Abstract
Increased nutrient loading associated with rapid population growth is the leading cause of deteriorating water quality in urbanized estuaries globally. Small estuaries are particularly sensitive to changes when connection with the marine environment is restricted, or lost, because of high water retention. The temporarily closed Hartenbos Estuary (South Africa) is an example of how such pressures can culminate in a severely degraded ecosystem. Wastewater treatment work (WWTW) discharges introduce substantial volumes of freshwater (8,000 m3 d–1) and nutrient loads (38 kg DIN d–1 and 22 kg DIP d–1) into this estuary. This constant inflow has necessitated frequent artificial breaching (inducing alternating states) of the estuary mouth to prevent flooding of low-lying developments and, occasionally, to mitigate against extreme events such as fish kills and sewage spills. This study investigated the efficacy of artificial mouth breaching practices in eliciting responses in selected abiotic and biotic parameters. Microalgal (phytoplankton and benthic diatoms), benthic macrofauna and fish community dynamics were assessed in response to mouth state and water quality conditions using a seasonal monitoring programme. The hypereutrophic nature of the Hartenbos Estuary was highlighted by persistent high-biomass phytoplankton accumulations (>100 μg Chl-a l–1), extreme dissolved oxygen conditions (0.4–20.5 mg O2 l–1) and the predominance of harmful algal bloom (HAB) events comprising Nannochloropsis sp. and Heterosigma akashiwo. Artificial breaching of the mouth facilitated limited tidal exchange and occurred approximately bimonthly once water levels exceeded 1.9 m above mean sea level (MSL). Current pressures and management interventions have culminated in an ecosystem void of natural fluctuations and instead characterised by low diversity and shifts between undesirable states. This is highlighted by the near year-round dominance of only a few opportunistic species/groups tolerant of adverse conditions (e.g., Nannochloropsis sp., Halamphora coffeiformis, oligochaetes, estuarine round herring Gilchristella aestuaria, and southern mullet Chelon richardsonii). Therefore, catchment-scale interventions such as the diversion of WWTW discharges and restoration of hydrodynamic variability are management priorities for improving the health and biodiversity of small, closed microtidal systems such as the Hartenbos Estuary.
Highlights
The issue of coastal eutrophication has been researched extensively (Nixon, 1995; Cloern, 2001; Bricker et al, 2008; Le Moal et al, 2019; Lemley and Adams, 2019; Malone and Newton, 2020) due to its worldwide prominence and close linkages with anthropogenic global change pressures
The considerable socio-economic value and long history of anthropogenic perturbations in the Hartenbos Estuary prompted the development of such an estuarine management plans (EMP), with the second generation thereof completed in 2018 (Mossel Bay Local Municipality, 2018)
Some of the key management objectives identified in the Hartenbos EMP were to (1) restore a degree of estuarine functionality and health, (2) improve water quality (e.g., wastewater treatment work (WWTW) discharges), (3) effectively manage mouth interventions, and (4) promote research and monitoring
Summary
The issue of coastal eutrophication has been researched extensively (Nixon, 1995; Cloern, 2001; Bricker et al, 2008; Le Moal et al, 2019; Lemley and Adams, 2019; Malone and Newton, 2020) due to its worldwide prominence and close linkages with anthropogenic global change pressures (e.g., nutrient enrichment, warming, hydrological modifications). The continued deterioration of coastal and estuarine ecosystems has led to widespread restoration efforts that aim to alleviate the pressures that cause degradation (Duarte and Krause-Jensen, 2018; Boesch, 2019; Le Moal et al, 2019) Such efforts, typically centred around a combination of nutrient reduction strategies and engineering solutions, tend to emanate from legal policies (e.g., Clean Water Act, United States; Water Framework Directive and Marine Strategy Framework Directive, Europe; National Water Act, South Africa) that prompt management interventions when an ecosystem is classified as being in an undesirable condition (Lemley and Adams, 2019). Recovery typically requires a greater reduction in pressure compared to that which initiated degradation to reduce or remove negative feedback loops (e.g., internal nutrient loading, altered food webs) that act to maintain the degraded state This was exemplified by Duarte et al (2009) that highlighted the convoluted recovery trajectories, and inability to return to past norms, of four European estuaries that underwent significant nutrient load reductions (oligotrophication). The recovery of estuarine ecosystems is closely linked to the (1) severity of the disturbance/pressure, and (2) degree of connectivity with adjacent healthy ecosystems
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