Oceanologia (Sep 2010)
Fluorescence measured in situ as a proxy of CDOM absorption and DOC concentration in the Baltic Sea
Abstract
This study presents results from field surveys performed in 2008 and 2009 in the southern Baltic in different seasons. The main goal of these measurements was to identify the empirical relationships between DOM optical properties and DOC. CDOM absorption and fluorescence and DOC concentrations were measured during thirteen research cruises. The values of the CDOM absorption coefficient at 370 nm aCDOM(370) ranged from 0.70 m-1 to 7.94 m-1, and CDOM fluorescence intensities (ex./em. 370/460) IFl, expressed in quinine sulphate equivalent units, ranged from 3.88 to 122.97 (in filtered samples). Dissolved organic carbon (DOC) concentrations ranged from 266.7 to 831.7 µM C. There was a statistically significant linear relationship between the fluorescence intensity measured in the filtered samples and the CDOM absorption coefficient aCDOM(370), R2 = 0.87. There was much more scatter in the relationship between the fluorescence intensity measured in situ (i.e. in unprocessed water samples) and the CDOM absorption coefficient aCDOM(370), resulting in a slight deterioration in the coefficient of determination R2 = 0.85. This indicated that the presence of particles could impact fluorometer output during in situ deployment. A calibration experiment was set up to quantify particle impact on the instrument output in raw marine water samples relative to readings from filtered samples. The bias calculated for the absolute percentage difference between fluorescence intensities measured in raw and filtered water was low (-2.05%), but the effect of particle presence expressed as the value of the RMSE was significant and was as high as 35%. Both DOM fluorescence intensity (in raw water and filtered samples) and the CDOM absorption coefficient aCDOM(370) are highly correlated with DOC concentration. The relationship between DOC and the CDOM absorption coefficient aCDOM(370) was better (R2 = 0.76) than the relationship between DOC and the respective fluorescence intensities measured in filtered and raw water (R2 = 0.61 and R2 = 0.56). The seasonal cycle had an impact on the relationship between DOC and CDOM optical properties. The hyperbolic relationships between aCDOM(370) vs. carbon-specific absorption coefficient a*CDOM(370), and IFl vs. the ratio of fluorescence intensity to organic carbon concentration IFl/DOC were very good. The discharge and mixing of riverine waters is a primary driver of variability in DOC and CDOM optical properties in the surface waters of the southern Baltic Sea, since all the parameters considered are negatively correlated with salinity. It was found that there was a positive trend of increasing values of DOM optical parameters with salinity increase (within a range of 8-12) in deep water below the permanent pycnocline. Evidence is also presented to show that late-summer photodegradation was responsible for the depletion of CDOM florescence intensities in the mixed layer above the seasonal thermocline. It was further demonstrated that the DOC concentration increases in the stagnant waters of the Baltic Sea deeps. The Integrated Optical-Hydrological Probe, which registers high-resolution vertical profiles of salinity, temperature, CDOM and the optical properties of water, confirmed that DOM optical proxies can be used in studies of DOM biogeochemical cycles in the Baltic Sea.
