Inorganic nutrient removal efficiency of a constructed wetland before discharging into an urban eutrophic estuary.

This study investigated the nutrient removal efficiency of a constructed wetland (CW) piloted to treat urban runoff before entering an estuary. Physico-chemical, dissolved inorganic nutrient (DIN and DIP), and stormwater inflow volume data were measured. The CW removal efficiency of DIN was negligible (5% uptake), while it acted as a consistent source of DIP (68% efflux) to the receiving estuarine waters. There was low water residency within the small CW (0.8 ha) that has been compounded by a 10-fold increase in flow volume since 2016. The surface area of the CW would need to be increased to 46 ha to cope with current daily inputs (ca. 6300 m3 d-1). The lack of maintenance (e.g., macrophyte harvesting, sediment desludging) has reduced nutrient uptake and increased autochthonous inputs. The conversion of an abandoned saltpan into an extension of the CW has been considered to manage the high flow volume and remove nutrients.

Assessing and planning future estuarine resource use: A scenario-based regional-scale freshwater allocation approach.

Rapid urbanisation and industrial growth in South Africa increases the need for proactive allocation of freshwater resources on a regional scale. A nine-step method is described that sets long-term targets for water resource condition and future use with a focus on estuary water quantity and quality requirements. The approach specifically focuses on the environmental flow allocation to estuaries, nested within a broader, regional (multi-catchment and multi-estuary) water resource landscape. The method differs to most other approaches in that the responses of multiple estuaries to escalating future development in a region are coherently quantified (versus only considering a single estuary in a single catchment). A case study that assessed the health, biodiversity importance and resilience to current and future pressures of 64 estuaries is used to illustrate the method. Projected growth in the study area was integrated into a range of future dam development and wastewater discharge scenarios. The results showed that estuaries around the urban centres were in poor condition, but those in the more rural areas in a more natural state. As a result of their small size, most of the estuaries in the region had little resilience to changes in freshwater quantity and nutrient loading. In contrast, the larger systems, targeted for dam development, only showed sensitivity to water abstraction during low-flow periods when base-flow reduction caused mouth closure and changes in nutrient processes. Broadly, the approach aimed to find a balance between ecological requirements and socio-economic development, which meant that maintaining larger systems in relatively good condition would be at the expense of smaller systems that are already in a poor condition. The approach developed was successful in quantifying the responses of multiple estuaries to escalating future pressures on a regional scale, and could be replicated to assist in managing water resources elsewhere in data-limited environments.

An environmental flow determination method for integrating multiple-scale ecohydrological and complex ecosystem processes in estuaries.

This paper presents an environmental flow methodology that was developed to accommodate shallow, highly dynamic micro-tidal estuaries found along the wave-dominated coast of South Arica. This method differs to most other approaches that primarily focus on larger permanently open systems having unrestricted inlets. Following an adaptive, design science research approach, the 7-step method adopted both ecohydrological and ecosystem-based concepts, encapsulating key hydrologicalhydrodynamic-biogeochemical processes, as well as biotic responses. The procedure also addresses a key challenge often encountered in applying these approaches to complex estuarine systems - the mismatch of temporal and spatial scales between abiotic processes and biotic responses. The method simplifies and aggregates abiotic processes to appropriate scales suitable for analysis of biotic responses, by introducing concepts such zoning and major physical states that characterize an estuary. The method's flexibility in data requirements lends itself to applications in countries where data is limited or where differences exist in data quality between systems. Essential in any environmental flow determination process, however, is long-term monitoring to incrementally improve confidence of the input data, but also to evaluate whether allocated flows achieve desired objectives set. Future challenges include refining the method to accommodate flow changes within much shorter timeframes and in conjunction with escalating global change pressures amongst other; pollution, living resource exploitation and physical destruction of habitat.

Comparative assessment of two agriculturally-influenced estuaries: Similar pressure, different response.

This study compared the spatio-temporal dynamics in two agriculturally-influenced South African estuaries - Gamtoos and Sundays - to investigate how contrasting hydrological alterations influence physical, chemical and biological responses. With the Gamtoos Estuary experiencing regular high flow conditions, a key difference between the two systems is the propensity for natural flushing events to occur; a mechanism largely eliminated from the highly-regulated Sundays Catchment. Phytoplankton blooms (>20Chl-aµgl-1) were persistent and seasonal in the Sundays, inducing summer bottom-water hypoxia (<2mgl-1), whilst those in the Gamtoos were episodic and flow-dependent. Of concern in the Sundays Estuary, was the magnitude (>550µgl-1) and recurrent nature of two harmful algal bloom (HAB) species. This study provides the first account of HAB persistence and seasonal hypoxia in a South African estuary, demonstrating the possible consequences of shifting an ecosystem into a new stable state.