Functional analysis of Microcystis vertical migration: a dynamic model as a prospecting tool. II. Influence of mixing, thermal stratification and colony diameter on biomass production

Abstract

YOYO is a deterministic model developed to represent the growth and vertical movement of Microcystis sp. colonies in the water column. Migration of colonies is represented in the model through the dynamics of carbon-reserve metabolism during photosynthesis and biosynthesis. It was used to quantify combined impacts of light, temperature and mixing on the growth and migration of colonies with different diameters. These 3 factors characterise a vertically non-homogeneous water column. Although conditions of simulation are schematic, they allow identification of situations in which the ability to regulate buoyancy gives Microcystis an advantage over non-mobile phytoplankton. Even if some vertical structures appear unfavourable for the exploitation of light energy, they nevertheless offer some advantages for the access to nutrients. Each diameter has particular benefits, and the increase in diameter observed throughout the year is probably an ecological adaptation to fluctuations in the environment at a seasonal scale.KEY WORDS: Microcystis · Photosynthesis · Carbohydrates · Vertical migration · Hydrodynamics · Mixing zone · Thermocline · 1-D mathematical model Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 39: [281][282][283][284][285][286][287][288][289][290][291][292] 2005 ( Rabouille et al. 2003). It simulates, at a daily scale, changes in growth and vertical migration of colonies due to the dynamics of the metabolism of carbon reserves during photosynthesis and biosynthesis. A previous analysis of this model under permanent conditions of light and temperature in a non-turbulent water column permitted us: (1) to independently explore the influence of light and of temperature on the migration pattern and (2) to test the variability of these migratory responses as a function of colony diameter (Rabouille et al. 2005). In the present study, we have analysed the combined influence of light, temperature and mixing on the growth and migration of colonies with different diameters. Most of the simulated diameters were chosen according to field observations in the Grangent reservoir (Loire, France) and are thus representative of colony sizes actually found in this reservoir throughout spring and summer. The largest diameters, though much less common, were nevertheless tested in the model, in order to cover the largest possible range of colony sizes and to estimate the behaviour of such colonies. Present simulations are not intended to represent realistic conditions. They are part of a model sensitivity analysis with regard to specific forcing conditions. BRIEF DESCRIPTION OF THE MODELYOYO is a deterministic model (1-dimensional vertical) intended to represent the growth and vertical movements of a population of Microcystis sp. colonies in the water column of a lake. A complete description of the model structure is given in Rabouille et al. (2005); here, we briefly outline the main components and relations. The growth model is based on a mass balance. It focuses...

Artificial mixing to control cyanobacterial blooms: a review

“…The latter two studies built upon laboratory experiments, which showed that cyanobacteria change their density in response to the diurnal light cycle, as they accumulate carbohydrate ballast during the daytime and respire these carbohydrates during the night. Gas vesicles to cell volume ratios and cell volume to colony volume ratios were added by Rabouille and Salençon (2005) and Aparicio Medrano et al (2013), while non-uniform colony size distribution was included by Chien et al (2013). However, no models to date include explicitly the buoyancy response to gradients in nutrients.…”

Vertical Heterogeneity of Genotypic Structure and Toxic Potential within Populations of the Harmful Cyanobacterium Microcystis aeruginosa

“…According to the literature, several factors could potentially drive such a vertical heterogeneity by influenc-ing the distribution of the colonies of M. aeruginosa into the water column: 1) interactions with the benthic compartment, by recruiting benthic colonies or by the continuous sedimentation of planktonic ones [29,40]; 2) vertical migration of the colonies, which can regulate their position in the water column by balancing their internal stock of gas vacuoles and carbon reserves [13,30,41,42]; and 3) fitness-related natural selection, which could allow some particular genotypes or phenotypes to better survive or develop at a specific depth because of particular conditions such as light intensity, nutrients availability or benefits of microcystin production [10,23,[43][44][45]. Since the qualitative shifts we observed were mainly restricted to the surface water layers of the deep reservoirs we studied, the influence of the interactions with the benthic compartment through recruitment or sedimentation can be minimized.…”

“…Through water column mixing, one can expect a homogeneous vertical distribution of the main genotypes constituting a population. On the contrary, considering that colonies can migrate more or less deep and more or less fast depending on their size [30], and considering that different genotypes may present various tolerances to deep environments conditions, one could predict a vertical heterogeneity within a population. Considering these hypotheses, such a vertical heterogeneity could represent another source of genotypic succession observed on top of the water column.…”

“…However, because of water column mixing events, of the continuous sedimentation of some colonies of M. aeruginosa throughout the bloom period [29], and of the ability of the colonies to migrate in the water column [30], the colonies of M. aeruginosa are observed throughout the water column. Then the question arises whether this vertical distribution could result in a vertical structuration of natural populations of M. aeruginosa, regarding their genetic structure and toxic potential.…”

Sedimentation Patterns of Toxin-Producing Microcystis Morphospecies in Freshwater Reservoirs

“…Analyzing such biological factors is especially interesting if considering that colonies within a single Microcystis bloom often differ widely in morphology (e.g., different colony sizes and shapes), physiological status and chemical properties [19,20]. According to several mathematical models, the colony diameter influences vertical migration during daily sinking-ascending cycles of Microcystis [21,22] with large colonies putatively showing higher sinking and ascending velocities than smaller ones. However, the influence of colony size on irreversible sedimentation during or after blooms remains poorly understood.…”