Detritus particles interact with phytoplankton growth through light attenuation and nutrient retention. A model is described for predicting abundance of cyanobacteria and microalgae in relation to the detritus dynamics in shallow lakes with varying phosphorus load. Steady-state P distribution among cyanobacterial biomass, detritus and the dissolved pool depending on mortality and detritus mineralisation rate was predicted from kinetics for P-limited growth and refractory fraction of the detritus. The depth-averaged irradiance was computed for a constant background attenuation and specific attenuation coefficients of biomass and detritus estimated from field and laboratory data. This light was compared to the light critical to net growth of microalgae based on initial slope of growth vs. irradiance relationship, maximal growth rate and constant maintenance rate. It was predicted that the probability of abundance of microalgae relative to that of cyanobacteria is high at low population loss rates, coinciding with the lowest depth-averaged irradiances at a particular P load. This prediction is consistent with observations on many shallow lakes.