Mangrove habitats support taxonomically diverse benthic communities, yet their effects on microphytobenthos remain understudied. This study investigated the benthic diatom community structure of 17 ...microtidal mangrove estuaries situated along the east coast of South Africa. High sediment organic content (≤ 18.1 %) and clay fraction (≤ 17.5 %) measured in the Avicennia marina and Rhizophora mucronata habitats favoured diverse communities (H′ > 3) and the presence of unique benthic diatom species such as Gyrosigma balticum. The habitat complexity and stabilisation provided by the mangrove forests served as microrefugia for colonisation of benthic diatoms. Taxa displayed a broad range of nutrient tolerance, with some nutrient tolerant species (e.g., Halamphora acutiuscula, Navicula clausii, Navicula gregaria, and Navicula radiosa) dominating the communities in the Mbashe and Mnyameni estuaries. The presence of these nutrient tolerant taxa in relatively pristine systems serves as an early warning for possible anthropogenic nutrient enrichment.
•Low-nutrient conditions supported a diverse benthic diatom community.•Benthic diatom diversity was higher (H′ >3) in the mangrove habitats.•The finer cohesive mangrove sediments favoured the proliferation of select taxa.•Lower diversity (H′ < 2) due to higher flow were unique to the control habitat.•Some nutrient tolerant taxa were recorded in select estuaries.
Low-inflow estuaries are naturally more susceptible to anthropogenic stressors, compared to well flushed systems, with excessive nutrient loading posing a particular threat. This study investigated ...the benthic diatom community structure of two eutrophic, microtidal estuaries impacted by daily wastewater effluent discharges. It was hypothesised that the community structure would be similar between the warm-temperate Hartenbos and subtropical uThongathi estuaries due to disproportionally high dissolved inorganic nitrogen (H: 38 kg DIN d
−1
; uT: 67.5 kg DIN d
−1
) and phosphorus (H: 22 kg DIP d
−1
; uT: 29.7 kg DIP d
−1
) inputs from wastewater treatment works (WWTWs). Taxa tolerant of high nutrient conditions proliferated in both systems. However, the dominant taxa differed with the brackish
Halamphora coffeaeformis
species occurring in the Hartenbos Estuary and the freshwater
Navicula rostellata
,
Sellaphora pupula
and
Navicula gregaria
species in the uThongathi Estuary. The overall benthic diatom diversity in both systems was low (
H
’ > 0 but ≤ 1.5) and indicative of a degraded health state. Temporal differentiation driven by salinity was evident in the Hartenbos Estuary, while changes in community structure were limited to periods of increased river inflow in the uThongathi Estuary. Therefore, while the trophic status of the dominant taxa was determined by the nutrient stress (primary stressor), changes in salinity and river inflow (secondary stressors) shaped the distinct community assemblages observed in each estuary. This study provides insight into the impact of similar anthropogenic-induced pressures in different biogeographical regions and the importance of managing towards a natural dynamic state of microtidal, low-inflow estuaries.
Urbanization and human-led development have increased more rapidly along shorelines and in coastal watersheds than inland regions over the past century. The result of major land use changes for both ...urban tracts and agriculture to serve the urban areas, as well as infrastructure development is increased runoff carrying sediments, nutrients, pollutants, pharmaceuticals, and toxins downstream to estuarine systems. The increased runoff levels are only the tip of the iceberg, with human development resulting in increased fecal bacteria from urbanization and excess nutrients from agriculture leading to harmful algal blooms. Estuaries act as a natural filter between land and sea, but have been overloaded by the influx of sediments and pollutants in recent decades. As a result, there have been a variety of impacts to estuarine ecosystems and water quality including increased sediment load, eutrophication, harmful algal blooms, fecal bacteria, as well as shellfish and fisheries declines. In some estuarine systems, the reduction in light penetration to the benthos has led to the loss of seagrasses. In others, seasonal hypoxia is a visible symptom of prolonged eutrophication. There is a need to augment long-term monitoring techniques with new technologies and data processing methods to better understand the current state of estuaries and work towards mitigating human impacts on estuarine ecosystems and water quality.
