Abstract
The Reserve concept, introduced in the National Water Act, has led to an increasing use of
biomonitoring techniques to assess the current ecological status of rivers in South Africa. The
ecological status of a river provides vital information necessary to establish the amount and
quality of water needed by the aquatic ecosystem. If the amount and quality of water is
inadequate, then the water resource will not be able to provide the necessary goods and
services to their respective water users.
The biomonitoring of rivers gained impetus with the establishment of the River Health
Programme (RHP) and its various objectives. The RHP led to the development and
refinement of a range of biomonitoring indices to assess various components of the aquatic
ecosystem. The components used to assess the current ecological status of a river include the
physical habitat, riparian vegetation, water quality, as well as the macro-invertebrate and fish
communities.
The Mvoti River, in the vicinity of Stanger, is subjected to extensive water abstraction, which
is then utilised for irrigation, industrial use, urban water requirements and various domestic
uses by informal settlements. Previous studies on the river indicated that it is in a severely
degraded state, especially below the confluences of the Nchaweni and Mbozambo rivers.
Thus, this assessment of the current ecological status of the Mvoti River was undertaken to
possibly identify the main causes of the degraded state. The study was undertaken during the
high (February) and low flow (August) periods in 2005. Selected monitoring sites included
sites used in previous studies on the Mvoti River as well as sites on the Nchaweni and
Mbozambo tributaries.
The methodology for this study was to implement the various indices used in the RHP for the
different ecosystem components. The habitat indices implemented in this study were the
Habitat Quality Index (HQI) and Integrated Habitat Assessment System (IHAS). Physicochemical
water analyses were done during each sampling period to assess the water quality
against the South African Water Quality Guidelines. The biotic indices used included the
South African Scoring System 5 (SASS5) index to assess the macro-invertebrate community
and the Fish Assemblage Integrity Index (FAII) to assess the fish community.
Additionally, the newly developed EcoStatus indices for the fish and macro-invertebrate
communities were implemented on the available fish and macro-invertebrate data. These
indices were developed mainly to be used in Reserve determination assessments, where the
assessments are built around the integration of all ecosystem components into a single value
for the ecological status. The fish index is termed the Fish Response Assessment Index
(FRAI) and the macro-invertebrate index the Macro-invertebrate Assessment Index
(MIRAI)….
Multivariate statistical analyses were performed on the water quality as well as the macroinvertebrate
and fish communities to establish any temporal and spatial trends together with
any dominating water quality variables responsible for species composition at the different
sites. PRIMER and CANOCO computer software were used to construct hierarchical clusters
and NMDS plots for the biotic communities, while RDA and PCA bi-plots were used to
represent the water quality variables and their effects on the biotic community structure.
The Mvoti River is in a seriously degraded state and this degradation was particularly evident
during the low flow period below the Sappi Stanger mill and the Stanger Sewage works
effluent discharge points. The degraded state of the river is caused by the multitude of
impacts on the river, which includes the local land-use, upstream land-use and effluent
discharges in the vicinity of Stanger.
The water quality in the vicinity of Stanger was in a poor state during the low flow, while the
high flow was only slightly better. The upstream site is in a fair condition but, after the
Nchaweni and Mbozambo rivers enter the Mvoti River, the water quality decreases. The
variables found to be problematic included microbiological variables, chlorides and electrical
conductivity in the Mvoti River. High nutrient values were found together with very high
electrical conductivity in especially the Nchaweni River tributary.
The lower Mvoti River contains very little habitat of good quality to support the biotic
communities. The habitat is generally degraded due to the destruction of the riparian zone
and the dominance of alien vegetation in the form of reeds. This, together with land-use and
water abstraction activities, has caused high sediment loads in the lower Mvoti River which
are continually moving. The IHAS and HQI results indicated the habitat in the lower Mvoti
River is in a modified condition.
The statistical analysis of the macro-invertebrate data showed that a definite spatial variation
existed while no significant temporal variation was identified. There is a difference in the
community structure between the Mvoti River and its tributaries, with the tributaries
containing a very poor diversity. This lowered species diversity was attributed to the effects
of, specifically, chlorides on the community structure but the origin of the chlorides could not
be linked specifically to the Sappi Stanger mill’s activities. The SASS5 was similar with the
Mvoti River sites having slightly higher scores, placing it in either a B or C class while most
of the tributaries had a Class D. The community structure in the tributaries responded to
nutrients and chlorides and these variables reduced the numbers of sensitive species and
allowed hardy taxa to flourish. The results of the MIRAI index provided the same categories
as identified by the SASS5 index.
The fish community structure showed the same spatial differences identified in the macroinvertebrate
communities, with the tributary sites containing different community structures
than those found in the Mvoti River. The Mvoti River fish community is in a modified state
with the majority of fish sampled being tolerant with very few sensitive species present. The
FAII scores for the Mvoti River was a Category C, while the tributary sites scored a Category
D or lower. The fish community is affected by the poor water quality, habitat and flow
modifications in the lower Mvoti River. The FRAI index provided the lower Mvoti River and
its tributaries with the same categories as identified with FAII.
Overall, the Mvoti River is in a seriously degraded state with even the reference site being
subjected to impacts that could potentially be harmful to the ecosystem. The ecological state
of the river decreases as it moves past Stanger and is subjected to the impacts from the
Nchaweni and Mbozambo rivers in the form of nutrients and salinity concentrations. The
impacts on the Mvoti River have a multitude of different sources and if the aquatic ecosystem
is to improve, only a collective effort will be of any value.
Prof. V. Wepener