. The impact of large dams is a popular topic in environmental
science, but the importance of altered water quality as a driver of ecological impacts is
often missing from such discussions. This is partly because information on the
relationship between dams and water quality is relatively sparse and fragmentary,
especially for low-latitude developing countries where dam building is now concentrated.
In this paper, we review and synthesize information on the effects of damming on water
quality with a special focus on low latitudes. We find that two ultimate physical
processes drive most water quality changes: the trapping of sediments and nutrients, and
thermal stratification in reservoirs. Since stratification emerges as an important driver
and there is ambiguity in the literature regarding the stratification behavior of water
bodies in the tropics, we synthesize data and literature on the 54 largest low-latitude
reservoirs to assess their mixing behavior using three classification schemes. Direct
observations from literature as well as classifications based on climate and/or
morphometry suggest that most, if not all, low-latitude reservoirs will stratify on at
least a seasonal basis. This finding suggests that low-latitude dams have the potential
to discharge cooler, anoxic deep water, which can degrade downstream ecosystems by
altering thermal regimes or causing hypoxic stress. Many of these reservoirs are also
capable of efficient trapping of sediments and bed load, transforming or destroying
downstream ecosystems, such as floodplains and deltas. Water quality impacts imposed by
stratification and sediment trapping can be mitigated through a variety of approaches,
but implementation often meets physical or financial constraints. The impending
construction of thousands of planned low-latitude dams will alter water quality
throughout tropical and subtropical rivers. These changes and associated environmental
impacts need to be better understood by better baseline data and more sophisticated
predictors of reservoir stratification behavior. Improved environmental impact
assessments and dam designs have the potential to mitigate both existing and future
potential impacts.

/pdf/reviews-and-syntheses-dams-water-quality-and-tropical-4mp01cy5s8.pdf

Reviews and syntheses: Dams, water quality and tropical reservoir stratification

Freshwater resources in South Africa are under severe pressure from existing anthropogenic impacts and global climate change is likely to exacerbate this stress. This review outlines the abiotic drivers of climate change, focusing on predicted changes in temperature and precipitation. The consequences of global climate change for freshwater ecosystems are reviewed, with effects grouped into those related to water quantity, water quality, habitat and aquatic biological assemblages. Several guiding principles aimed at minimising the potential impact of climate change on freshwater ecosystems are discussed. These guidelines include those focused on water quantity and the maintenance of appropriate environmental flows, integration of global climate change into water quality management, conservation planning for freshwater biodiversity, the promotion of ecosystem resilience, and extending climate change science into policy and public discourse. Proactive assessment and monitoring are seen as key as these will allow for the identification of ecological triggers and thresholds, including thresholds of vulnerability, which may be used to monitor and inform decisions, as well as to improve the ability to forecast based on this knowledge.

/pdf/ecological-consequences-of-global-climate-change-for-4cu02hnqpp.pdf

Ecological consequences of global climate change for freshwater ecosystems in South Africa

Land use and land cover change are major drivers of water quality deterioration in watercourses and impoundments. However, understanding of the spatial and temporal variability of land use change characteristics and their link to water quality parameters in catchments is limited. As a contribution to address this limitation, the objective of this study is to assess the linkages between biophysico-chemical water quality parameters and land use and land cover (LULC) classes in the upper reaches of the uMngeni Catchment, a rapidly developing catchment in South Africa. These were assessed using Geographic Information Systems tools and statistical analyses for the years 1994, 2000, 2008 and 2011 based on changes over time of eight LULC classes and available water quality information. Natural vegetation, forest plantations and cultivated areas occupy 85% of the catchment. Cultivated, urban/built-up and degraded areas increased by 6%, 4.5% and 3%, respectively coinciding with a decrease in natural vegetation by 17%. Variability in the concentration of water quality parameters from 1994 to 2011 and an overall decline in water quality were observed. Escherichia coli (E. coli) levels exceeding the recommended guidelines for recreation and public health protection was noted as a major issue at seven of the nine sampling points. Overall, water supply reservoirs in the catchment retained over 20% of nutrients and over 85% of E. coli entering them. A relationship between land use types and water quality variables was found. However, the degree and magnitude of the associations varies between sub-catchments and is difficult to quantify. This highlights the complexity and the site-specific nature of relationships between land use types and water quality parameters in the catchment. Thus, this study provides useful findings on the general relationship between land use and land cover and water quality degradation, but highlights the risks of applying simple relationships or adding complex relationships in the management of the catchment.

