Spectral absorption and fluorescence characteristics of phytoplankton in different size fractions across a salinity gradient in the Baltic Sea.

Spectral absorption and fluorescence properties of living phytoplankton were measured across a salinity gradient in the northern Baltic Sea in August 2000. The discrete water samples (n=22) were size-fractionated (<2 μm and <20 μm) to study the origins of different optical signals. The concentration of chlorophyll-a (Chla) ranged from 1.6 mg m^-3 to 6.0 mg m^-3, from which picophytoplankton (<2 μm) contributed 18–54%. Phytoplankton community structure had a clear gradient; from the northern part, dominated by chlorophytes, to the southern part, dominated by cyanophytes (mainly Aphanizomenon sp.). The regional variability in the Chla-specific absorption coefficient was partly explained by variations in phytoplankton community size-structure, or species composition. Phycoerythrin absorption was notable, especially for picophytoplankton. Accordingly, phycoerythrin fluorescence arose mainly from picophytoplankton (average 74%, range 35–100%). Thus, pico-sized cyanophytes - predominated by phycoerythrin-rich cells - were the main source of the phycoerythrin signal. Large filamentous cyanobacteria, in turn, were responsible for the phycoerythrocyanin and phycocyanin signals. The observed spectral differences between size fractions contribute to the development of operational optical detection systems for cyanobacterial blooms and provide the background for other absorption and fluorescence-based algal detection systems and for remote sensing algorithm development. [ABSTRACT FROM AUTHOR]