitrate – were added to one of the chemostat’s 0.2 µm filtered media reservoir. This methodology was compared to a traditional batch culture, where nutrients were added to the growth chamber once, at time-zero. Cultures followed a 16-hour on/8-hour off light/dark cycle using LEDs, simulating the equatorial Pacific day/night cycle. Community responses were measured by continuous optical density measurements (OD), with endpoint subsamples analyzed for microbial abundance and DNA content using flow cytometry. Distinct day/night responses were observed in all cases, with the nutrient-enriched chemostat showing the most pronounced response. Overall, the results provide new insight into the linkages between marine autotrophic and heterotrophic microbes, while demonstrating an effective new methodology for examining microbial community responses to added nutrients. Thus, this study not only advances our understanding of microbial community dynamics in the oligotrophic equatorial Pacific but also introduces a novel experimental method that can be applied across a diversity of marine and aquatic environments.

Net primary productivity (NPP) is a major component of the carbon cycle. NPP is defined as the amount of carbon biomass produced by primary producers over a given period of time and area. The NPP exceeds 100 billion tons of carbon per year on Earth and half of it comes from the ocean through phytoplankton. The equatorial Pacific ocean is the largest tropical ocean on Earth and subsequently the largest oceanic source of CO2 to the atmosphere. Despite its importance, NPP in the west equatorial Pacific is poorly characterized due to the lack of data. Previous research suggests that strong upwelling is associated with increased nutrient concentration in the euphotic zone leading to an increase in primary productivity. However, the western equatorial Pacific is known for weaker upwellings compared to the eastern and central equatorial Pacific.This study was conducted aboard the R/V Thomas G. Thompson from December 28, 2023, to January 12, 2024, with the goal of identifying and quantifying the critical variables that have a substantial impact on NPP in the region including temperature, chlorophyll, dissolved nutrients, and current speed. NPP was measured using in-situ oxygen incubations, and was compared to NPP calculated from satellite data, which tend to typically overestimated or underestimated NPP in the region. Results of this study provide important information for refining satellite models to comprehend CO2 emissions into the atmosphere.