PARTITIONING PARTICULATE LIGHT-ABSORPTION - A BUDGET FOR A MEDITERRANEAN BAY

The contribution of different components of the plankton (autotrophs a nd heterotrophic bacteria, heterotrophic flagellates, and mixo- and he terotrophic ciliates) and suspended inorganic particles to light absor bed 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 fracti on (53-73%, depending on the wavelength examined) of this variability could be accounted for by changes in the abundance of inorganic suspen ded matter, as well as planktonic organisms. The specific light absorp tion by autotrophs was less variable over the three spectral bands con sidered than those of microheterotrophs and inorganic particles, which dropped sharply with increasing wavelength. Inorganic particles contr ibuted, on the average, 48 and 74% of the total particulate absorption for the PAR waveband and at 400 nm, respectively, with their contribu tion to light absorption at 675 Mn being negligible. Autotrophs domina ted light absorption at 675 nm ton average 45.8% of total particulate absorption), whereas mixo- and heterotrophic ciliates and bacteria tog ether contributed, on average, 22.5% of the total light absorption at this wavelength. The combined light absorption coefficient of microhet erotrophs at 400 nm (0.0126 m(-1)) was similar to that of autotrophs ( 0.013 m(-1)). These results documented the dominant role that inorgani c particles play in the absorption of blue light in the Bay of Planes and showed that the particulate light absorption by autotrophs was oft en comparable to that of heterotrophs.