Effective Management of Closed Hypereutrophic Estuaries Requires Catchment-Scale Interventions

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.

Body condition of larval roundherring,Gilchristella aestuaria(family Clupeidae), in relation to harmful algal blooms in a warm-temperate estuary

“…However, little is known about the effects of HABs on larval fishes in estuaries. There are isolated studies on HAB implications for zooplankton, larval fishes ( Black et al ., 2016 ; Smit et al ., 2021 ) and adult fishes ( Gannon et al ., 2009 ; Bornman et al ., 2021 , 2022 ; Lemley et al ., 2021 ), but more comprehensive knowledge on these effects is needed.…”

Water filtration by endobenthic sandprawns enhances resilience against eutrophication under experimental global change conditions

“…Beyond thermal optima though, rising temperature may impair consumer physiology, increase mortality and alter distribution 19 , 20 . Similarly, at high levels of eutrophication, oxygen depletion, pathogenic microbes, harmful algae and released toxins may increase the incidence of disease or mortality in consumers, negating their contribution to ecosystem resilience 11 , 12 , 20 , 21 . Thus, impaired physiology and health, in addition to greater mortality at high temperature and eutrophication levels, may have the effect of weakening total resource consumption and hence top-down impact.…”

“…Thus, impaired physiology and health, in addition to greater mortality at high temperature and eutrophication levels, may have the effect of weakening total resource consumption and hence top-down impact. Shifts to phytoplankton taxa that are less edible in response to nutrient enrichment and high temperature can also potentially weaken top-down impacts 21 , 22 . To complicate matters, coastal warming and eutrophication can directly impact phytoplankton through metabolic changes 16 , 23 , 24 .…”

“…Likely due to the semi-arid climate, these systems periodically close-off from oceans 26 , 27 , although this may be exacerbated by drought and increasing water abstraction for human needs. Temporarily closed estuaries also occur in Australia, south-eastern Brazil and Uruguay, south-western India, Sri Lanka 27 , Portugal, as well as along the coastlines of California and Texas in the United States of America 21 . Consistent with global trends 11 , 12 , eutrophication is a key process impacting South African estuaries, but the consequences of inorganic nutrient inputs (such as from fertilisers) are compounded by organic matter inputs from wastewater treatment works that lack operational skills and resources for maintenance and upgrades 13 .…”

Polymerizable Oxygen Probe Derived from Platinum-Based Porphyrins for Oxygen Sensing and Pressure-Sensitive Paints

“…Oxygen-level detection is highly important for many fields, such as biology, , environment, aerospace industry, and so on. In the aerospace industry, the surface pressure on the aircraft can be reflected by a visualization techniquepressure-sensitive paints (PSPs), which are based on optical oxygen sensors. − The working mechanism of PSP is oxygen quenching .…”