Effects of land use and land cover changes on water quality in the uMngeni river catchment, South Africa

Water temperature is a key variable affecting aquatic organisms. Understanding their response to elevated water temperatures is important for estimating upper thermal limits, and ultimately for assisting with setting defendable, biologically-relevant water temperature guidelines for lotic systems. Sublethal effects impacting on an individual organism or species may manifest at higher levels of the hierarchy, namely, populations, communities and entire ecosystems. Sublethal effects typically include those affecting an organism's physiology and metabolism (e.g. growth rates, secondary productivity, respiration); phenology (e.g. development time, voltinism, emergence); reproductive success and fitness (e.g. fecundity, rates and success of egg development and hatching); behaviour (e.g. migration, movement, drift); and broad-scale ecological effects (e.g. species richness, composition, density, distribution patterns). Sublethal effects are discussed with examples drawn from freshwater studies, in particular those focused on aquatic insects. Commonly-used methods, which vary from simple, cost-effective, laboratory-based methods to more elaborate, expensive, laboratory- and field-based studies, are assimilated to serve as a toolbox for future thermal research. Ultimately, the method adopted depends largely on the question(s) being asked and available resources.

http://www.scielo.org.za/pdf/wsa/v41n5/15.pdf

Sublethal effects of temperature on freshwater organisms, with special reference to aquatic insects

River temperature research and practice: Recent challenges and emerging opportunities for managing thermal habitat conditions in stream ecosystems.

Variable seasonal stream temperatures are a critical factor in maintaining aquatic invertebrate community patterns. We investigated whether the degree of predictability in a stream's water temperature profile provides insights into the structure and functional predictability of macroinvertebrate communities. Quarterly macroinvertebrate sampling over 11 months at five paired sites along the longitudinal axes of the Keurbooms River, Western Cape, and the Kowie/Bloukrans River, Eastern Cape, was undertaken in conjunction with collection of hourly water temperatures. Macroinvertebrate taxonomic turnover across seasons was higher for sites having lower water temperature predictability values than for sites with higher predictability, while temporal partitioning was greater at sites with greater temperature variability. Macroinvertebrate taxa responded in a predictable manner to changes in their environment. Specialist taxa preferred sites with greater temperature variability, and generalist taxa those with les...

Relationship between water temperature predictability and aquatic macroinvertebrate assemblages in two South African streams

Stream flow and associated seasonal hydrologic variation is a critical driver affecting the structure of aquatic macroinvertebrate communities. Quarterly aquatic macroinvertebrate sampling was carried out for a single year along the longitudinal axes of two streams in the southern Cape of South Africa. Stones-in-current, marginal vegetation, gravel-sand-mud, and stones-out-of-current biotopes were sampled for aquatic macroinvertebrates. Associations between macroinvertebrate communities and hydrographs were analyzed to investigate whether a non-perennial stream with greater flow variability had a higher prevalence of common or opportunistic species than a perennial one with less variable flow. Macroinvertebrate species richness was greater in the perennial as opposed to the non-perennial stream, where, on a seasonal basis, species richness increased from winter to autumn. Temporal species turnover differed between sites and streams, where reduced flows transformed the more dominant aquatic biotopes from s...

Bruce R. Eady

Papers

Relationship between water temperature predictability and aquatic macroinvertebrate assemblages in two South African streams

Shifts in aquatic macroinvertebrate community structure in response to perenniality, southern Cape, South Africa

Linking land use to water quality for effective water resource and ecosystem management