The accumulation and degradation of carbohydrates (aldoses) were investigated during diatom blooms in two mesocosms. The

effects of macrozooplankton were explored by addition of zooplankton to one mesocosm (+Z). Aldoses accumulated at a steady rate

of 4.9 µM C d-1 from day 9 in the mesocosm without zooplankton (-Z), while zooplankton induced an increased rate to 10.3 µM C d-1.

The surplus of 14 µM dissolved combined carbohydrates (DCCHO) in the +Z mesocosm after 22 days was caused by higher

concentrations of arabinose, galactose and rhamnose. The increase was 50, 25 and 25% respectively in the +Z mesocosm, compared

with the -Z mesocosm. The introduction of zooplankton had a significant effect on the rate of biodegradation and the fraction of

refractory aldoses. The accumulated aldoses became less labile and a larger fraction was refractory with the presence of

zooplankton. During degradation the fraction of dissolved organic carbon (DOC) explained by aldoses decreased in the -Z

mesocosm while it increased with the presence of grazers. In the +Z mesocosm, two new peaks appeared on the chromatograms and

contributed about 4% to the total area of aldoses. It is hypothesised that these two peaks are an indicator of the presence of grazers.