Seasonal and spatial variability of phytoplankton and non-algal absorption in the surface layer of the Baltic

Abstract

The paper addresses the seasonal variability of coefficients of light absorption by suspended particulate matter ap(λ) in different regions of the Baltic Sea: open-sea waters, coastal waters, the Gulf of Gdańsk, the Pomeranian Bay and river mouths. We analysed 865 spectra of light absorption by phytoplankton aph(λ) and non-algal particles aNAP(λ) in different seasons during 35 cruises of r/v Oceania in the southern Baltic from 2006 to 2012. Depending on season and region, we found distinct differences in the absorption properties of particles suspended in the water with respect both to the spectral shapes of the coefficients ap(λ), aph(λ) and aNAP(λ), and to the values at the characteristic wavelength of 440 nm. The proportions of phytoplankton governing the absorption properties of the suspended matter varied from 50 to 75%, whereby the greatest spread of these proportions was found in open waters. In contrast, the greatest seasonal variability in the overall absorption coefficients ap occurred in coastal waters.The absolute values of coefficients aph(λ) and aNAP(λ) at wavelength 440 nm in the different types of water and seasons varied respectively over three and four orders of magnitude. In river mouth waters the mean values of aph(440) were nearly twelve times and aNAP(440) over twenty times greater than in open sea waters. The absorption properties of non-algal particles exhibited a greater regional than seasonal variability, achieving the lowest values in open and coastal waters (0.006–0.31 m−1), and the highest ones in river mouths (0.199–4.196 m−1).