Diatoms as indicators of water quality change from drought to flood conditions in the Kruger National Park

Abstract

Water as a resource is important for all living organisms and forms the very basis of life. The transportation of water, around the globe, forms the hydrologic cycle. This cycle includes precipitation, evaporation, and transpiration. Anthropogenic accelerated global warming and climate change have altered the timing, magnitude, and frequency by which these elements occur, thereby causing instability in the hydrologic cycle. Natural events such as droughts and floods can occur more frequently with more devastating impact. South Africa is a water-stressed country that frequently experiences drought due to the low annual precipitation across most of its interior. The country is considered semi-arid and has a growing human population, which places stress on water as a resource for South Africa, since its quality must be suitable for human consumption and use. This limited amount of water is unfortunately subject to anthropogenic pollution and heavy exploitation. Also, there are virtually no natural lentic systems and so our rivers are used for abstraction, by impounding them along their flow direction to improve yield. Rivers and their impoundments therefore serve as major contributors to freshwater for personal consumption as well as for irrigation and industry, and thus, the state of these rivers in terms of water quantity and quality is important with regard to water security. Changes in seasonal rainfall intensity and duration can also alter the discharge of rivers by changing the contribution to surface water or runoff. Low seasonal rainfall decreases the contribution of surface runoff to discharge, while higher seasonal rainfall increases the surface runoff. El Niño Southern Oscillation (ENSO) influences the amount of seasonal rainfall for the greater interior of South Africa. During El Niño years, South Africa is prone to lower-than-normal summer rainfall and consequently at risk of drought. La Niña in turn creates higher than normal rainfall for South Africa and can increase the risk of floods. Seasonal rainfall patterns, the ENSO effect, flow regulation, and water abstraction are drivers of riverine flow regime changes. Additionally, wastewater discharge, stormwater discharge and agricultural discharge increase streamflow through a higher contribution of surface water, especially during dry seasons. When the water quantity of a river decreases, there are consequent changes in the water quality as well. Periods of prolonged low streamflow can lead to eutrophic conditions, where surface water temperatures increase, dissolved oxygen concentration decrease, and nutrients and/or pollutants are further increased in concentration

due to less dilution. A prolonged period of increased discharge can also alter water quality by increasing nutrient flux from surrounding agricultural areas. Large amounts of water flowing downstream can also remove much sediment, which holds elements including nutrients, pollutants and/or toxicants. Natural seasonal flooding generally improves water quality by removing toxicants that have been retained in the sediment, thereby „flushing‟ the system. Habitat scouring can also alter the biotope composition consequently altering ecosystem resilience through niche occupancy. These conditions further complicate the effect of flooding on water quality. Aquatic biota have proven to be the preferred method for monitoring integrated water quality changes within rivers over time, rather than using once-off chemical and physical analysis. Within aquatic biomonitoring, bioindicator organisms are used to infer the water quality of aquatic ecosystems, in the present study diatoms were used. Diatoms have many advantages as bioindicators, they respond reliably and predictably to a range of pollutants and toxicants. Their short generation time allows for cross-generation studies. Collecting diatoms are also quick and relatively cheap due to their ubiquitous nature, however, the analysis of diatom samples requires specialised equipment and training, and this can add cost to the process due to the time-comsuming nature of the analysis, thereby prohibiting diatoms from being implemented on a greater scale. The Kruger National Park (KNP) has experienced many years of drought throughout its history, many of which had devastating effects. From 2016 to 2019 the park was still experiencing the devastating effect of a recent drought. From 2020, however, an increase in the annual summer rainfall, due to La Niña was evident, alleviating to a large extent the impact of the drought experienced. La Niña still prevails during 2022, and will likely persist into 2023, thereafter a short return to neutral conditions is expected. The KNP has therefore seen a steady increase in average annual seasonal rainfall from 2018 to 2021 due to the phase shift of El Niño and La Niña. This provides an invaluable opportunity to use diatoms as an inference of water quality change during this period. The use of diatoms in the park is further important since it is on the cusp of entering the biomonitoring arsenal of the park, and this will only increase its efficacy as bioindicator. Many diatom indices have been developed of which the Indice de Polluosensibilité Spécifique (IPS) was used in the present study, based on previous studies from the same region. The parameter values used for the IPS are based on the autecological preferences of diatom taxa to

ionic composition and organic pollution. Increased levels of nutrients increase the ionic load of water, and consequently also aids in the increase in primary production of a system, leading to eutrophication – also an important component of the IPS calculation. The IPS index was used to indicate water quality change from 2018 to 2022 within five large perennial rivers within the KNP in terms of ionic composition (electrical conductivity), trophic status and organic pollution. Sites sampled were selected as part of the park‟s internal monitoring program and most sites were sampled across the entire period. Most rivers sampled experienced a temporal improvement in water quality from 2018 to 2022, except the Olifants River which experienced a deterioration in water quality over time, possibly due to an increased nutrient load from diffuse sources and/or an increase in point-pollution from the Palabora mining company. Spatially, the water quality for the Sabie River increased with distance downstream, however, from the second last site to the last, water quality severely decreased, although it did improve over time. The Crocodile, Sabie, Letaba and Olifants rivers all exhibited spatial increases in water quality with flow direction, while the Luvuvhu River experienced a decrease in water quality with flow direction. The IPS index indicated an increase in water quality in terms of ionic composition, trophic state and organic pollution temporally and spatially for the Crocodile, Sabie and Letaba rivers due to the increase in average annual seasonal precipitation across the entire park. The Luvuvhu River exhibited a temporal increase and spatial decrease in water quality, while the Olifants River exhibited a temporal decrease and spatial increase in water quality. These rivers did not respond in a similar fashion to other rivers toward the increase in seasonal rainfall and may suggest the influence of other sources of pollution. Diatoms were therefore successfully implemented to indicate water quality change in response to increased seasonal rainfall, by using the IPS index. This index was not expressly designed for the purpose of determining water quality changes in response to water quantity changes as a result of rainfall, nevertheless it was successful in this regard. The use of diatoms as indicator organisms is therefore highly recommended within the KNP. The implementation thereof would increase the ease of sampling, and decrease the overall cost of monitoring. It is well established that diatom communities are correlated with water quality variables and, therefore, the further use of physico-chemical analysis in addition to diatom analysis in the park is redundant, unnecessary and costly. Diatoms can, therefore, form part of the biomonitoring tools used in the

KNP, however, as always, it is recommended that multiple bioindicators be used to infer water quality when possible to improve the accuracy of results.