The importance of climatic variability and human influence in driving aspects of temporarily open‐closed estuaries

Abstract

Freshwater availability is a major topic in scientific literature and the public domain.Estuaries are complex and dynamic ecosystems that depend on freshwater inflow from their catchments and exchange with the marine environment. Catchment forcings are subject to shifts in frequency and intensity due to regional hydrological variability over time. Human manipulation of freshwater regimes may further alter this beyond natural variability through flow abstraction and additional inputs in the form of wastewater. To elucidate the effect of climate variability and human activities, we conducted a time series investigation on hydrological drivers into a small South African estuary (uMdloti), representative of temporarily open-closed estuaries. Subsequently, we investigated the ability of the hydrological variables to drive physical parameters and microalgae in the estuary. Freshwater abstraction caused a disconnect between rainfall and river flow, and increased lag times during dry spells, peaking at 115 days. Annual seasonal peaks in rainfall were not represented in river flow due to the dam. Wastewater discharges equate to 4% of the mean annual runoff from the catchment, which has reduced from a natural runoff of 85.8 to 71.9 Mm −3 year −1 . Due to this and damming of catchment flows, wastewater discharging into the estuary was the most important driver of water quality in the system, resulting in increased nutrient and decreased dissolved oxygen concentrations, whereas river flow was an important driver at high flow volumes. An understanding of interaction effects between climatic variability and catchment flow manipulation is vitally important to estuarine management and their ecological viability. K E Y W O R D S climatic variability, estuarine water quality, freshwater abstraction, hydrological drivers, intermittently open-closed lagoons, temporarily open-closed estuaries, wastewater treatment works

How drought affects range and variability of environmental conditions in subtropical South African estuaries

“…Drought conditions restricted the available salinity niches that are typical of estuaries, evident in the absence of higher salinity regions in the LTCE (uMdloti) and lower salinity regions in the PrOE (uMlalazi). When the mouth is closed, the uMdloti Estuary slowly turns fresh due to its perched nature and low but present freshwater input that partially arises from wastewater treatment work outflow (Brooker and Scharler, 2020). These shifts were not only apparent in the case of salinity, which is perhaps the most obvious parameter to change.…”

“…Occasional blooms of the water hyacinth, Eichhornia crassipes , observed during the drought period further compete for nutrients and block out light. Therefore, although previously both riverflow gauged below the dam and the wastewater treatment work flows were shown to significantly influence phytoplankton chl-a concentrations (Brooker and Scharler 2020), the exact contribution and diel cycles of production and respiration of various primary producers and concomitant changes in DO concentration and pH in the estuary remain to be investigated. What can be concluded, however, is that both phytoplankton chl-a concentrations and the oxygen concentration environment are more variable with higher frequencies at extreme conditions during prolonged closed mouth periods during drought.…”

Ten Principles to Determine Environmental Flow Requirements for Temporarily Closed Estuaries

“…Input of sewage effluent is a major problem on the developed east coast around Durban, where this is often the only flow into small estuaries [31]. River flow into the Mdloti Estuary was found not to be a predictor of physico-chemical state as the system is driven by effluent inflows [67]. The minimum discharge standards of municipal effluents are extremely high (21 mg L −1 (ca.…”

Human Health and Ocean Pollution

“…All of these changes compromise the nursery function of estuaries. These changes can result in increased or decreased salinity, more frequent or less frequent flooding, changes in energy supplies, frequent closures of inlets that hinder migration of marine species in and out of estuaries, and changes in the timing of inlet closure and opening such that they no longer synchronize with fish life stages [ 475 476 477 478 ].…”