The effects of diel changes in photosynthetic coefficients and depth of Planktothrix rubescens on the daily integral of photosynthesis in Lake Zürich

Abstract

In late summer and autumn, before the vertical circulation reaches the thermocline, the phytoplankton population of Lake Zürich is dominated by the red-coloured filamentous cyanobacterium Planktothrix rubescens, which stratifies in the metalimnion at depths close to the photosynthetic compensation point. The filament volume concentration reached a maximum of 12 cm 3 m -3 ; the depth of the maximum varied from 10.5 to 12.5 m. Changes in the depth distribution were attributed to a combination of (1) seiche movements, which raised or lowered the thermocline by up to 2 m over 36 h, and (2) flotation by the buoyant filaments relative to the isotherms, by up 0.4 m d -1 . These changes caused a 2-fold change in insolation at the Planktothrix peak. Estimates were made of the daily integral of photosynthetic O 2 -production, SS(NP), by the population of P. rubescens over a period of four cloudless days. The estimates were calculated from measurements of surface irradiance (at 5-min intervals), vertical light attenuation, temperature, filament volume concentration and the photosynthesis/irradiance (P/I) curves of filaments concentrated from the metalimnion. Despite the similar, high insolation on each of the four days, the calculated values of SS(NP) varied from 9 to 53 mmol m -2 d -1 , owing to the changing depth distribution of the filaments. Measurements of P/I curves of lakewater samples incubated at a depth of 11 m showed changes in the photosynthetic coefficients during the day. These also generated large changes in calculated values of SS(NP). The computer spreadsheet used to calculate SS(NP) was modified to incorporate timebased changes in the photosynthetic coefficients and vertical distribution of the organism. These refinements provide a more accurate description of photosynthesis by the deep-living P. rubescens, which adjusts its position by buoyancy regulation to exploit the light field in the metalimnion, where it outcompetes other phytoplankton.

The interaction between climate warming and eutrophication to promote cyanobacteria is dependent on trophic state and varies among taxa

“…We found the opposite, however; temperature explained almost all of the variation in Oscillatoria or Planktothrix abundance in the EPA NLA dataset. This may be because the EPA NLA dataset only classifies cells to genus, not species, and other Oscillatoria or Planktothrix species, such as P. rubescens (Reynolds group R), are highly tolerant to light deficient and mixed conditions (Dokulil and Teubner 2000;Walsby et al 2001). P. rubescens produces gas vesicles resistant to high external pressure and allows them to maintain buoyancy even when carried to deeper layers by convective mixing (D'Alelio et al 2011).…”

“…Under increased thermally stratified conditions, which are anticipated with global warming, these cyanobacterial taxa might be able to migrate between wellilluminated surface layers and nutrient-rich hypolimnetic waters (Ganf and Oliver 1982;Walsby 1994;Bouterfas et al 2002), escaping the increasingly nutrient-depleted epilimnion of lakes during extended stratification periods (Livingstone 2003). Cyanobacteria may also take direct advantage of warming because their growth rate will increase with temperature, while the growth rates of many other phytoplankton taxa decline over 20uC (Reynolds 2006;Litchman et al 2010); however, see Lü rling et al (2013).…”

Health hazards for terrestrial vertebrates from toxic cyanobacteria in surface water ecosystems

“…Specific physiologic capabilities of cyanobacteria enable them to compete very efficiently with other photosynthetic microorganisms. Most cyanobacterial species regulate their buoyancy (by means of gas-vacuoles) and this allows them to colonize different depths in the water column depending on the localization of nutrients and the availability of light [63,122]. Possession of accessory pigments, such as phycoerythrin, allows several species to carry out photosynthesis at depths that receive only green light and where, in addition, nutrients are more abundant than on the surface (surface waters are rapidly depleted following spring algal proliferations).…”

Short‐term displacement of Planktothrix rubescens (cyanobacteria) in a pre‐alpine lake observed using an autonomous sampling platform

“…Not only does P. rubescens thrive in the metalimnion during late summer and early autumn, but the cyanobacterium also outcompetes other phytoplankton species under the low-irradiance conditions that prevail in that layer by maximizing the absorption of green light with phycoerythrin pigments (Davis and Walsby 2002;Oberhaus et al 2007). P. rubescens can also regulate its buoyancy with gas vesicles to maintain and adjust its vertical position according to the irradiance, preferably at depths above its photosynthetic compensation point where it has the biggest competitive advantage (Walsby et al 2001). The prevalence of P. rubescens in Lake Zurich, a drinking water reservoir for 1.5 million people, is a serious issue because it produces cyclic hepatotoxins called microcystins, including the very acute form [D-Asp 3 ,(E)-Dhb 7 ]microcystin-RR (Blom et al 2001).…”