Computer simulations of seasonal outbreak and diurnal vertical migration of cyanobacteria |
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Authors: | Hiroshi Serizawa Takashi Amemiya Axel G Rossberg Kiminori Itoh |
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Institution: | (1) Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan;(2) Evolution and Ecology Program, International Institute for Applied Systems Analysis (IIASA), Schlossplatz 1, 2361 Laxenburg, Austria |
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Abstract: | Algal blooms caused by cyanobacteria are characterized by two features with different time scales: one is seasonal outbreak
and collapse of a bloom and the other is diurnal vertical migration. Our two-component mathematical model can simulate both
phenomena, in which the state variables are nutrients and cyanobacteria. The model is a set of one-dimensional reaction-advection-diffusion
equations, and temporal changes of these two variables are regulated by the following five factors: (1) annual variation of
light intensity, (2) diurnal variation of light intensity, (3) annual variation of water temperature, (4) thermal stratification
within a water column and (5) the buoyancy regulation mechanism. The seasonal change of cyanobacteria biomass is mainly controlled
by factors, (1), (3) and (4), among which annual variations of light intensity and water temperature directly affect the maximum
growth rate of cyanobacteria. The latter also contributes to formation of the thermocline during the summer season. Thermal
stratification causes a reduction in vertical diffusion and largely prevents mixing of both nutrients and cyanobacteria between
the epilimnion and the hypolimnion. Meanwhile, the other two factors, (2) and (5), play a significant role in diurnal vertical
migration of cyanobacteria. A key mechanism of vertical migration is buoyancy regulation due to gas-vesicle synthesis and
ballast formation, by which a quick reversal between floating and sinking becomes possible within a water column. The mechanism
of bloom formation controlled by these five factors is integrated into the one-dimensional model consisting of two reaction-advection-diffusion
equations. |
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Keywords: | Ballast formation Buoyancy regulation Diurnal vertical migration Reaction-advection-diffusion equations Seasonal outbreak and collapse |
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