POLLUTION INCIDENT IN THE ROODEPLAAT DAM, May 2000

4.1.4 Trophic Status of the Roodeplaat Dam

Although decreasing water quality can be attributed to the increasing levels of a number of chemical species, the process of eutrophication describes nutrient enrichment of the water body (Henderson-Sellers and Markland, 1987). The location of a water body along an eutrophication scale from oligotrophic (nutrient poor conditions) to hyper-eutrophic (extreme nutrient enrichment) is called its trophic status.

The trophic status classification method in this report is based on the following variables and is based on the cut-off values as stated in Table 1.

The mean annual chlorophyll a concentration, since it is a symptom of eutrophication. Chlorophyll a is a direct indicator of the primary production within a system, and represents one of the variables that are used to determine the trophic state of an impoundment or water source.

The proportion of the time that the actual chlorophyll-a concentrations are greater than 30µg/l (expressed as per cent), as formulated for South African conditions by Walmsley and Butty (1980) and Walmsley (1984) as the cut-off value for eutrophic systems. This statistic is used as it indicates the potential duration of algal blooms in an impoundment.

The mean annual total phosphorus (TP) concentration. Walmsley and Butty (1980) proposed cut-off points for mean annual TP concentrations at which different trophic levels are indicated. The TP concentrations are a direct indicator of the extent of eutrophication that exists within an impoundment.

The presence of cyanobacteria (mean annual percentage cyanobacteria in the phytoplankton population).

The mean annual transparency (as determined as Secchi disc depth). Du Plessis et al. (1990) proposed transparency categories for the determination of enriched turbid systems. The transparency plays a major role in the development of phytoplankton in the system, and can explain low chlorophyll-a concentrations in co-existence with high nutrient concentrations when turbidity is high. This variable is, therefore, included to help in the understanding of the system, but not as a direct indicator of eutrophication.

Table 1. Trophic status indicators and the appropriate ranges used to classify the impoundment.

Variable

Oligotrophic

Mesotrophic

Eutrophic

Hyper-eutrophic

Mean Chl a (µg/l )

0 –10

10 – 20

20 – 30

> 30

Proportion of time Chl a

> 30 µg/l (%)

< 8

8 – 50

>50

Total Phosphorus (mg/l )

< 0.015

0.015 – 0.047

0.047 – 0.130

> 0.130

Mean annual % cyanobacteria in phytoplankton population

0 - 1

1 – 10

10 - 50

> 50

Mean annual transparency

Highly turbid

Turbid

Clear

Secchi disc depth (m)

< 0.2

0.2 – 0.8

> 0.8

- Even where the first four variables may be indicative of a certain trophic status, the fifth variable (transparency) may play a significant role in the develoment of eutrophication symptoms.

The trophic status of the Roodeplaat Dam was eutrophic in 1989, after which the dam became hyper-eutrophic (Table 2). The mean annual TP concentrations were constantly above the PMO value of 130 µg/l , except for 1989 when TP concentrations were below the PMO threshold value. Of particular concern is the steady increase above the PMO threshold value of the TP concentrations that has occurred since 1991. The mean annual chlorophyll-a concentrations were above the 30 µg/l threshold for hyper-eutrophic systems for most of the period and of all the chlorophyll-a samples, between 29 and 64 per cent were higher than 30 µg/l . The Roodeplaat Dam is thus in serious need of a management strategy to improve the situation, particularly in terms of inorganic, and thus available, nutrients.

Table 2. Trophic status indicators in the Roodeplaat Dam for the period 1989 –1999 (as extracted from Van Ginkel et al. 2000).

Pretty

The shading in the Table is proportional to the level of eutrophication in the impoundment.

O = oligotrophic M = mesotrophic

E = eutrophic HE = hyper-eutrophic

	Department of Water Affairs and Forestry
	Institute for Water Quality Studies