Nutrient pollution is facilitating the encroachment of invasive aquatic plants in various water bodies globally. This study investigated seasonal aquatic macrophyte responses in two temporarily ...closed estuaries with different nutrient inputs. Consistent effluent discharge from the upstream wastewater treatment works (WWTW) facilitated the establishment of numerous freshwater invasive alien aquatic plants (IAAPs) in the uThongathi Estuary. IAAPs (Myriophyllum aquaticum, Pistia stratiotes and Pontederia crassipes) were only displaced from the estuary after high flow events (>5 m3 s−1). In the less polluted uMdlotane Estuary nutrient pulses (>1 mg/L DIN) associated with high rainfall changed the aquatic macrophyte composition. The nutrient tolerant non-rooted Ceratophyllum demersum outcompeted the rooted submerged macrophyte Stuckenia pectinata. Species composition changed in response to flow and nutrient inputs, with the exception of emergent macrophytes, such as the grass Echinochloa pyramidalis, that remained consistent in cover and distribution in the uMdlotane Estuary. This study demonstrated that aquatic macrophytes are more responsive to nutrient inputs in unimpacted estuaries compared to consistently nutrient-rich systems where flow is an important driver of IAAPs community dynamics. Many temporarily closed estuaries are subjected to nutrient pollution from WWTWs and restoration efforts such as diversion of discharges to constructed wetlands needs urgent implementation.
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•Invasive aquatic plants are abundant in nutrient rich oligohaline closed estuaries.•Consistent wastewater input to the uThongathi Estuary resulted in numerous IAAPs.•M. aquaticum, Pistia stratiotes and P. crassipes were absent during high flow.•Peaks in freshwater inflow introduced nutrients to the pristine uMdlotane Estuary.•The non-rooted Ceratophyllum demersum replaced the rooted S. pectinata.
Upstream anthropogenic perturbations can dramatically affect estuarine water quality, especially in small systems where water retention times are high. Management of these modified ecosystems often ...requires direct interventions to prevent detrimental long-term effects. The heavily urbanized and temporarily closed Zandvlei Estuary in South Africa has a long history of anthropogenic manipulation, and therefore acts as an ideal case-study to gauge reactive management practices. Continued deterioration of estuarine water quality and ecological functioning prompted the adoption of an adaptive management approach. Subsequent management interventions to increase salinity variability and nutrient removal included the (1) adoption of a mouth management plan, (2) regular harvesting of submerged vegetation, and (3) removal of flow obstructions. Physico-chemical and nutrient monitoring data (2010–2018) were assessed to document the response of phytoplankton to these interventions. Time-series analysis indicated an upward trend in salinity throughout the estuary. This corresponded with a temporal decline in phytoplankton biomass levels and was supported by model results that highlighted a strongly inverse relationship with salinity. The frequency of high-biomass events (>80 μg Chl-a l−1) also declined in each of the designated estuarine zones. In April 2012, an extensive harmful algal bloom (HAB) of Prymnesium parvum (Prymnesiophyceae) was recorded (~530 μg Chl-a l−1) in the estuary, culminating in hypoxic conditions (O2 < 2 mg l−1) and fish kills. However, the subsequent mechanical breaching of the mouth has prevented reoccurrences of P. parvum. Similarly, more saline conditions arising from increased marine connectivity reduced the abundance of the previously dominant Bacillariophyceae and Chlorophyceae classes. The overall improvement in water quality validates the efficacy of management interventions aimed at restoring and conserving ecosystem functionality. Yet, these efforts are a deviation from natural estuarine functioning and thus a ‘catchment to coast’ approach that incorporates upstream nutrient mitigation measures is needed to achieve sustainable long-term management objectives.
