Trophic Status of Impoundments

Eutrophication is a process of nutrient enrichment of a system and it is used to classify the stage at which this process is at any given time in a particular water body. The 'trophic status' of the water body is thus used as a description of the water quality status of water body, with regard to nutrient enrichment. The following classification terms and colour coding are provided for easy reference.

Bear in mind that aquatic systems are large and dynamic, with great variability. These classes therefore serve as a guideline rather than comprising a set of hard and fast rules. Please see the reports and publications pages of the NEMP programme, and Water Research Commission publications, e.g. The Determination of Annual Phosphorus Loading Limits for South African Dams for more detail.

Table 1. Colour codes, classification naming and description:

ClassDescription
Oligotrophic Low in nutrients and not productive in terms of aquatic animal and plant life.
Mesotrophic Intermediate levels of nutrients, fairly productive in terms of aquatic animal and plant life and showing emerging signs of water quality problems.
Eutrophic Rich in nutrients, very productive in terms of aquatic animal and plant life and showing increasing signs of water quality problems.
Hypertrophic Very high nutrient concentrations where plant growth is determined by physical factors. Water quality problems are serious and can be continuous.

Table 2. Simplified method for classifying trophic status classes, used in the maps and tables on this web site. Note the use of medians rather than means to reduce the effect of irregular monitoring and occasional outliers on the results.

Statistic Unit Current trophic status:
Median annual chlorophyll a g/L 0<x<10 10<x<20 20<x<30 >30
Oligotrophic
(low)
Mesotrophic
(moderate)
Eutrophic
(significant)
Hypertrophic
(serious)
Potential for algal and plant productivity:
Median annual total phosphorus mg/L x<0.015 0.015<x<0.047 0.047<x<0.130 >0.130
negligible moderate significant serious

Table 3. Detailed method for determining trophic status statistics.

Statistic Unit Current trophic status:
Mean annual chlorophyll a g/L 0<x<10 10<x<20 20<x<30 >30
Oligotrophic
(low)
Mesotrophic
(moderate)
Eutrophic
(significant)
Hypertrophic
(serious)
Current nuisance value of algal bloom productivity:
% of time chlorophyll a> 30g/l % 0 0<x<8 8<x<50 >50
negligible moderate significant serious
Potential for algal and plant productivity:
Mean annual total phosphorus mg/L x<0.015 0.015<x<0.047 0.047<x<0.130 >0.130
negligible moderate significant serious

Go to table of seasonal eutrophication data reports.
National Eutrophication Monitoring Programme home.
Last edited 2013-06-12 15:21