Abstract
Faced with an environment of accelerated change, the long-term dynamics of biotic communities can be approached to build a consistent and causal picture of the communities’ life. We have undertaken a 25-year monthly-sampling study on the phytoplankton of a meso-oligotrophic lake, paying attention to controlling factors of overall biomass (TB) and taxonomical group biomass (TGBs). Long-term series included decreased trends of TB and TGBs, and multi-scale periodicity. A decadal TB periodicity emerged related to nitrogen concentration and Cryptophytes. Annual periodicities were mainly related to air and water temperature controlling the abundance of Chlorophytes or Dinoflagellates. Intra-annual cycles could arise from autogenic processes. The analysis by periods revealed relevant dynamics (for example, Diatom periodicities), hidden in the analysis of the complete series. These results allow us to establish that: i) two organizational levels of phytoplankton change differently in time scales from months to decades; ii) controlling factors (climate, water physics and chemistry) act at different time scales and on different TGBs, and iii) different combinations of the “taxonomical group-control factor-trend and periodicity” set throughout the studied time explain total biomass dynamics. A holistic approach (multiple complementary analyses) is necessary to disentangle the different actors and relationships that explain non-stationary long-term phytoplankton dynamics.
Highlights
Faced with an environment of accelerated change[1], the long-term dynamics of biotic communities can be approached to build a long-term, consistent and causal picture of the communities’ life
There have been many studies dealing with long-term freshwater phytoplankton dynamics, addressing parts of those aforementioned questions
Chlorophytes and Dinoflagellates had annual cycles and the latter exhibited an intra-annual periodicity; Cryptophytes had interannual cycle (Table 2). Wavelet analysis complemented this information by highlighting that: i) annual periodicities took place in the early period, ii) Diatom dynamics had weaker annual periodicity over the whole 25-year period, and iii) Cryptophyte periodicity vanished after 1998
Summary
Faced with an environment of accelerated change[1], the long-term dynamics of biotic communities can be approached to build a long-term, consistent and causal picture of the communities’ life They often respond to instability, and this fact makes it necessary to disentangle the time scales involved to predict their future behaviour. Time spans much longer than 10 years are needed to suggest an insightful picture of organismic responses and their controlling factors, providing that the sampling frequency is high enough to disclose some parts of community effects. A variety of suggested causes for the trajectory of long-term (>10 years) phytoplankton biomass exists (Table 1). They include the often expected covariation with phosphorus concentration[5,6] which is considered the most likely. Loch Leven Luzern Maggiore Müggelsee Neusiedlersee North Pine Dam Ontario (Bay of Quinte) Ontario (main lake) Ontario (main lake) Orta Pyhäjärvi Saidenbach
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