Dinoflagellate growth rate in water columns of varying turbidity as a function of migration phase with daylight

Photosynthetic dinoflagellates occasionally exhibit nonphototacticphase relationships between their diurnal vertical migrationand the daylight cycle. The cause of this variability in specificfield cases is generally unclear. A computer model, MIGTIM,is reported that incorporates light dependence of the initialslope () and maximum photosynthesis (P_max) of the photosynthesisversus light intensity (P-I) curve. MIGTIM is used to determinethe migration phase that optimizes growth rate of sinusoidallymigrating populations under different conditions of photosyntheticlight dependence, turbidity and migration depth. Migration phaseprovides little selective advantage for shallow populationsor for populations in low turbidity water columns. Migrationphase provides a strong selective advantage for deep populationsor for populations in high turbidity water columns. A comparisonbetween MIGTIM output and published field observations suggeststhat field populations experienced optimum growth rate underthe observed environmental and biological conditions. The analysissupports the contention that, under appropriate conditions,water column turbidity is an important component of the complexequation that determines migration phase and that this componentacts through the organism's cumulative diurnal photosyntheticresponse.