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 pulses of allochthonous humic matter (5-29% of the ambient dissolved organic carbon concentration) and phosphorus (PO4-P was increased from 1-2 to 15-20 mu g . L(-1)) on the plankton. The addition of phosphorus, alone or with humic matter, clearly raised the primary production in summer (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 controlled primarily by organic substrates released by algae rather than by phosphorus or humic matter. In the control enclosure, bacterial production constituted 23-32% of primary production while with extra phosphorus and humic matter the proportion rose to 59%, indicating that humic matter was used by bacteria as an additional energy and carbon source besides algal exudates. Simultaneous enrichment with phosphorus and humic matter resulted in substantially higher bacterial production than with humic matter alone. In autumn, the addition of humic matter alone increased bacterial production, indicating that humic matter contained 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-zooplankton communities were less obvious.