Original paper
Late summer phytoplankton responses to experimental manipulations of nutrients and grazing in unlimed and limed Lake Njupfatet, central Sweden
Blomqvist, Peter
Archiv für Hydrobiologie Volume 137 Number 4 (1996), p. 425 - 455
51 references
published: Oct 18, 1996
DOI: 10.1127/archiv-hydrobiol/137/1996/425
ArtNo. ESP141013704000, Price: 29.00 €
Abstract
Phytoplankton biomass and community composition were studied in Lake Njupfatet during two consecutive years, one before and one after whole-lake calcite treatment. As a result of the liming, total phosphorus decreased by 30 %, and the average epilimnetic biomass of phytoplankton decreased by almost 40 %. This decrease was mainly due to the complete loss of the dominant species, the cyanobacterium Merismopedia tenuissima which, prior to liming, constituted up to 80 % of the late summer biomass. Enclosure experiments were performed to assess the role of nutrients and zooplankton in regulating late summer phytoplankton community in the unlimed and limed lake. The Merismopedia-dominated community of the acidic lake did not respond to nutrient additions and, with the exception of dinoflagellates, most of its constituents were not sensitive to grazing. The lack of response by Merismopedia was probably due to its inability to utilize nitrate as a nitrogen source. The lack of response by other species is attributed to allelopathic control of the system by Merismopedia. SinceMerismopedia was not grazed, this species may act as a dead-end in energy flow towards higher trophic levels. Furthermore, structural changes at higher trophic levels during early stages of acidification, e.g., the loss of daphnids, may be due to acidification-induced dominance of Merismopedia and other cyanophytes. The phytoplankton community of the alkaline lake showed a significant and positive response to nutrient additions. Community composition also changed from chrysophytes and green algae to cryptophytes and diatoms. Unless combined with nutrient additions, liming may have very little effect on the pelagic ecosystem at least during the first year after liming.
Keywords
Phytoplankton • Lake Njupfatet • Sweden