EFFECTS OF PHOSPHORUS AND ALLOCHTHONOUS HUMIC MATTER ENRICHMENT ON THE METABOLIC PROCESSES AND COMMUNITY STRUCTURE OF PLANKTON IN A BOREAL LAKE (LAKE-PAAJARVI)

During the ice-free period, three experiments each of 16-17 days were performed in four enclosures to evaluate the effects of moderate pulse s of allochthonous humic matter (5-29% of the ambient dissolved organi c carbon concentration) and phosphorus (PO4-P was increased from 1-2 t o 15-20 mu g . L(-1)) on the plankton. The addition of phosphorus, alo ne or with humic matter, clearly raised the primary production in summ er (54-56%), and to a lesser extent in spring (17-23%) and in autumn ( 17-58%). A significant correlation existed between primary production and bacterial production indicating that bacterial production was cont rolled primarily by organic substrates released by algae rather than b y phosphorus or humic matter. In the control enclosure, bacterial prod uction constituted 23-32% of primary production while with extra phosp horus and humic matter the proportion rose to 59%, indicating that hum ic matter was used by bacteria as an additional energy and carbon sour ce besides algal exudates. Simultaneous enrichment with phosphorus and humic matter resulted in substantially higher bacterial production th an with humic matter alone. In autumn, the addition of humic matter al one increased bacterial production, indicating that humic matter conta ined more easily utilizable organic material at that point than it did earlier. Some algal taxa responded positively to phosphorus and (or) humic matter enrichment, but the effects on proto- and meta-zooplankto n communities were less obvious.