Particulate Organic Matter Distributions in the Water Column of the
Chukchi Sea during Late Summer
Abstract
Two oceanographic cruises were completed in September 2016 and August
2017 to investigate the distribution of particulate organic matter (POM)
across the northeast Chukchi Shelf. Both periods were characterized by
highly stratified conditions, with major contrasts in the distribution
of regional water masses that impacted POM distributions. Overall,
surface waters were characterized by low chlorophyll fluorescence (Chl
Fl<0.8 mg m-3) and particle beam attenuation
(cp<0.3 m-1) values, and low
concentrations of particulate organic carbon (POC<8 mmol
m-3), chlorophyll and pheophytin
(Chl+Pheo<0.8 mg m-3), and suspended
particulate matter (SPM~2 g m-3).
Elevated Chl Fl and Chl+Pheo (~2 mg
m-3) values measured at mid-depths below the
pycnocline defined the subsurface chlorophyll maxima (SCM), which
exhibited moderate POC (~10 mmol m-3),
cp (~0.4 m-1) and SPM
(~3 g m-3). In contrast, deeper waters
below the pycnocline were characterized by low Chl Fl and Chl+Pheo
(~0.7 mg m-3), high cp
(>1.5 m-1) and SPM (>8 g
m-3) and elevated POC (>10 mmol
m-3). POM compositions from surface and SCM regions of
the water column were consistent with contributions from active
phytoplankton sources whereas samples from bottom waters were
characterized by high Pheo/(Chl+Pheo) ratios (>0.4)
indicative of altered phytoplankton detritus. Marked contrasts in POM
were observed in both surface and mid-depth waters during both cruises.
Increases in chlorophyll and POC were measured in mid-depth waters
during the September 2016 cruise following a period of
downwelling-favorable winds, and in surface waters during the August
2017 cruise following a period of upwelling-favorable winds.