Durkin, C. | Search bioRxiv

bioRxiv2025

Solitary phytoplankton cells sink in the mesopelagic ocean

Phytoplankton, and their carbon, are typically exported from the surface ocean when packaged inside larger, sinking detrital particles. This process draws carbon out of the atmosphere, where it can be sequestered for long time periods in the deep ocean. Phytoplankton can also sink as solitary cells, but direct observations are scarce and the ecological significance is unknown. We collected unprecedented observations of solitary sinking cells during month-long observations in the upper 500 m at two contrasting ocean locations. While these cells account for only a small fraction of the total particulate organic carbon flux (<5%), they provide essential nutrients and a persistent source of food to deep sea ecosystems while preserving a seed bank for future phytoplankton blooms. In one case, observed depth changes over time allowed calculation of a sinking speed of 6 m d-1. The disaggregation of detrital aggregates collected at the same time could not account for the magnitude or composition of individually sinking cells in our samples, although in some instances their fluxes were correlated. Instead, the data imply that these solitary cells were transferred through the upper mesopelagic as individually sinking particles and play key roles in ocean ecology and biogeochemistry.

ecology↗

bioRxiv2023

Quantitative analysis of food web dynamics in a low export ecosystem

Food webs trace the flow of organic matter and energy among producers and consumers; for pelagic marine food webs, network complexity directly influences the amount and form of carbon exported to the deep ocean via the biological pump. Here we present a synoptic view of mixed layer food web dynamics observed during the late summer 2018 EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign in the subarctic Northeast Pacific at the long-running time-series site, Ocean Station Papa. Carbon biomass reservoirs of phytoplankton, microzooplankton, and bacterioplankton, were approximately equal while mesozooplankton biomass was 70% lower. Live organisms composed [~]40% of the total particulate organic carbon within the mixed layer: the remainder was attributed to detritus. Rates of carbon transfer among reservoirs indicated production and assimilation rates were well balanced by losses, leaving little organic carbon available for export. The slight positive net community production rate generated organic carbon that was exported from the system in the form of food web byproducts, such as large fecal pellets generated by mesozooplankton. This characteristically regenerative food web had relatively slow turnover times with small-magnitude transfers of carbon relative to standing stocks that occurred amidst a high background concentration of detrital particles and dissolved organic matter. The concurrent estimation of food web components and rates revealed that separated processes dominated the transfer of carbon within the food web compared to those that contributed to export. Plain Language SummaryThe biological carbon pump drives a downward flux of organic matter from the sunlit surface ocean to the vast ocean interior. Ecological interactions in the surface ocean directly affect the amount and type of carbon that is exported to the deep ocean. In this study, we present a synthesis of the late summer mixed layer food web in the Northeast Pacific that was extensively characterized during the 2018 EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign. We found the majority of carbon was recycled within the mixed layer by microbes through multiple transfers between producers and consumers. Larger organisms, mesozooplankton and salps, only consumed a small amount of carbon but through the formation of sinking fecal pellets were the main mechanism of transporting carbon out of the system. The study highlights the need to concurrently study microbial and large organism dynamics to develop a predictive understanding of the fate of organic carbon in the oceans. Key PointsO_LIThe microbial loop dominated carbon flow in the late summer mixed layer food web of the North Pacific, most net production was respired leaving little carbon available for export. C_LIO_LIActive production and consumption of organic carbon occurred amid a high background of detrital particulate organic carbon (58% of total) with slow turnover time, 66 d. C_LIO_LIMesozooplankton which had relatively minor carbon consumption rates created the majority of export production due to efficient repackaging of consumed material. C_LI

ecology↗