Award: OCE-1756442

This research was part of the NSFs 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 ocean biology in transferring carbon from the atmosphere to the deep sea through a process known as the biological pump. The biological pump starts with the conversion of carbon dioxide to organic matter via photosynthesis by phytoplankton in the surface ocean. Phytoplankton act as the grass of the sea supporting the entire ocean food web. As the organic carbon in phytoplankton moves through the food web, some sinks or is otherwise transferred to the deep ocean in various forms such as fecal material or dead organisms. This process exp[orts or pumps carbon to depth where it can remain sequestered from the atmosphere for decades or longer. The focus of this project was a group of phytoplankton called diatoms. Diatoms account for up to 40% of the photosynthesis in the ocean making them important to the biological pump. Diatom also form a shell of opal which means they require the element silicon to thrive. Our studies examined how the silicon requirement of diatoms affects their contribution to the biological pump. The EXPORTS program conducted two research expeditions. One was to the Subarctic Pacific and the other to the North Atlantic. The Subarctic Pacific is known to be a region where phytoplankton productivity is low due to a lack of iron to support their growth, while the North Atlantic has one of the most prolific spring blooms of phytoplankton in the global ocean, but it is known to have low dissolved silicon. These contrasting conditions allowed the EXPORTS program to examine how changes in food web structure alters the efficiency of the biological pump. The expedition to the Subarctic Pacific occurred in late fall of 2018. Diatom abundance determined from measures of biogenic silica concentration (bSi) were low, being in the tens of nanomolar range. On average, large diatoms dominated Si dynamics, accounting for 65% of bSi stocks and 81% of Si uptake compared to the small diatoms. The growth of small diatoms was limited by iron, while their ability to utilize dissolved silicon was restricted by low dissolved silicon concentrations. Larger diatoms were only growth-limited by iron and appear to be able to obtain ample silicon from the environment. About a third of bSi produced by diatoms was exported out of the upper 100 m. The contribution of diatoms to carbon export (913%) was about twice their contribution to photosynthesis (37%). The North Atlantic was sampled in spring 2021 following the demise of the main diatom bloom allowing mechanisms that sustain continued diatom contributions to be examined. Diatom biomass was initially relatively high with bSi up to 2.25 mol Si L-1. Low initial dissolved silicon concentration of 0.1 to 0.3 M imposed severe silicon limitation of diatom growth rate. Four storms over the next three weeks entrained dissolved silicon from deeper waters into the mixed layer, partially relieving silicon limitation. Diatom growth was modest and dominated by large diatoms. Over 3.5 weeks the resupply of silicic acid via entrainment and silica dissolution supported a cumulative post-bloom silica production that was 32 % of that estimated during the main bloom event. Estimated diatom contribution to total organic-carbon export following the main bloom remained high at 40 70 %. Thus, diatoms can significantly contribute to regional biogeochemistry following initial dissolved silicon depletion, but that contribution relies on physical processes that resupply the nutrient to surface waters. Comparing these two studies the influence of phytoplankton nutrition on the contribution of diatoms to the biological p[ump is clear. In the Subarctic Pacific where low iron restricted phytoplankton growth the contribution of diatom to carbon export was lower than in the North Atlantic where storm events partially alleviated silicon limitation by mixing nutrients, including dissolved silicon, from depth to surface waters. The contribution of small diatoms was more important in the subarctic Pacific where iron limitation held back the larger diatoms compared to the North Atlantic where large diatoms were overwhelmingly dominant. This work supports the idea that large diatoms follow efficient export pathways in the biological pump. When growth conditions are favorable, they tend to dominate carbon export. The larger effort by the EXPORTS research team involved assessments of many other aspect of the food web and environmental conditions during these two expeditions. The data from all projects have been made available for an upcoming synthesis effort that will look across studies to gain a more predictive understanding of how the biological pump operates to enable assessments of how carbon sequestration by the biological pump may change with shifts in climate. Last Modified: 07/03/2024 Submitted by: MarkABrzezinski