The goal of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign is to develop a predictive understanding of the export, fate, and carbon cycle impacts of global ocean net primary production. To accomplish this goal, observations of export flux pathways, plankton community composition, food web processes, and optical, physical, and biogeochemical (BGC) properties are needed over a range of ecosystem states. Here we introduce the first EXPORTS field deployment to Ocean Station Papa in the Northeast Pacific Ocean during summer of 2018, providing context for other papers in this special collection. The experiment was conducted with two ships: a Process Ship, focused on ecological rates, BGC fluxes, temporal changes in food web, and BGC and optical properties, that followed an instrumented Lagrangian float; and a Survey Ship that sampled BGC and optical properties in spatial patterns around the Process Ship. An array of autonomous underwater assets provided measurements over a range of spatial and temporal scales, and partnering programs and remote sensing observations provided additional observational context. The oceanographic setting was typical of late-summer conditions at Ocean Station Papa: a shallow mixed layer, strong vertical and weak horizontal gradients in hydrographic properties, sluggish sub-inertial currents, elevated macronutrient concentrations and low phytoplankton abundances. Although nutrient concentrations were consistent with previous observations, mixed layer chlorophyll was lower than typically observed, resulting in a deeper euphotic zone. Analyses of surface layer temperature and salinity found three distinct surface water types, allowing for diagnosis of whether observed changes were spatial or temporal. The 2018 EXPORTS field deployment is among the most comprehensive biological pump studies ever conducted. A second deployment to the North Atlantic Ocean occurred in spring 2021, which will be followed by focused work on data synthesis and modeling using the entire EXPORTS data set.
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Direct Observations of Biological Carbon Export From Profiling Floats in the Subtropical North Atlantic
The complex interplay of biological and physical mechanisms comprising the ocean's biological carbon pump has not been well characterized to date, due to the difficulty of observing these mechanisms in situ at adequate spatial and temporal resolution. An annual time series is presented of direct measurements of export production and particle properties collected using optical sediment trap‐equipped profiling floats cycling every 1.5–2.5 days. The observations indicate strong variability in particle export and bio‐optical properties, influenced by the spring bloom, mesoscale eddy activity, and the mixed layer pump. Temporal and vertical decoupling of fluxes at depths ranging from 150 to 1,000 m was also observed, and remineralization length scales were more variable than predicted by temperature‐ and oxygen‐based models. Net primary production was computed from float observations using a modification of the Carbon‐based Productivity Model and used to estimate export and export (e‐) ratios, which were compared to predictions of literature export models. Mechanistic models explicitly incorporating ecosystem processes and their depth dependence may perform better at reproducing regional observations collected at high temporal resolution.
more » « less- NSF-PAR ID:
- 10374491
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 33
- Issue:
- 3
- ISSN:
- 0886-6236
- Page Range / eLocation ID:
- p. 282-300
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Carbon export out of the surface ocean via the biological pump is a critical sink for atmospheric carbon dioxide. This process transports organic carbon to the deep ocean through sinking particulate organic carbon (POC) and the downward transport of suspended POC and dissolved organic carbon (DOC). Changes in the relative contribution of each pathway can significantly affect the magnitude and efficiency of carbon export to depth. Net community production (NCP), an analog of carbon export under steady state assumptions, is typically estimated using budgets of biologically important chemical tracers in the upper ocean constrained by ship‐board or autonomous platform observations. In this study, we use measurements from biogeochemical profiling floats, the Ocean Station Papa mooring, and recently developed algorithms for carbonate system parameters to constrain budgets for three tracers (nitrate, dissolved inorganic carbon, and total alkalinity) and estimate NCP in the Northeast Pacific from 2009 to 2017. Using our multiple‐tracer approach, and constraining end‐member nutrient ratios of the POC and DOC produced, we not only calculate regional NCP throughout the annual cycle and across multiple depth horizons, but also partition this quantity into particulate and dissolved portions. We also use a particle backscatter‐based approach to estimate POC attenuation with depth and present a new method to constrain particle export across deeper horizons and estimate in situ export efficiency. Our results agree well with previously published estimates of regional carbon export annually and suggest that the approaches presented here could be used to assess the magnitude and efficiency of carbon export in other regions of the world's oceans.
