JOURNAL OF PLANKTON RESEARCH | VOLUME 17 | NUMBER 6 | PAGES 1363-1379 | 1995
© Oxford University Press
research-article |
Direct and indirect effects of grazing on the phytoplankton seasonal succession in an oligotrophic lake
Departamento de Biología Animal y Ecología 18071 Granada, Spain 1Departamento de Biología Vegetal, Facultad de Ciencias 18071 Granada, Spain 2Institute del Agua, Universidad de Granada 18071 Granada, Spain
Received on April 18, 1994; accepted on February 28, 1995 We examined the simultaneous dynamics of phytoplankton and zooplankton in an oligotrophic lake, as well as the algal response to experimental manipulations of herbivore population structure and density. The seasonal succession of phytoplankton is characterized by a shift in dominance from eukaryotic species to prokaryotic cyanobacteria, as in a eutrophic lake. This unusual pattern for an oligotrophic lake is related to the release of zooplankton phosphorus, rather than to the amount of total phosphorus. The highest estimated values of this released phosphorus occur during the co-dominance of small-sized species and of one calanoid population, both under natural and experimental conditions. Our experimental results clearly reveal that a blue-green species replaces non-blue-green ones when environmental conditions render low or intermediate values for released phosphorus, irrespective of the grazing pressure. The absence of predators or high levels of released phosphorus stimulate non-blue-green growth. These results agree with the successional pattern observed for the algal assemblage under natural conditions. Nutrient recycling plays an essential role in the maintenance of the metabolism of the pelagic system, and therefore the prevalence of a topdown regulation mechanism as proposed for such low-nutrient environments should be reconsidered.