Planet WGSW-24010 User Manual page 21

considered to be the element which limits aquatic plant growth in fresh waters because of its low
availability in relation to plant requirements. Where it is limiting, an increase in the level of
phosphate in the water should cause accelerated growth of those plants present or a change in the
species composition of the plant community to reflect the change in phosphate concentration.
At very high concentrations of phosphate, the plant community is usually species poor because
of the excessive growth of filamentous and unicellular algae and some high phosphate tolerant
macrophytes. In these cases where the biological symptoms of nutrient enrichment are manifest,
eutrophication as defined in the UWWTD and the Agency's proposed Eutrophication Strategy
can be deemed to be taking place. In contrast, where the availability of a nutrient is sufficient so
as not to limit plant growth, such as is usually considered to be the case for compounds of
nitrogen, any increase in the concentration of that nutrient will not lead to changes in the plant
community and thus eutrophication cannot be deemed to be taking place.
Evidence suggests that the MTR is particularly responsive to change in nutrient status at
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concentrations less than 1.0 mg l P or 10 mg l N, and even more so at less than 0.5 mg l P or
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5 mg l N (Dawson et al 1999b). This implies that the MTR system may be most useful at
detecting eutrophication impacts when the nutrient concentration upstream of (or prior to) the
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nutrient input, is less than 1.0 mg l or 10 mg l N, particularly when supported by other
biological and/or chemical evidence. The level of eutrophication attributable to nitrate rather
than phosphate at any one site cannot be established using MTR at this stage of method
development. In many cases, however, the nitrate concentration is unlikely by itself to be limiting
to plant growth, and phosphate is likely to be the limiting factor (Dawson et al 1999b).
2.1.5 Basis of oper ation
The method involves the survey of the macrophyte flora and physical character of defined lengths
(100m) of watercourse using a standard checklist. The presence, absence and percentage area
covered by each macrophyte are recorded and the data relating to scoring species (those assigned
an STR) are then used to calculate the MTR score. Physical parameters are recorded to aid
interpretation of results. Detailed procedural guidance is given in Chapters 3–8.
R&D Technical Report E38 7