This research was conducted as part of the NSF contribution to the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program sponsored by NASA. The overall goal of the project was to gain a predictive understanding of the role of diatom physiology in transferring carbon from the atmosphere to the deep sea through the biological pump. In the biological pump, carbon dioxide is converted to organic matter via photosynthesis by phytoplankton in the surface ocean. These phytoplankton serve as the basis of the marine food web, and as organic carbon moves through the food web, some sinks or is otherwise transferred to the deep ocean as fecal material or dead organisms. This process exports or pumps carbon to depth where it can remain sequestered from the atmosphere for decades or longer. The focus of this project was diatoms, which are an important group of phytoplankton that account for up to 40% of the photosynthesis in the ocean and thus play an important role in the biological pump. Diatom growth in the open ocean is impacted by nutrients, especially by silicon, which is required for the frustules or shells that diatoms form, and by iron, an essential trace element that limits diatom growth in as much as one third of the surface ocean.

The EXPORTS program conducted two research expeditions, one in the subarctic Pacific and one in the eastern North Atlantic. The study site in the subarctic Pacific is known to be a region where iron limits primary production, whereas the study site in the North Atlantic is host to one of the larges spring blooms of phytoplankton in the global ocean but is also known to have low dissolved silicon. This project leveraged the contrasting conditions of these two EXPORTS campaigns to examine how nutrient limitation and resulting diatom physiology impacted carbon export through the biological pump. Trace metal clean techniques were used to collect water through the euphotic zone of each site to conduct shipboard bioassay experiments probing the iron, silicon, and iron-silicon limitation of diatom communities.

In the subarctic Pacific, increases in dissolved iron concentrations in surface waters of the study area were spatially associated with increases in new primary production, and iron additions to incubation experiments led to significant increases in large (>5 µm) diatom populations, reflecting the pervasive iron limitation of large diatom communities in this study region. Upon relief of iron limitation, the demand for dissolved nickel by large diatoms was most closely tied to exhaustion of nitrate and silicic acid concentrations in the bioassay experiments, leading to an apparent delay in dissolved nickel uptake that did not fully deplete background dissolved nickel concentrations even though nickel speciation measurements indicated that most of the dissolved nickel was labile and likely bioavailable.

In the North Atlantic, the EXPORTS campaign sampled the decline of the main diatom bloom. Diatom biomass was initially relatively high but declined early in the study as silicic acid concentrations became exceedingly low. Dissolved iron concentrations were also depleted, and the first bioassay experiment indicated iron limitation of large diatoms may have contributed to the decline of the bloom and to subsequent shifts in phytoplankton community composition. In the remaining bioassay experiments, dissolved silicon concentrations primarily limited diatom growth rates, with gradual resupply of silicon from storms over the next three weeks partially relieving silicon limitation. Concentrations of dissolved iron, on the other hand, remained low (<0.2 nM) in surface waters following the storms, with resulting iron:nitrate concentrations ratios that were generally less than 0.05 nmol:µmol throughout the study and consistent with ratios that would be expect to limit the growth of large diatoms in the study area.

Broader impacts of this project have included the training and mentoring of undergraduate and graduate students, and of early-career researchers in ocean biogeochemistry, the mentoring of middle-school girls in an ocean science summer program, and the dissemination of project activities with visiting high school students and teachers during lab tours. All data from the project have been made publicly available and will contribute to an upcoming synthesis effort that will look across the EXPORTS program studies to gain a more predictive understanding of how the biological pump operates and to enable assessments of how carbon sequestration by the biological pump may change with shifts in climate.

Last Modified: 12/17/2024
Modified by: Kristen N Buck