Alexander M. Chekalyuk

Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction

Significance Transport of organic carbon from the sunlit surface ocean to deeper depths drives net oceanic uptake of CO2 from the atmosphere. However, mechanisms that control this carbon export remain poorly constrained, limiting our ability to model and predict future changes in this globally important process. We show that the flux of sinking particles (typically considered the dominant form of downward transport of organic carbon) is twice as high at a frontal system, relative to surrounding waters or to nonfrontal conditions. Furthermore, downward transport by subduction leads to additional carbon export at the front that is similar in magnitude to the sinking flux. Such enhanced C export at episodic and mesoscale features needs to be incorporated into biogeochemical forecast models. Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from 238U:234Th disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C⋅m−2⋅d−1) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front was mechanistically linked to Fe-stressed diatoms and high mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional ∼225 mg C⋅m−2⋅d−1 was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.

Photo-physiological variability in phytoplankton chlorophyll fluorescence and assessment of chlorophyll concentration

Photo-physiological variability of in vivo chlorophyll fluorescence (CF) per unit of chlorophyll concentration (CC) is analyzed using a biophysical model to improve the accuracy of CC assessments. Field measurements of CF and photosystem II (PSII) photochemical yield (PY) with the Advanced Laser Fluorometer (ALF) in the Delaware and Chesapeake Bays are analyzed vs. high-performance liquid chromatography (HPLC) CC retrievals. It is shown that isolation from ambient light, PSII saturating excitation, optimized phytoplankton exposure to excitation, and phytoplankton dark adaptation may provide accurate in vivo CC fluorescence measurements (R2 = 0.90-0.95 vs. HPLC retrievals). For in situ or flow-through measurements that do not allow for dark adaptation, concurrent PY measurements can be used to adjust for CF non-photochemical quenching (NPQ) and improve the accuracy of CC fluorescence assessments. Field evaluation has shown the NPQ-invariance of CF/PY and CF(PY-1-1) parameters and their high correlation with HPLC CC retrievals (R2 = 0.74-0.96), while the NPQ-affected CF measurements correlated poorly with CC (R2 = -0.22).

Next generation Advanced Laser Fluorometry (ALF) for characterization of natural aquatic environments: new instruments

The new optical design allows single- or multi-wavelength excitation of laser-stimulated emission (LSE), provides optimized LSE optical collection for spectral and temporal analyses, and incorporates swappable modules for flow-through and small-volume sample measurements. The basic instrument configuration uses 510 nm laser excitation for assessments of chlorophyll-a, phycobiliprotein pigments, variable fluorescence (F(v)/F(m)) and chromophoric dissolved organic matter (CDOM) in CDOM-rich waters. The three-laser instrument configuration (375, 405, and 510 nm excitation) provides additional Fv/Fm measurements with 405 nm excitation, CDOM assessments in a broad concentration range, and potential for spectral discrimination between oil and CDOM fluorescence. The new measurement protocols, analytical algorithms and examples of laboratory and field measurements are discussed.

Aquatic laser fluorescence analyzer: field evaluation in the northern Gulf of Mexico.

The new Aquatic Laser Fluorescence Analyzer (ALFA) provides spectral and temporal measurements of laser-stimulated emission (LSE) for assessment of phytoplankton pigments, community structure, photochemical efficiency (PY), and chromophoric dissolved organic matter (CDOM). The instrument was deployed in the Northern Gulf of Mexico to evaluate the ALFA analytical capabilities across the estuarine-marine gradient. The robust relationships between the pigment fluorescence and independent pigment measurements were used to validate the ALFA analytical algorithms and calibrate the instrument. The maximal PY magnitudes, PYm = PY(1-1.35·10⁻⁴PAR⁻¹, were estimated using the underway measurements of PY and photosynthetically active radiation (PAR). The chlorophyll (Chl) spatial patterns were calculated using the ratio of Chl fluorescence to PY to eliminate the effect of non-photochemical quenching on the underway Chl assessments. These measurements have provided rich information about spatial distributions of Chl, PYm, CDOM, and phytoplankton community structure, and demonstrated the utility of the ALFA instrument for oceanographic studies and bio-environmental surveys. The data suggest that the fluorescence measurements with 514 nm excitation can provide informative data for characterization of the CDOM-rich fresh, estuarine, and coastal aquatic environments.

Analysis of spectral excitation for measurements of fluorescence constituents in natural waters

Field measurements of chlorophyll-a (Chl), phycoerythrin (PE), chromophoric dissolved organic matter (CDOM), and variable fluorescence (F(v)/F(m)) in diverse waters of the California Current, Mediterranean Sea and Gulf of Mexico using 375, 405, 510 and 532 nm laser excitation wavelengths (EW) are analyzed. EW = 375 and 405 nm were found more suitable for Chl assessment in high-Chl (> 10 μg/l) waters. Both EW = 532 and 510 nm can be used to efficiently stimulate PE fluorescence for structural characterization of phytoplankton communities. EW = 375 nm and 405 nm can provide best results for CDOM assessments in offshore oceanic waters; the green EWs can be also used for CDOM measurements in fresh and estuarine water types in conjunction with spectral discrimination between CDOM and PE fluorescence. Both EW = 405 and 510 are suitable for photo-physiological F(v)/F(m) assessments, though using EW = 405 nm may result in underestimation of PE-containing phytoplankton groups present in mixed phytoplankton assemblages.