[Variation of spectrum of photosynthesis available radiation absorbed by underwater optical medium with depth in Lake Taihu in summer].

ABSTRACT

Normalized spectrum of photosynthesis available radiation (PAR), which was absorbed by all kinds of medium per unit volume of water at different depths, was calculated by the data from downward and upward irradiance and absorption coefficients of phytoplankton, non-algae particles and chromophoric dissolved organic matter (CDOM) in 41 locations in Lake Taihu from July 29 to August 1, 2006. The results showed that there were two peaks at about 450 nm and 675 nm in the normalized spectrum of PAR absorbed by phytoplankton per unit volume of water, respectively. With the increasing of the depth, the peak at about 450 nm gradually weakened, and red-shift appeared from 450 nm. The above phenomena were much more obvious in the algae-type zone and the center of Lake Taihu. The PAR absorbed by phytoplankton per unit volume of water was shift to 600-700 nm which mainly originated from 400-500 nm to 600-700 nm. The transitional speed was slower in the grass-type zone but it was faster in Meiliang Bay(algae-type zone)and the center of Lake Taihu. In the above water, the PAR absorbed by non-algae particles per unit volume of water mainly originated from 400-500 nm, but energy source gradually shifted to 500-600 nm with the increasing of depth. The variable characteristics of normalized spectrum of PAR absorbed by CDOM per unit volume of water were similar to that of non-algae particles. However, the advantage in grass-type zone which CDOM per unit volume of water absorbed PAR between 500 and 600 nm was less significant than that of non-algae particles per unit volume of water.