The Tale of Three Systems: Fate of Primary Production in the Chukchi Sea

Part 1

Single-celled photosynthetic primary producers generate most of the fixed carbon that supports Arctic Ocean food webs and drives numerous biogeochemical cycles. Three main sources of net primary production (NPP) have been identified in the Arctic Ocean that differ primarily in their timing and location within the water column. These include (1) single-celled microalgae growing within the sea ice, (2) phytoplankton that bloom in water under the sea ice (UIB), and (3) phytoplankton that bloom in open water, including in the marginal ice zone (MIZ) and at the depth of the subsurface chlorophyll maximum (SCM).

Increasing our knowledge of the relative rates of NPP and particle export efficiency from these three systems is critical if we are to understand how continued sea ice loss, and the associated shifts in ice algal and phytoplankton bloom dynamics, will differentially impact benthic and pelagic food webs in shallow seas throughout the Arctic Ocean, including many organisms on which Indigenous human populations rely. The particle flux aspects of this proposal could be truly transformative.

We know very little about the relative carbon fluxes from sea ice, UIBs, and open water phytoplankton blooms and virtually nothing about the processes that control those fluxes. Our ability to characterize, for the first time, buoyancy regulation by individual members of each of these three critical microalgal communities will shed new light on how these organic matter fluxes vary as a function of algal cell size, taxonomy, and physiological state, as well as environmental conditions.

This information will contribute greatly to the foundation of knowledge needed to understand potentially significant ecosystem changes, both now and in the future. Furthermore, the project broadens the participation of underrepresented groups through participation in Stanford University’s Summer Undergraduate Research in Geoscience and Engineering (SURGE) program.

The Arrigo lab also takes part in Stanford’s School of Earth Sciences high school internship program. Results of this work will be integrated into lecture and laboratory classes at Stanford exploring the effect of UIBs on the biology and biogeochemistry of the Arctic Ocean. Finally, this project will contribute to the education and training of two Ph.D. students (one female African American) and one post-doc.

Training of undergraduates will be an integral part of this project, and we expect 4-6 to participate in this cruise. Part 2

Detailed observations of microalgal blooms in the Chukchi Sea have been largely made at a time of year when most of the Chukchi shelf is already in open water. As such, the relative rates of daily net primary production (NPP) by sea ice microalgae, under-ice phytoplankton blooms (UIBs), and open water phytoplankton blooms are not well known. Additionally, the fate of the organic matter associated with these three NPP sources is poorly known.

Hence, there is a strong need for comprehensive measurements beginning in late spring that capture all three blooms, with particular emphasis on particle export events. The primary objectives of this proposed study are to utilize data obtained from a ship-based field program to the Chukchi Sea to 1) measure rates of NPP associated with the sea ice, UIBs, and open water phytoplankton in both the MIZ and SCM, 2) use sediment traps to characterize and quantify the vertical sinking fluxes of bulk particulate matter from the sea ice, UIBs, and open water phytoplankton blooms, and 3) use our newly developed Hydrodynamic treadmill to determine how sinking speeds of microalgae from all three NPP sources vary by community composition, physiological state, and environmental condition.

The centerpiece of our proposed program is a 45-day process study cruise that will take place in June 2023. The cruise will consist of a series of hydrographic surveys/sections that will repeatedly transition between the deep ice pack and the marginal ice zone. 150 hydrographic survey (HS) stations will include vertical profiles of temperature, salinity, currents, light, nutrients, and Chl a concentration.

At 90 of these (referred to as biological sampling (BS) stations), we will also collect samples to measure algal and zooplankton abundance/species/size (the latter using the underwater vision profiler, UVP), algal physiology, NPP, POC, particle sinking speeds (Hydrodynamic treadmill), deploy a Haps corer to measure sediment Chl a, and deploy sediment traps to measure vertical fluxes of ice algae, phytoplankton, detritus, and fecal pellets (at 1/3 of the BS stations). During transits, we will continuously measure atmospheric conditions as well as sea surface temperature, salinity, and Chl a fluorescence from the various ship’s systems to provide detailed maps of these parameters above and below the ice.

About every second day, we will conduct a 6-hour sea ice (SI) station near solar noon where we will measure the physical, chemical, and biological properties (see above) of the ice pack and underlying surface water, as well as deploy sediment traps to measure sinking fluxes of algae, detritus, and fecal pellets from the sea ice and from the UIB. The ship will also be used as a platform for characterizing buoyancy regulation and measuring sinking speeds of microalgal cells and aggregates under various manipulated environmental conditions using the Hydrodynamic treadmill.

This instrument provides unlimited scope for active vertical tracking of particles (scale-free), while concurrently providing micron and millisecond-scale spatiotemporal resolution. The Hydrodynamic treadmill also directly measures the flow field around the diatom to measure any bound matrix materials (not visible optically) associated with a single cell and can provide estimates of particle porosity and density.

At each BS and SI station, measurements of sinking speed will be combined with detailed analyses of algal taxonomic composition, physiology, and size structure, as well as environmental characteristics (e.g., light, temperature, nutrients) to determine functional relationships and assess the relative importance of each of the three primary bloom communities to sinking particle fluxes.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.