CS26 Organic Carbon Dynamics
Date: Thursday, June 13, 2002
Time: 10:00:00 AM
Location: Carson C
SųndergaardM, Freshwater Biological Laboratory, Hillerod, Denmark, MSondergaard@zi.ku.dk
Borch, N, H, Freshwater Biological Laboratory, Hillerod, Denmark, NHBorch@zi.ku.dk
Kragh, T, , Freshwater Biological Laboratory, Hillerod, Denmark, TKragh@zi.ku.dk
Biodegradability of autochthonous DOC: Effects of algal growth phases and zooplankton
The accumulation and fate of DOC during an experimental freshwater diatom bloom with and without macrozooplankton grazers were investigated. A mixed community of macrozooplankton was added to one of the experimental containers during exponential algal growth. Long-term bacterial degradation experiments were used to characterize the DOC as bioavailable or refractory. The algal biomass peaked at 103 and 148 µg chl./L in the grazed and non-grazed experiment, respectively. Prior to the peak in algal biomass about 100 µM DOC had accumulated. During the decline of the bloom the rate of net DOC production increased and when the bloom started to decay the concentrations of DOC were 235 and 284 µM with and without grazing, respectively. The biodegradation measurements showed that DOC produced during the exponential growth phase was completely degraded. Thus, bacterial metabolism did not produce a measurable refractory pool of DOC. Contrary, about 25-30% of the DOC sampled after the collapse of the bloom was found to be biologically recalcitrant in a 230 days perspective. We hypothesize a change in the composition and biodegradability of DOC produced by exponentially growing and decaying diatoms to explain the observed difference. The presence of grazers did not change the biodegradability of DOC. However, the time course in degradation changed. The initial decomposition of DOC from the experiment with grazers was slower than for DOC produced without grazers. The fate of autochthonous DOC is apparently affected by both the algal growth phase and macrozooplankton grazing.