Properties of suspended particulate matter in the southern Baltic Sea* doi:10.5697/oc.53-3.691 OCEANOLOGIA, 53 (3), 2011. pp. 691–729. ©C Copyright b

This study presents three alternative models for estimating the absorption properties of chromophoric dissolved organic matter a_CDOM(λ). For this analysis we used a database containing 556 absorption spectra measured in 2006–2009 in different regions of the Baltic Sea (open and coastal waters, the Gulf of Gdańsk and the Pomeranian Bay), at river mouths, in the Szczecin Lagoon and also in three lakes in Pomerania (Poland) – Obłęskie, Łebsko and Chotkowskie. The variability range of the chromophoric dissolved organic matter (CDOM) absorption coefficient at 400 nm, a_CDOM(400), lay within 0.15–8.85 m⁻¹. The variability in a_CDOM(λ) was parameterized with respect to the variability over 3 orders of magnitude in the chlorophyll a concentration Chl a (0.7–119 mg m⁻³). The chlorophyll a concentration and a_CDOM(400) were correlated, and a statistically significant, nonlinear empirical relationship between these parameters was derived (R² = 0.83). On the basis of the covariance between these parameters, we derived two empirical mathematical models that enabled us to design the CDOM absorption coefficient dynamics in natural waters and reconstruct the complete CDOM absorption spectrum in the UV and visible spectral domains. The input variable in the first model was the chlorophyll a concentration, and in the second one it was a_CDOM(400). Both models were fitted to a power function, and a second-order polynomial function was used as the exponent. Regression coefficients for these formulas were determined for wavelengths from 240 to 700 nm at 5 nm intervals. Both approximations reflected the real shape of the absorption spectra with a low level of uncertainty. Comparison of these approximations with other models of light absorption by CDOM demonstrated that our parameterizations were superior (bias from −1.45 to 62 %, RSME from 22 to 220 %) for estimating CDOM absorption in the optically complex waters of the Baltic Sea and Pomeranian lakes.

Bio‐optical properties of surface waters were characterized off western and northern Spitsbergen in the summers of 2013, 2014, and 2015. We observed statistically significant year‐to‐year differences in spatial distribution of spectral absorption (a(λ)) and beam attenuation (c(λ)). Highest a(λ) and c(λ) were located in the frontal zone between water masses and co‐varied strongly with chlorophyll a fluorescence. Phytoplankton pigments dominated the absorption budget at 443 nm (50%). The contribution of chromophoric dissolved organic matter to total nonwater absorption was highest at 412 nm (42%), and detrital absorption contributed most at 550 nm (37%). Almost all inherent optical properties, except chromophoric dissolved organic matter, were highly correlated with the chlorophyll a concentration (Chla, R² > 0.81). Relationships between Chla and the particulate and phytoplankton pigments absorption coefficients at 443 and 676 nm were characterized by significant determination coefficients (R² > 0.73). The phytoplankton pigments line height absorption a_LH(676) was found to be the most reliable optical proxy for determination of Chla, compared to total nonwater absorption, a_pg(676), and stimulated in situ chlorophyll a fluorescence intensity, I_Chla. In the presence of sea ice melt the water column was stratified and the vertical distribution of inherent optical properties was characterized by a surface minimum followed by a distinct subsurface maximum, aligned with a subsurface chlorophyll a maximum. We surmise that prevailing regional wind patterns affect sea ice and surface drift in central Fram Strait, and thus the location of sea ice meltwater, which affects the vertical stratification and occurrence of subsurface chlorophyll a maximum.