JOURNAL OF PLANKTON RESEARCH | VOLUME 16 | NUMBER 10 | PAGES 1411-1432 | 1994
© Oxford University Press
research-article |
Factors controlling production of phytoplankton and bacteria under ice in a humic, boreal lake
University of Helsinki, Lammi Biological Station 16900 Lammi, Finland
Received on February 22, 1994; accepted on June 3, 1994 The factors controlling pelagic primary and bacterial production of a humic, boreal lake in winter were investigated, combining laboratory and field experiments where some of the predicted consequences of the climate change, i.e.the increased load of phosphate-phosphorus and dissolved organic matter (DOM), were simulated. In situ incubations were performed in eight acrylic tubes lowered underneath the ice cover for 1–4 months. In the lake, production of phytoplankton (0.03–0.33 µg C l–1 day–1) was lower than that of bacteria (0.2–2.3 µg C l–1 day–1) from the end of January to mid-April. Later in spring, the light conditions improved due to the disappearance of snow and finally ice itself, and primary production was revived. The importance of light as the factor controlling primary production in winter was confirmed in laboratory experiments where additions of phosphorus and DOM did not enhance the primary production. These same enrichments resulted in higher growth rates and production of bacteria. However, bacterioplankton was simultaneously controlled by heterotrophic flagellates, as in the laboratory experiments the exclusion of flagellates always resulted in higher yields of bacteria of increased cell size. There was also some evidence that the quality of substrates is of importance for microbial activities. The importance of temperature was highlighted with the significant correlation (r2 = 0.59) between bacterial production and temperature within the range 0.6–2.1°C. Thus, on the boreal zone the possible warming of climate may lead to enhanced activities in winter, but under unchanged light conditions no effects on primary production will be expected.