Absorption properties along the 70-m isobath in the southeastern Bering Sea during July 2008

The spectral absorption coefficients of particulate matter are important parameters in bio-optical models and to understand the behavior of underwater light field. Particulate (a_P(?)), phytoplankton (a_PHY(?)) and non-algal particles (NAP) (a_NAP(?)) absorption were measured along the 70-m isobath in the southeastern Bering Sea. The surface distribution of a_P(?), a_PHY(?) and a_NAP(?) at 443 nm did not reveal any distinct trends, but over the entire water column the northern part showed higher particulate absorption relative to the southern part of the 70-m isobath. The a_P(?) at 443 nm was higher at middle depths due to chlorophyll maxima relative to surface samples. The normalized a_PHY(?) spectra were variable within stations as well as between stations with variability larger in the blue region compared to the red region of the spectrum which suggested change in phytoplankton species or pigment composition or package effect. The generally higher values of the blue to red peak ratios of normalized a_PHY(?) (> 2) indicated that smaller phytoplankton cells dominated in surface waters relative to middle depths. The spectral slope of a_NAP(?) did not portray any trend with a_NAP(?), so parameterization of a_NAP(?) is possible along the 70-m isobath by using the mean value of spectral slopes. High correlation (R² =3D 0.81) found between a_P(?) and a_PHY(?) at 443 nm, indicated that a_PHY(?) dominated changes in a_P(?). The a_NAP(?) also correlated well with a_P(?) at 443 nm (R² = 0.64) though not as good as a_PHY(?), so a_NAP(?) also drives changes in a_P(?) to a certain extent. The relative contribution of a_PHY(?) to a_P(?) was greater at 667 nm (35-97%) then at 443 nm (18-88%), however for some samples the contr- ibution of a_NAP(?) was almost equal to a_PHY(?) corresponding to low chlorophyll biomass samples.