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Abstract Surface phytoplankton communities were linked with the carbon they export into the deep ocean by comparing 18 S rRNA gene sequence communities from surface seawater and individually isolated sinking particles. Particles were collected in sediment traps deployed at locations in the North Pacific subtropical gyre and the California Current. DNA was isolated from individual particles, bulk-collected trap particles, and the surface seawater. The relative sequence abundance of exported phytoplankton taxa in the surface water varied across functional groups and ecosystems. Of the sequences detected in sinking particles, about half were present in large (>300 μm), individually isolated particles and primarily belonged to taxa with small cell sizes (<50 µm). Exported phytoplankton taxa detected only in bulk trap samples, and thus presumably packaged in the smaller sinking size fraction, contained taxa that typically have large cell sizes (>500 μm). The effect of particle degradation on the detectable 18 S rRNA gene community differed across taxa, and differences in community composition among individual particles from the same location largely reflected differences in relative degradation state. Using these data and particle imaging, we present an approach that incorporates genetic diversity into mechanistic models of the ocean’s biological carbon pump, which will lead to better quantification of the ocean’s carbon cycle.
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Abstract To better quantify the ocean's biological carbon pump, we resolved the diversity of sinking particles that transport carbon into the ocean's interior, their contribution to carbon export, and their attenuation with depth. Sinking particles collected in sediment trap gel layers from four distinct ocean ecosystems were imaged, measured, and classified. The size and identity of particles was used to model their contribution to particulate organic carbon (POC) flux. Measured POC fluxes were reasonably predicted by particle images. Nine particle types were identified, and most of the compositional variability was driven by the relative contribution of aggregates, long cylindrical fecal pellets, and salp fecal pellets. While particle composition varied across locations and seasons, the entire range of compositions was measured at a single well‐observed location in the subarctic North Pacific over one month, across 500 m of depth. The magnitude of POC flux was not consistently associated with a dominant particle class, but particle classes did influence flux attenuation. Long fecal pellets attenuated most rapidly with depth whereas certain other classes attenuated little or not at all with depth. Small particles (<100 μm) consistently contributed ∼5% to total POC flux in samples with higher magnitude fluxes. The relative importance of these small particle classes (spherical mini pellets, short oval fecal pellets, and dense detritus) increased in low flux environments (up to 46% of total POC flux). Imaging approaches that resolve large variations in particle composition across ocean basins, depth, and time will help to better parameterize biological carbon pump models.
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Abstract Modern observations indicate that variations in marine phytoplankton stoichiometry correlate with the boundaries of major surface waters. For example, phytoplankton in the oligotrophic subtropical gyres typically have much higher C:N:P ratios (i.e., higher C:P and higher N:P ratios) than those in eutrophic upwelling regions and polar regions. Such a spatial pattern points to nutrient availability as a key environmental driver of stochiometric flexibility. Environmental dependence of phytoplankton C:N:P opens unexplored possibilities for modifying the strength of the biological pump under different climate conditions. Here we present a power law formulation of C:N:P flexibility that is driven by nutrients, temperature, and light. We embed the formulation in a global ocean carbon cycle model with multiple phytoplankton types and explore biogeochemical implications under glacial conditions. We find three key controls on export C:N:P ratio: phytoplankton physiology and community structure as well as the balance in regional production at the global level. Glacial inputs of iron and sea ice expansion are important modifiers of these three controls. We also find that global export C:N:P increases substantially under glacial conditions, and this strongly buffers global carbon export against decrease and draws down approximately 20 μatm of atmospheric CO2. These results point to the importance of including phytoplankton C:N:P flexibility in a mix of mechanisms that drive atmospheric CO2over glacial‐interglacial time scale. Finally, our simulations indicate decoupling of nutrients, which may provide a resolution to the longstanding disagreement regarding nutrient utilization in the glacial Southern Ocean derived from different nutrient proxies.
