Overview Article Photosynthetic carbon allocation: Effects of planktivorous fish and nutrient enrichment

Abstract. The proportions of lipid, polysaccharide and protein profoundly influence the nutritional value of food for planktonic organisms. Here, we investigated the ef-fects of nutrients (N and P) and planktivorous fish (Phox-inus sp.) on the partitioning of photosynthetically fixed 14C-labelled bicarbonate into macromolecules (lipids, polysaccharides and proteins) and low molecular weight (LMW) metabolites of epilimnetic phytoplankton living in large (8 m diameter) enclosures from July to mid-Oc-tober 1991. The enclosures were situated in an olig-otrophic lake (north of Montreal, Canada) in both shallow (3.5 m) and deep (11 m) locations. The fractions were cal-culated using the sequential extraction of 14C-labeled phytoplankton samples incubated for short duration (~2 hours) at maximum photosynthetic rates. Labelling pat-terns of all end products exhibited great temporal varia-tions. Overall, the LMW, polysaccharide, lipid and pro-tein fractions accounted for 32.1 ± 10.5%, 31.0 ± 8.8%, 19.2 ± 5.3 % and 17.8 ± 6.1 % of the total 14C fixed respectively. Percent 14C in the LMW fraction was, in general, lower in the nutrient-enriched enclosure and was usually lower in the deep than the shallow enclosures. The effect of fish on the labelling pattern of LMW varied throughout the sampling period but, overall, its impact was non-significant. The proportions of 14C into lipids and polysaccharides did not show the expected increase under nutrient deficiency. In contrast, the polysaccharide fraction increased, for the most part, with nutrient enrichment and was higher in the deep than the shallow enclosures with little changes in the presence of plank-tivorous fish. Incorporation of 14C into lipids remaine