Abstract
The cyanoprokaryote Planktothrix rubescens has been studied for over four decades in an alpine lake, Mondsee, Austria. We hypothesise that impacts of climate change can be distinguished from other environmental constraints controlling its population dynamics. During thermal stratification, P. rubescens formed a deep chlorophyll maximum in the metalimnion. Seasonal and diurnal depth distributions indicated that Planktothrix lived well below the euphotic zone, at low light levels and moderate temperatures. Photosynthetic parameters indicated a shade adapted population. The eutrophication and rehabilitation periods of Mondsee were characterised by fluctuations controlled by phosphorus, with periods below the P-threshold associated with low biovolumes and relatively stable populations. Positive net changes of Planktothrix biovolume occurred during the spring–summer transition and in autumn. During summer, the population usually declined to an annual minimum. The standardised residuals of annual biovolumes responded positively to the climate signal of the Winter North Atlantic Oscillation and the timing of the onset of stratification. An inverse relationship existed between off-set and persistence of stratification. P. rubescens only benefits from climate warming early in the year, during late spring overturn and early summer. Longer periods of summer stratification did not favour biovolume development.
Highlights
Most freshwater habitats in the world today are affected by eutrophication (Smith & Schindler, 2009), with the problem of nutrient enrichment further exacerbated by the effects of climate change (Dokulil & Teubner, 2011)
In deep lakes that stratify during summer, problems may be hidden in deeper water because some planktonic cyanobacteria thrive in the metalimnion where they build up deep chlorophyll maxima (DCM)
After its first appearance in autumn 1968 (Findenegg, 1969), P. rubescens rapidly increased in biovolume and reached a first peak in 1971 in Mondsee (Fig. 1)
Summary
Most freshwater habitats in the world today are affected by eutrophication (Smith & Schindler, 2009), with the problem of nutrient enrichment further exacerbated by the effects of climate change (Dokulil & Teubner, 2011). In deep lakes that stratify during summer, problems may be hidden in deeper water because some planktonic cyanobacteria thrive in the metalimnion where they build up deep chlorophyll maxima (DCM). This behaviour seems paradoxical because the epilimnion should be more attractive for autotrophic organisms (Davis et al, 2003). One of the best and most widespread examples of this behaviour is the cyanoprokaryote Planktothrix rubescens (de Candolle ex Gomont) Anagnostidis et Komarek (1988) This ‘paradoxical’ species is seen by others as an efficient ‘ecosystem engineer’ acting as a major organizing force (Padisak et al, 2010). The species may even produce surface blooms (e.g. Almodovar et al, 2004)
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