Methodology used to observe effects of fluid flow on Microcystis colonies.

(A) Experimental setup consisting of a (cone- and-plate) controlled flow setup combined with inverted microscopy. The conical upper surface was rotated by a rheometer head, while the stationary glass slide below the sample allowed optical access for the microscope. (B) Examples of microscopy images. Colony size distributions were calculated after image processing of the captured frames. (C) Changes in size distributions (and other complementary measurements) over time were used to identify aggregation and fragmentation of cyanobacterial colonies. (D) The majority of the measurements were conducted using a laboratory culture of Microcystis strain V163. Colonies collected from Lake Gaasperplas (Netherlands), dominated by the morphospecies M. aeruginosa were also used.

Kinetics of fragmentation of Microcystis strain V163 colonies under cone-and-plate shear flow. The laboratory culture was filtered to select mainly large colonies, and the total biovolume fraction was adjusted to ϕ = 10−4. Suspensions were subjected to an intense dissipation rate () in panels A-E. (A) The initial size distribution of colonies, expressed as biovolume fraction of the relative colony diameter (normalized by single cell diameter). The size distribution had a bimodal shape, with large colonies (in yellow) and small colonies (in green, composed mostly of single cells, dimers, and some trimers, as depicted by the inset). (B) Median diameter of small and large colonies as a function of time. Bars indicate limits of 25th and 75th percentiles. (C) Most large colonies have been fragmented after 1 hour of shear flow, but the bimodal shape remained. (D) The size distribution shifted from large to small colonies as the shear flow induced fragmentation. (E) The rate of change in biovolume distribution at t = 0h. Negative values indicate loss of colonies by fragmentation, while positive values indicate newly created fragments. The distribution suggests an erosion mechanism, as depicted by the cartoon inside the plot. (F) Fragmentation frequency as a function of the relative diameter of colonies for three values of dissipation rate. The inset shows details for the low dissipation rates. Error bars indicate the standard deviation. Dashed lines and shaded regions in panels B-D indicate predictions from the population model given by Eq. (1), and the lines in panel F indicate the predictions by Eq. (4). Best fit parameters: α1 = 0.023, S1 = 0.034 , q1 = 4.5, S2 = 31 , q2 = 4.1.

Kinetics of aggregation for a single-cell suspension of Microcystis strain V163 at a moderate dissipation rate of = 0.019 m2/s3 and a total biovolume fraction of ϕ = 10−4. (A) Initial size distribution of the suspension as a function of the relative diameter, composed mostly of single cells. (B) Median diameter of colonies formed by aggregation of single cells as a function of time. Bars indicate limits of 25th and 75th percentiles. (C) After 1 hour of shear flow, the size distribution has shifted towards slightly larger diameters. (D) Time behavior of a suspension of large division-formed colonies under the same moderate dissipation rate and total biovolume fraction. The bimodal size distribution is separated into large (yellow) and small (green) colonies. Dashed lines and shaded regions in panels B-D indicate predictions from the population model given by Eq. (1). Best fit parameters: α1 = 0.023, S1 = 0.034 , q1 = 4.5, S2 = 31 , q2 = 4.1.

Kinetics of the fragmentation of colonies in field samples of Microcystis spp. at an intense dissipation rate () and total biovolume fraction of ϕ = 10−4. (A) Comparison of the fragmentation frequency as a function of colony size for the laboratory culture (Microcystis strain V163) and the field samples (Microcystis spp.). Error bars indicate the standard deviation. (B) Brightfield microscopy images of colonies in a Nigrosin-dyed medium (dark region) show evidence of a thick EPS layer (bright region) surrounding a field colony. (C) Initial size distribution of colonies in field samples as a function of the relative diameter. The size distribution had a bimodal shape, with small colonies (green) and large colonies (yellow). (D) The median diameter of the colonies in each subpopulation as a function of time. Bars indicate 25th and 75th percentiles. (E) After 1 hour of shear flow, the small colonies have aggregated slightly, while the large colonies keept their size distribution. (F) The fraction of small colonies remained nearly constant during the experiment.

