Evolution and Natural Selection in Nutrient TMDL Development
Over the past several years, EPA and the various states have attempted to develop nutrient TMDLs for streams with increasingly restrictive limitations on phosphorus by targeting a range of endpoints. Prior to 2005, nutrient TMDLs typically were developed as part of an overall control
strategy for CBOD, ammonia and nutrients to address excursions of the dissolved oxygen water quality standard. In this context, it is fairly well understood that nutrients, combined with a multitude of other factors, stimulate plant growth. As the plant biomass increases, it results in pronounced
diurnal variation in dissolved oxygen levels in response to the photosynthesis – respiration cycle, with the primary concern being DO depletion prior to dawn, when the effects of plant respiration reach a peak. Although the underlying science is understood, implementation of nutrient
TMDLs in river and stream systems was still problematic because the dynamics of periphyton, benthic algae that colonize rocks, is very complicated and good predictive models are not available. Consequently, TMDLs developed to address nuisance algae concerns have been largely unsuccessful.
More recent nutrient TMDLs have targeted instream nutrient concentrations without consideration for algal growth. This new approach does not address use impairments and its utility in a TMDL setting is doubtful. Readily available information confirms that nutrients are not solely responsible
for the impairments attributed to them. Other factors, such as shading, are known to exert greater control on nuisance plant growth than nutrient levels and these factors may be more easily and cost effectively addressed to mitigate the impairment of concern. We anticipate that, as the science
evolves, these other factors will play a greater role in addressing stream eutrophication issues.
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