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ESTIMATING DIFFUSE PHOSPHORUS LOADS TO
LAKES: IMPLICATIONS FOR THE CALCULATION OF
TOTAL MAXIMUM DAILY LOADS
LH Defew
Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian, EH26 0QB,
UK, E-mail: ldef@ceh.ac.uk
SUMMARY
Nutrients and the concentrations in which they are present within a water body
are important factors regulating the growth of phytoplankton and other plants.
Problematic overgrowth of these is generally recognised as the biological
manifestation of eutrophication and poor water quality, often resulting in hypoxia,
loss of biodiversity, fish kills, and release of algal toxins in the water body. As many
freshwater environments are considered to be phosphorus limited, controlling and
regulating the input of this particular nutrient is often one of the targets set under the
total maximum daily load (TMDL) management approach.
A TMDL is the sum of the allowable loads of a single pollutant from all contributing
point and non-point (diffuse) sources, including natural background levels. The major
steps in the American TMDL initiative are: (1) to develop load calculations for nonpoint
sources of pollution; (2) to calculate natural loads; (3) to calculate maximum
total loads from all sources; and (4) to determine how to reduce the maximum load
in order to meet water-quality standards. A key requirement of this approach is the
determination of accurate and reliable pollutant loads to the receiving water from
non-point sources.
Many of the methods that are currently being developed in the UK to assess nutrient
inputs to lakes from non-point sources are based on data collected at weekly, monthly
or even quarterly intervals. Previous studies have shown that such infrequent data
collection results in pollutant load estimates that are of low accuracy and precision
(Webb et al., 1997; Soerens and Nelson, 2001). There is now mounting evidence
(e.g. May et al., 2005) that calculations based on such data may underestimate the
pollutant load to water bodies from diffuse sources by up to 80%. This is because
they fail to take account of the influence of storm events on the mobilisation and
transport of pollutants. The method used to estimate the nutrient loading to a lake
will have important implications to the final TMDL that is calculated. To be reliable,
TMDLs must take into account the impact of high intensity, low frequency rainfall
events on nutrient loadings to water bodies.
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