IMPLICATIONS OF INCREASED UV-B INDUCED PHOTOREDUCTION - IRON(II) ENRICHMENT STIMULATES PICOCYANOBACTERIAL GROWTH AND THE MICROBIAL FOOD-WEB IN CLEAR-WATER ACIDIC CANADIAN SHIELD LAKES

Ozone depletion and associated increases in UV-B radiation could increase the photoreduction of iron in Canadian Shield lakes of the boreal forest zone. Since photoreduced iron(II) is more soluble than iron(III), and the reoxidation rate slower in acidic (pH = 5-6) lakes, phytoplanktonic growth and(or) species composition may be altered where iron is growth limiting. This hypothesis was tested by amending herbivore-free lake-water enclosures with phosphorus, phosphorus and iron(II), or phosphorus, ammonium, and iron(II) in lac Tantare, a clear-water acidic lake in Quebec. In herbivore-free unenriched control enclosures, phytoplanktonic abundance doubled within 48 h, demonstrating unequivocally that herbivore grazing (top-down control) intensity controls phytoplanktonic abundance in early spring in this lake. However, at chlorophyll a concentrations 2-3 times the ambient lake levels, picocyanobacterial production and abundance were greater in the phosphorus and iron enclosures, relative to the phosphorus-only enclosures. This resulted in enhanced growth of the indigenous chrysophytes presumably through increased mixotrophy. In spite of a 10-fold increase in nanoflagellate abundance, we were unable to discern any strong competitive interactions among the mixotrophic chrysophytes. This strongly suggests that most if not all of their iron quotas are obtained from their picoplanktonic prey.