Light‐dependent growth rate determines changes in the population of Planktothrix rubescens over the annual cycle in Lake Zürich, Switzerland

Abstract

Summary• Analyses were made to determine which changes in a Lake Zürich population of Planktothrix rubescens were dependent on light-and temperature-dependent growth rates, and when growth was limited by the mixing depth.• Changes in vertical distribution of the cyanobacterium, determined weekly from August 1998 to September 1999, were related to growth increments calculated at 1-h time and 1-m depth intervals from values of irradiance, attenuance, temperature and biomass in the lake, using algorithms based on growth rates in culture.• Population biovolume varied annually from 1.2 to 120 cm 3 m −2 . During summer, modelled growth in the metalimnion matched the 50-fold population increase. Modelled growth exceeded the observed increase when Planktothrix was mixed into the nutrient-depleted epilimnion, suggesting nutrient limitation. The measured increase ceased when the mixed depth exceeded the critical depth for growth in autumn (Sverdrup's principle). Light limitation explained the gradual decrease of the population in winter. The steep decline in spring had other causes.• Population changes were largely determined by interactions of light and depth distribution; decreases in nutrient loading have had little impact on Planktothrix growth in Lake Zürich.

Modelling the plankton groups of the deep, peri-alpine Lake Bourget

“…5) notably coincides well with the observations in Lake Bourget (Jacquet et al, , 2014bLe Vu et al, 2011). In reality, this phenomena is usually attributed to buoyancy regulation (Walsby et al, 2004), whereas in this study, no active mobility mechanism has been considered and their sub-surface growth largely re ects the parameterized low-light tolerance and high-light inhibition of the species (Walsby and Schanz, 2002). The depth-dependent mortality we assumed for P. rubescens (eq.14) does not play a signi cant role in the occurrence of such thin layers, whereas it certainly pulls the depth of maximum concentration a few meters upwards.…”

“…P. rubescens was assigned as a separate algal group because of its distinctive eco-physiology characterized by low-light tolerance (Walsby and Schanz, 2002), slow growth (Bright and Walsby, 2000), and grazing defense (Kurmayer and Jüttner, 1999) as well as the fact that it represented about half of the phytoplankton assemblage in some years ( Fig. 1,3).…”

“…Analysis of the longterm changes in individual lakes and inter-comparison between lakes suggest that phosphorus (and nitrogen, see Posch et al, 2012) availability is the primary determinant (Dokulil and Teubner, 2012;Jacquet et al, 2014b;Anneville et al, 2015). P. rubescens is characterized by their slow growth rates (Bright and Walsby, 2000) and tolerance to low light conditions (Walsby and Schanz, 2002). As a result of the latter, it often develops thin and intense layers within the metalimnion during the growing season (eg., Jacquet et al, 2014b), where they are the rst to harvest the nutrients leaking from the nutrient-rich layers beneath the thermocline.…”

Strict coupling between the development ofPlanktothrix rubescensand microcystin content in two nearby lakes south of the Alps (lakes Garda and Ledro)

“…The increase of P. rubescens during the summer months is partly due to its ability to form strong thickening in the metalimnetic layers provided that the light intensity is not below the Lim. 49 (2013) 309-318compensation point (Walsby and Schanz, 2002). The control of the vertical position is associated with the action of gas vesicles, cellular structures providing positive buoyancy (D'Alelio et al, 2011).…”

Impact of internal waves on the spatial distribution of Planktothrix rubescens (cyanobacteria) in an alpine lake

“…Among cyanobacteria proliferating in temperate areas, Planktothrix rubescens is characterized by its localization in the metalimnic layer of the water column of deep lakes. This species is particularly well adapted for growing at low light intensities and also at low temperatures (Oberhaus et al, 2007) and has been found in many prealpine lakes (Walsby and Schanz, 2002;Ernst et al, 2009). In addition, P. rubescens is able to produce microcystins , which are dangerous hepatoxins that threaten both human and animal health Briand et al, 2003).…”