Phase diagram indicating the dominant colony formation mechanism as a function of the dissipation rate and the cyanobacterial abundance (expressed by the total biovolume fraction ϕ). (I) Colonies grow only by cell division at low dissipation rates and total biovolume fractions. (II) As the biovolume fraction increases, aggregation enhances colony growth. (III) For moderate dissipation rates, aggregated colonies are fragmented, and only cell division can increase colony size. (IV) Fragmentation of colonies dominates at intense dissipation rates, irrespective of whether these colonies were formed by aggregation or cell division. Bars on the right side indicate typical values of dissipation rate. The horizontal arrowed line indicates the transition from the absence of a bloom to a dense scum formation under typical wind mixing, with the bullets indicating the total biovolume fraction for WHO alert level 1 [59] (left) and for a typical scum layer [61] (right).

(A-D) Micrographic images of phytoplankton field samples collected from the surface layer of Lake Gaasperplas. Microcystis spp. are dominant in the sample.

Kinetics of the fragmentation of Microcystis strain V163 colonies under a moderate dissipation rate () and various values of total biovolume fraction. The laboratory culture was filtered to select mainly large colonies, and total biovolume fraction was adjusted. Plots in the left column depict the median diameter of each size population as a function of time, where bars and shaded regions indicate limits of 25th and 75th percentiles. Plots in the right column depict the fraction of small colonies as a function of time. The total biovolume fraction is (A-B) ϕ = 10−4, (C-D) ϕ = 2 · 10−4, (E-D) ϕ = 5 · 10−4. Best fit parameters: α1 = 0.023, S1 = 0.034 , q1 = 4.5, S2 = 31 , q2 = 4.1.

Kinetics of the fragmentation of strain V163 colonies under a moderate total biovolume fraction (ϕ = 10-4) and various values of dissipation rate. The laboratory culture was filtered to select mainly large colonies, and total biovolume fraction was adjusted. Plots in the left column depict the median diameter of each size population as a function of time, where bars and shaded regions indicate limits of 25th and 75th percentiles. Plots in the right column depict the fraction of small colonies as a function of time. The dissipation rate is (A-B) , (C-D) , (E-D) . Best fit parameters: α1 = 0.023, S1 = 0.034 , q1 = 4.5, S2 = 31 , q2 = 4.1.

Kinetics of the aggregation of Microcystis strain V163 colonies under a moderate dissipation rate () and various values of total biovolume fraction. The laboratory culture was filtered to select mostly single cells, and the total biovolume fraction was adjusted. Plots in the left column depict the median diameter of each size population as a function of time, where bars and shaded regions indicate limits of 25th and 75th percentiles. Plots in the right column depict the fraction of small colonies as a function of time. The total biovolume fraction is (A-B) ϕ = 10−4, (C-D) ϕ = 2 · 10−4, (E-D) ϕ = 5 · 10−4. Best fit parameters: α1 = 0.023, S1 = 0.034 , q1 = 4.5, S2 = 31 , q2 = 4.1.

Rheology of concentrated colonies of Microcystis strain V163. (A) Shear stress as a function of the shear rate obtained for a steady shear test. Solid line indicates a Herschel–Bulkley fit, , where τy = 4.3 ± 0.3Pa (SD from best fit) is the dynamical yield stress. Other fit parameters are k = 0.55 ± 0.13 and n = 0.85 ± 0.05 (B) Storage G’ and loss G’’ moduli as a function of the deformation strain amplitude γ for a oscillatory shear with angular frequency of 1 rad/s.

Comparison of hypothesis for the fragment distribution of category C1 colonies. Bars show the experimental results for normalized biovolume distribution of filtered single cells of Microcystis strain V163 after 1 hour under a moderate dissipation rate () and a total biovolume fraction of ϕ = 10−4. Lines depict the prediction of the sectional method using an equal fragment hypothesis (dashed) and an erosion hypothesis (solid).

Cone-and-plate setup used to generate a turbulent shear in a suspension of Microcystis colonies. (A) Schematics of the components with the main dimensions indicated in millimeters. (B) Average energy dissipation rate as a function of the angular velocity of the conical probe. Bullets indicate the measured data, and the solid lines indicate the best fit of the laminar regime (~ ω2) and the inertial regime (~ ω5/2).