skip to main content


Title: The Importance of Subsurface Productivity in the Pacific Arctic Gateway as Revealed by High‐Resolution Biogeochemical Surveys
Abstract

Following sea‐ice retreat, surface waters of Arctic marginal seas become nutrient‐limited and subsurface chlorophyll maxima (SCM) develop below the pycnocline where nutrients and light conditions are favorable. However, the importance of these “hidden” features for regional productivity is not well constrained. Here, we use a unique combination of high‐resolution biogeochemical and physical observations collected on the Chukchi shelf in 2017 to constrain the fine‐scale structure of nutrients, O2, particles, SCM, and turbulence. We find large O2excess at middepth, identified by positive saturation () maxima of 15%–20% that unambiguously indicate significant production occurring in middepth waters. Themaxima coincided with a complete depletion of dissolved inorganic nitrogen (DIN = NO3 + NO2 + NH4+). Nitracline depths aligned with SCM depths and the lowest extent ofmaxima, suggesting this horizon represents a compensation point for balanced growth and loss. Furthermore, SCM were also associated with turbulence minima and sat just above a high turbidity bottom layer where light attenuation increased significantly. Spatially, the largestmaxima were associated with high nutrient winter‐origin water masses (14.8% ± 2.4%), under a shallower pycnocline associated with seasonal melt while lower values were associated with summer‐origin water masses (7.4% ± 3.9%). Integrated O2excesses of 800–1,200 mmol m−2in regions overlying winter water are consistent with primary production rates that are 12%–40% of previously reported regional primary production. These data implicate short‐term and long‐term control of SCM and associated productivity by stratification, turbulence, light, and seasonal water mass formation, with corresponding potential for climate‐related sensitivities.

 
more » « less
Award ID(s):
1949593
NSF-PAR ID:
10396432
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Oceans
Volume:
128
Issue:
2
ISSN:
2169-9275
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    In this study, we report on turbulent mixing observed during the annual stratification cycle in the hypolimnetic waters of Lake Michigan (USA), highlighting stratified, convective, and transitional mixing periods. Measurements were collected using a combination of moored instruments and microstructure profiles. Observations during the stratified summer showed a shallow, wind‐driven surface mixed layer (SML) with locally elevated dissipation rates in the thermocline () potentially associated with internal wave shear. Below the thermocline, turbulence was weak () and buoyancy‐suppressed (< 8.5), with low hypolimnetic mixing rates () limiting benthic particle delivery. During the convective winter period, a diurnal cycle of radiative convection was observed over each day of measurement, where temperature overturns were directly correlated with elevated turbulence levels throughout the water column (;). A transitional mixing period was observed for spring conditions when surface temperatures were near the temperature of maximum density (TMD3.98) and the water column began to stably stratify. While small temperature gradients allowed strong mixing over the transitional period (), hypolimnetic velocity shear was overwhelmed by weakly stable stratification (;), limiting the development of the SML. These results highlight the importance of radiative convection for breaking down weak hypolimnetic stratification and driving energetic, full water column mixing during a substantial portion of the year (>100 days at our sample site). Ongoing surface water warming in the Laurentian Great Lakes is significantly reducing the annual impact of convective mixing, with important consequences for nutrient cycling, primary production, and benthic‐pelagic coupling.

     
    more » « less
  2. Abstract

    The air‐sea exchange of oxygen (O2) is driven by changes in solubility, biological activity, and circulation. The total air‐sea exchange of O2has been shown to be closely related to the air‐sea exchange of heat on seasonal timescales, with the ratio of the seasonal flux of O2to heat varying with latitude, being higher in the extratropics and lower in the subtropics. This O2/heat ratio is both a fundamental biogeochemical property of air‐sea exchange and a convenient metric for testing earth system models. Current estimates of the O2/heat flux ratio rely on sparse observations of dissolved O2, leaving it fairly unconstrained. From a model ensemble we show that the ratio of the seasonal amplitude of two atmospheric tracers, atmospheric potential oxygen (APO) and the argon‐to‐nitrogen ratio (Ar/O2), exhibits a close relationship to the O2/heat ratio of the extratropics (40–). The amplitude ratio,/, is relatively constant within the extratropics of each hemisphere due to the zonal mixing of the atmosphere./is not sensitive to atmospheric transport, as most of the observed spatial variability in the seasonal amplitude ofAPO is compensated by similar variations in(Ar/). From the relationship between/heat and/in the model ensemble, we determine that the atmospheric observations suggest hemispherically distinct/heat flux ratios of 3.30.3 and 4.70.8 nmolbetween 40 andin the Northern and Southern Hemispheres respectively, providing a useful constraint forand heat air‐sea fluxes in earth system models and observation‐based data products.

