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

A model which was developed and calibrated for predicting algalgrowth in mass cultures, was modified for natural systems. Inturbid systems the ratio of euphotic to aphotic depth is usuallysmall and mixing may exceed the compensation depth. In orderto compensate for the various light regimes to which the phytoplanktonwould be subjected, the losses due to dark respiration weremodified so that the effective light history would determinethe actual rate. The efficiency of light utilization also changesunder different light regimes and the model was modified totake these variations into account. Both these modificationsresulted in different production profiles being generated forthe same surface conditions, but with different mixing depths,where the phytoplankton become more efficient as the light regimedeteriorates (i.e. less respiration and greater light utilizationefficiency). A further consequence is that the ‘criticalmixing depth’ is {small tilde}2.5 times greater than thatwhich was previously accepted, being {small tilde}20 times theeuphotic depth. The model predicted productivities to within90% of observed rates. It was also suggested how the predictionscould be used to determine the extent of nutrient limitation.The predictions also have biomanipulatory consequences, as alterationsof the light regime through the addition of non-photosynthesizingmaterials, under certain conditions, may even result in a stimulationof phytoplankton productivity.