Gradients 3 Underway Optics

Cruise Name: KM1906, Gradients 3.0.

Project: Simons Foundation - Gradients NPSG

A Wetlabs AC-S and ECO Triplet were plumbed to the underway uncontaminated seawater on the R/V KM.  Data for each instrument were averaged to the minute. For the first 10 min of every hour, incoming seawater was passed through a 0.2 µm filter (Graver ZTEC G series with Teflon O-rings) in order to collect dissolved ‘blanks’ that were subsequently used to correct whole water signals collected in the succeeding 50 min of each hour. Filtered values were linearly interpolated throughout the cruise and subtracted from raw measurements to provide corrected chlorophyll fluorescence (volts), the particulate volume scattering function at 117 degrees (β, m-1 sr-1), spectral particulate beam attenuation at 83 wavelengths (cp(λ), m-1), and spectral particulate absorption at 83 wavelengths (ap(λ), m-1).

Chlorophyll fluorescence (Flchl) was calibrated to discrete extracted chlorophyll measurements collected at nighttime. Particulate β values were converted to the particulate backscattering coefficient (bbp, m-1), following Boss and Pegau (2001) [bbp = 2𝜋𝜒p𝛽p] where 𝜒p is 1.1 for 117°. Particulate absorption was then corrected for residual temperature and scattering after Slade et al. (2010). The dissolved component of absorption (ag(λ), m-1) was calculated from filtered measurements using a pure freshwater blank and temperature and salinity corrected according to Slade et al. (2010). AC-S-derived chlorophyll a concentrations (Chla676) were estimated from the particulate absorption coefficients using the line-height method (Davis et al., 1997) assuming a chlorophyll specific absorption coefficient of 0.01178 m2 mg-1.

Plankton abundance and carbon content was measured by a combination of continuous flow cytometry via SeaFlow which measures auto-fluorescing particles ranging in size from ~0.4 to ~6 μm as well as an Imaging FlowCytobot (IFCB) with imaging of 4-100 mm particles triggered on both scattering and fluorescence.  For the IFCB data, particulate cell diameter was converted to carbon biovolume using allometric scaling (Menden-Deuer and Lessard, 2000) whereas the  forward light scattering-biovolume relationship from Ribalet et al. (2019) was used to convert SeaFlow scattering per cell to carbon biovolume. Additional details of data processing are described in Juranek et al. (2020).

Measurements of surface planar photosynthetically active radiation (PAR) were made using a LICOR quantum cosine collector (400-700 nm) and data logger (models LI-190R and LI-1500, respectively). The sensor was positioned 4 meters above the deck to minimize the influence of shadows from the ship’s superstructure. Spectrally integrated irradiance was logged throughout the day and averaged in 1 min intervals. The diffuse attenuation coefficient for PAR, KPAR, was calculated according to methods described in Wei and Lee (2013), which calculates the average daily KPAR as a function of the sun angle at local solar noon, the total absorption coefficient at 490 nm, and the total backscattering coefficient at 490 nm.