The effect of zooplankton on the dynamics and molecular composition of carbohydrates during an experimental algal bloom

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.