Partitioning particulate light absorption: A budget for a Mediterranean bay

The contribution of different components of the plankton (autotrophs and heterotrophic bacteria, heterotrophic flagellates, and mixo‐ and heterotrophic ciliates) and suspended inorganic particles to light absorbed by particles in a Mediterranean bay was examined based on a 2‐year time series of particulate light absorption (400–700, 400, and 675 nm), the biomass of planktonic microorganisms, and the mass of suspended inorganic particles. The average (±SE) particulate light absorption coefficient for the photosynthetically active radiation (PAR) range (0.035 ± 0.002 m −1 ) was characteristic of relatively clear coastal waters but showed great variability on occasions. A substantial fraction (53–73%, depending on the wavelength examined) of this variability could be accounted for by changes in the abundance of inorganic suspended matter, as well as planktonic organisms. The specific light absorption by autotrophs was less variable over the three spectral bands considered than those of microheterotrophs and inorganic particles, which dropped sharply with increasing wavelength. Inorganic particles contributed, on the average, 48 and 74% of the total particulate absorption for the PAR waveband and at 400 nm, respectively, with their contribution to light absorption at 675 nm being negligible. Autotrophs dominated light absorption at 675 nm (on average 45.8% of total particulate absorption), whereas mixo‐ and heterotrophic ciliates and bacteria together contributed, on average, 22.5% of the total light absorption at this wavelength. The combined light absorption coefficient of microheterotrophs at 400 nm (0.0126 m −1 ) was similar to that of autotrophs (0.013 m −1 ). These results documented the dominant role that inorganic particles play in the absorption of blue light in the Bay of Blanes and showed that the particulate light absorption by autotrophs was often comparable to that of heterotrophs.