Highlights
- Catchment geology was found to be the dominant driver of pH in river inflow.
- Catchment vegetation/anthropogenic pressures act as modifiers to ambient pH.
- Trends in inter-annual variability were linked to anthropogenic pressures.
- Relationship between pH and flow was present in systems with rainfall seasonality.
- Highlights the potential effects of upstream catchment practices on downstream.
Abstract
River inflow plays an integral role on the water quality characteristics of estuaries, including pH. This study aimed to investigate pH variability in river inflow to South African estuaries and how these might be influenced by catchment characteristics. Specifically, three hypotheses were tested: 1) catchment geology is a dominant influencing factor of pH in river inflow, 2) catchment vegetation and anthropogenic pressures, e.g., urban and agriculture, act as modifiers of geology-driven ambient pH, and 3) seasonality in river flow rates can alter pH levels (e.g., pH decreasing during periods of high flow). First, drivers of pH variability were explored in relation to electrical conductivity, total alkalinity, and catchment geology type, as well as vegetation and key anthropogenic pressures. Thereafter, temporal variability was evaluated considering both inter-annual and seasonal variability also including variability in flow rates. Values of pH displayed an exponential relationship with electrical conductivity and total alkalinity, and as hypothesised pH variability was primarily influenced by catchment geology. Results also indicated that catchment vegetation (e.g., peatlands and fynbos) and/or anthropogenic pressures (e.g., urban and agriculture) act as modifiers causing pH to deviate from geology-driven ambient equilibria, especially in Table Mountain Group sandstone-dominated systems. Only limited trends in inter-annual variability were observed, mostly linked to increases in pH with increases in anthropogenic pressures. Further, a significant inverse relationship between pH and river flows was present, mostly in systems showing marked seasonality in rainfall. This study adds to our understanding of the variability of pH in river inflows to estuaries, and highlights some of the key influencing factors. Indeed, results suggest that anthropogenic pressures in catchments potentially are leading to alkalinisation of river inflow, contrary to the potential effect of ocean acidification. Finally, this study highlights the ripple effects of upstream catchment practices on downstream coastal ecosystems such as estuaries, emphasising the need for integrated catchment-to-coast management.
Omarjee A., Taljaard S., Ramjukadh C.-L. & van Niekerk L., 2021. pH variability in catchment flows to estuaries – a South African perspective. Estuarine, Coastal and Shelf Science: 107605. doi: 10.1016/j.ecss.2021.107605. Article (subscription required).
