Modelling phytoplankton productivity in turbid waters with small euphotic to mixing depth ratios

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Abstract
A model which was developed and calibrated for predicting algal growth in mass cultures, was modified for natural systems. In turbid systems the ratio of euphotic to aphotic depth is usually small and mixing may exceed the compensation depth. In order to compensate for the various light regimes to which the phytoplankton would be subjected, the losses due to dark respiration were modified so that the effective light history would determine the actual rate. The efficiency of light utilization also changes under different light regimes and the model was modified to take these variations into account. Both these modifications resulted in different production profiles being generated for the same surface conditions, but with different mixing depths, where the phytoplankton become more efficient as the light regime deteriorates (i.e. less respiration and greater light utilization efficiency). A further consequence is that the ‘critical mixing depth’ is ˜2.5 times greater than that which was previously accepted, being ˜20 times the euphotic depth. The model predicted productivities to within 90% of observed rates. It was also suggested how the predictions could be used to determine the extent of nutrient limitation. The predictions also have biomanipulatory consequences, as alterations of the light regime through the addition of non-photosynthesizing materials, under certain conditions, may even result in a stimulation of phytoplankton productivity.