     
    more » « less
  3. Abstract

    Estimates of turbulence kinetic energy (TKE) dissipation rate (ε) are key in understanding how heat, gas, and other climate‐relevant properties are transferred across the air‐sea interface and mixed within the ocean. A relatively new method involving moored pulse‐coherent acoustic Doppler current profilers (ADCPs) allows for estimates ofεwith concurrent surface flux and wave measurements across an extensive length of time and range of conditions. Here, we present 9 months of moored estimates ofεat a fixed depth of 8.4 m at the Stratus mooring site (20°S, 85°W). We find that turbulence regimes are quantified similarly using the Obukhov length scaleand the newer Langmuir stability length scale, suggesting that ocean‐side friction velocityimplicitly captures the influence of Langmuir turbulence at this site. This is illustrated by a strong correlation between surface Stokes driftandthat is likely facilitated by the steady Southeast trade winds regime. In certain regimes,, whereis the von Kármán constant andis instrument depth, and surface buoyancy flux capture our estimates ofwell, collapsing data points near unity. We find that a newer Langmuir turbulence scaling, based onand, scalesεwell at times but is overall less consistent than. Monin‐Obukhov similarity theory (MOST) relationships from prior studies in a variety of aquatic and atmospheric settings largely agree with our data in conditions where convection and wind‐driven current shear are both significant sources of TKE, but diverge in other regimes.

     
    more » « less
  4. Abstract

    Prior investigations have attempted to characterize the longitudinal variability of the column number density ratio of atomic oxygen to molecular nitrogen (O/N2) in the context of non‐migrating tides. The retrieval of thermosphericO/N2from far ultra‐violet (FUV) emissions assumes production is due to photoelectron impact excitation on O and N2. Consequently, efforts to characterize the tidal variability inO/N2have been limited by ionospheric contamination from O+ + e radiative recombination at afternoon local times (LT) around the equatorial ionization anomaly. The retrieval ofO/N2from FUV observations by the Ionospheric Connection Explorer (ICON) provides an opportunity to address this limitation. In this work, we derive modifiedO/N2datasets to delineate the response of thermospheric composition to non‐migrating tides as a function of LT in the absence of ionospheric contamination. We assess estimates of the ionospheric contribution to 135.6 nm emission intensities based on either Global Ionospheric Specification (GIS) electron density, International Reference Ionosphere (IRI) model output, or observations from the Extreme Ultra‐Violet imager (EUV) onboard ICON during March and September equinox conditions in 2020. Our approach accounts for any biases between the ionospheric and airglow datasets. We found that the ICON‐FUV data set, corrected for ionospheric contamination based on GIS, uncovered a previously obscured diurnal eastward wavenumber 2 tide in a longitudinal wavenumber 3 pattern at March equinox in 2020. This finding demonstrates not only the necessity of correcting for ionospheric contamination of the FUV signals but also the utility of using GIS for the correction.

     
    more » « less
  5. Abstract

    Nitrification, the microbial conversion of ammonium to nitrite then to nitrate, occurs throughout the oceanic water column, yet the environmental factors influencing the production of nitrate in the euphotic zone (EZ) remain unclear. In this study, the natural abundances of N and O isotopes (δ15N and δ18O, respectively) in nitrate were used in an existing model framework to quantify nitrate contributed by EZ nitrification in the California Current Ecosystem (CCE) during two anomalously warm years. Model data estimated that between 6% and 36% of the EZ nitrate reservoirs were derived from the combined steps of nitrification within the EZ. The CCE data set found nitrification contributions to EZ nitrate to be positively correlated with nitrite concentrations () at the depth of the primary nitrite maximum (PNM). Building on this correlation, EZ nitrification in the southern California Current was estimated to contribute on average 20% ± 6% to EZ nitrate as inferred using the PNMof the long‐term California Cooperative Oceanic Fisheries Investigation (CalCOFI) survey record. A multiple linear regression analysis of the CalCOFI PNMtime series identified two conditions that led to positive deviations in. Enhanced PNM, and potentially enhanced EZ nitrification, may be linked to (1) reduced phytoplankton competition for ammonium () andas interpreted from particulate organic carbon:chlorophyll ratios, and/or (2) to increased supply of(and thenoxidation to) from the degradation of organic nitrogen as interpreted from particulate organic nitrogen concentrations.

     
    more » « less