Abstract
Ferreira, M., Dr.; Van Vuren, J.H.J., Prof.
The Wilge River is one of many adjoining tributaries of the Olifants River located in the
Olifants Water Management Area (WMA4) within the Highveld (11) – Lower Level 1
Ecoregion (Dallas, 2007). These river systems experience extreme demand for natural
resources, as they flow through heavily utilised economic hubs. They are closely associated
with land modification and pollution, primarily mining and industrial-related disturbances and
extensive agricultural activities, all of which are the primary cause of impairment to river
health.
The primary aim of this study was to study the aquatic macroinvertebrate communities at six
monitoring sites along the Wilge River, coupled with two monitoring sites on adjoining
tributaries, and to further identify the driving variables that influence these communities both
spatially and temporally. Functional Feeding Groups (FFGs) of the aquatic
macroinvertebrates and the surrounding land use in the project area was taken into account.
The analysis of in situ water quality measured during the period March 2010 to May 2013,
clearly illustrated high levels of variation both spatially and temporally. In situ water quality
was a limiting factor to the aquatic ecosystem from a Dissolved Oxygen (DO) and
Percentage Saturation (DO%) perspective. The remaining parameters were within the South
African guideline for aquatic ecosystems (DWAF, 1996).
Habitat availability illustrated clear seasonal variation as well, of which the wet season
indicated better habitat availability compared to the dry season. The dominant biotopes in
the study area were vegetation (VEG) and gravel, sand and mud (GSM). Site WIL04
illustrated the poorest habitat integrity overall primarily attributed to the site’s steep incised
banks and deep channel which lacks the stones biotope.
The South African Scoring System, Version 5 (SASS5) results indicated that there was a
change in the integrity of the aquatic macroinvertebrate community’s in the study area and
further illustrated variability both spatially and temporally. It was evident that the aquatic
macroinvertebrate communities within the Wilge River and two adjoining tributaries sampled,
were generally in a slightly to modified state with moderate variations. The lowest number of
taxa, SASS5 and average score per taxon (ASPT) values was recorded at site WIL04 and
this was mainly brought about due to changes in flow and habitat availability. The ASPT
score ranged from 3.8 at site WIL04 to 7.7 at site WIL02, indicating that the aquatic
macroinvertebrate communities were primarily composed of tolerant and moderately tolerant
taxa. Of these mostly tolerant taxa, predators and gathering collector populations were the
most dominant FFG, with the shredders being the least abundant within the study area. The Bray-Curtis cluster analysis of the aquatic macroinvertebrate communities clearly
illustrated a high level of similarity and seasonal variation among the communities. The high
similarity was an indication that similar taxa occurred at the sites within the groups identified.
However, in accordance with the Similarity Percentages (SIMPER) analysis, there was no
clear indication of dominant taxa. There was however a separation of sites TRI01 and
WIL04. This was expected due to differences in the physical stream condition (flow) and
other habitats / general biotopes, primarily at site WIL04. Stream bed composition is one of
the most important physical factors controlling the structure of freshwater invertebrate
communities (Mackay and Eastburn, 1990). The separation and similarity of these two sites
were not a consequence of dominant taxa, but rather a consequence of differing water
quality, habitat availability and common tolerant taxa driving the system. Inclusive, the
seasonal variation illustrated was contributing to the changes in the in situ water quality and
habitat availability, thus making seasonal variation also a driving variable, in the differences
between the sites.
The Redundancy Analysis (RDA) bi-plots indicated, as with the Bray-Curtis similarity
matrices and related NMDS plot, that there was a distinct seasonal separation. It further
illustrated a clear separation of site TRI01 and WIL04 due to reasons mentioned above. All
the environmental variables, with the exception of pH, was identified as significant drivers in
the river systems (p<0.05). This however varied seasonally. During the wet season, clarity,
DO and pH were the significant drivers, while clarity, TDS/EC, percentage saturation and pH
were the significant drivers during the dry season (p<0.05). These drivers were expected
due to possible sources namely intensive agriculture in the project area. The RDA tri-plots
further indicated the significant role that the ASPT, SASS5 score and the IHAS played within
the aquatic macroinvertebrate community (p<0.05). This confirms the importance of habitat
as a driving variable in aquatic macroinvertebrate community structures. Consequently, the
driving variables in the separation of the sites along the Wilge River and two adjoining
tributaries, appear to be a combination of variables (DO, percentage saturation, TDS/EC,
clarity and pH), including habitat availability (based on IHAS scores).
To determine the effects and relations between the primary driving changes, to the
surrounding land uses in the project area, further multivariate analyses were conducted,
which included the FFGs. It was clearly indicated that predators have a negative correlation
with the rest of the FFG’s which was expected. As the percentage of predators increase at a
site, the percentage of the other FFG’s decreased. Therefore, there was a large variation
and clear changes in the food sources constantly entering into the river system. This is
normally related to changes in the land use.