Microbial plankton response to contrasting climatic conditions:: insights from community structure, productivity and fraction stoichiometry

Climatic events are among the most important forces governing natural processes in ecosystems. Over a 3 yr period, we monitored changes in the hydrological, chemical and biological regimes of a high mountain lake severely affected by a drought that peaked in 1995. During the drought, the decrease in lake volume enhanced the contribution of the sediment to the lake nutrient budget. The community became more complex, with the appearance of ciliates and heterotrophic nanoflagellates, organisms rarely found prior to the drought. The impaired post-drought recovery of the zooplankton, which did not reach normal densities for this lake until 1997, resulted in a microbial-dominated food web in 1996 and a classical grazing food web in 1997. Besides the species composition or abundance and the autotrophic to heterotrophic ratio, food web changes associated with the drought and atmospheric nutrient inputs were reflected in the stoichiometry and contribution of both bacteria and phytoplankton fractions to the planktonic particulate matter. Specific productivity measurements showed that cellular N:P ratios could be used to evaluate the growth conditions of phytoplankton but not of bacteria, indicating that an element other than P may limit bacteria growth. Assessments of phytoplankton and bacteria productivity and of the photosynthetic carbon release relative to bacteria demand in microcosms revealed that after a phosphorus pulse, the phytoplankton N:P ratio determines the nature of the interaction between phytoplankton and bacteria as either commensalistic or competitive for inorganic P. If phytoplankton N:P is initially high (P-deficient algae), a P pulse can reduce photosynthetic extracellular release (PER) and bacteria can experience C limitation, whereas if phytoplankton N:P is initially low (P-sufficient algae) and bacteria N:P is high, increased PER is observed and competition for inorganic P may occur.