I. Reche et al., Relationship of trophic and chemical conditions to photobleaching of dissolved organic matter in lake ecosystems, BIOGEOCHEMI, 44(3), 1999, pp. 259-280
Dissolved organic matter (DOM) is a major light-absorbing substance, respon
sible for much of the color in water bodies. When sunlight energy is absorb
ed by DOM, some color can be lost by the process of photobleaching. We meas
ured rates of DOM photobleaching in thirty lakes that varied greatly in col
or, trophic status and ionic composition. Loss of color (measured as absorb
ance at 440 nm and expressed as absorption coefficients) was a first order
function of sunlight dose, and rates were nearly identical for 0.2 mu m- an
d GF/F-filtered samples suggesting that the process was predominantly abiot
ic. Photobleaching rates were rapid (color loss of 1-19% d(-1)) and varied
about seven-fold among lakes. Our method underestimated the actual rate by
15-20% based on comparisons between the glass bottles we used in the survey
and quartz containers. The large variation in photobleaching rates was exa
mined in relation to lake trophy and chemical conditions. The best predicto
r of this variability was acid-neutralizing capacity (ANC) (r(2) = 0.94; p
< 0.001) such that photobleaching was most rapid in the most alkaline lakes
. The relationship between ANC and photobleaching suggests that differences
in ionic conditions among lakes may influence the solubility and configura
tion of humic and fulvic acids and hence their susceptibility to photobleac
hing.