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•Artificial mouth breaching restored estuarine salinity gradient and variability.•Phytoplankton biomass and bloom frequency has declined with increased salinity.•Eutrophic symptoms (HABs and hypoxia) persist despite ad-hoc management practices.•Catchment-scale measures are required to achieve long-term sustainability goals.
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.
•Natural flow variability results in episodic and event-dependent in situ responses.•Removal of natural system dynamism causes persistent and undesirable disturbances.•Red-tide events were comprised of Heterosigma akashiwo and Heterocapsa rotundata.•Exceptional phytoplankton blooms (>550μgl−1) recorded in the Sundays Estuary.•First record of seasonal bottom-water hypoxia in a South African estuary.
Phytoplankton-mediated nutrient fluxes typically provide only pulsed relief to adjacent coastal waters during the productive period, with nutrient export increasing in the absence of substantial ...phytoplankton biomass. On the warm temperate coastline of South Africa, the Sundays Estuary is characterised by highly regulated freshwater inflow patterns, nutrient-enriched conditions, and resident harmful algal blooms (HABs). Given these attributes, the study objective was to investigate the effect of these phytoplankton blooms on fluvial inorganic nutrient dynamics. To assess uptake, we analysed inorganic nutrient (phosphate, ammonium, NO
x
) and phytoplankton concentrations in relation to salinity using data from 17 surveys. Property-salinity mixing diagrams and statistical analyses indicated a positive association between increasing phytoplankton biomass and decreasing NO
x
flux (p < 0.001), and to a lesser degree phosphate flux (p = 0.22), along the gradient from low-salinity inner estuary to high salinity outer estuary. High biomass HAB accumulations of
Heterosigma akashiwo
(>100 µg chl
a
l
-1
) represent significant removal of available NO
x
(~100%) and phosphate (>75%) during warmer conditions (>20°C). These events, together with continuous inorganic nutrient uptake during less severe bloom conditions, remove a substantial portion of annual NO
x
and phosphate loads (36.5 and 36.4% flux, respectively). Although this buffers inorganic nutrient loading to adjacent coastal waters, it also represents an emerging legacy pollution issue in the form of a benthic accumulation of organic material in bottom waters subject to recurrent hypoxia. Future management efforts should adopt an ecosystem-based approach centred around simultaneous restoration of hydrological variability and dual nutrient reduction strategies (N and P).
The increased frequency and magnitude of harmful algal blooms (HABs) from anthropogenic nutrient enrichment is of great concern in estuarine systems worldwide. Despite estuaries being important fish ...nurseries, a paucity of knowledge exists on the impacts of HABs on estuarine fishes due to the sporadic and unpredictable nature of HABs. Thus, this study aimed to determine the frequency and magnitude of HABs and the subsequent impact on the abundance of a common estuarine-dependent Mugilidae species,
Mugil cephalus
, in the shallow low inflow and agriculturally altered warm temperate Sundays Estuary in South Africa. The sampling design adopted a monthly (monitoring over 2 years between 2018 and 2020) and twice weekly (for 7 weeks during known spring/summer HABs in 2018) day and night sampling approach. It was hypothesised that
Mugil cephalus
abundance would be negatively related to phytoplankton biomass since HABs would result in unfavourable daytime supersaturated dissolved oxygen surface water and night-time bottom-water hypoxia. This study found that the Sundays Estuary was dominated by the HAB species,
Heterosigma akashiwo
, which significantly altered the dissolved oxygen in the estuary resulting in frequent supersaturated surface waters and hypoxic bottom waters.
Mugil cephalus
abundances were negatively related to phytoplankton biomass, with instances of high abundance (CPUE > 10) only being found at sites characterised as not experiencing severe bloom conditions (< 60 Chl-
a
µg.L
−1
). This avoidance of HABs suggests that if unsustainable levels of agricultural fertilisation continue, resulting in more severe and frequent HABs, a major decline in fish nursery quality in the estuary may occur.
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.
•Nutrient uptake in the small study CW is limited by excessive stormwater volumes.•The lack of primary producer diversity reduces nutrient removal efficiency.•Routine maintenance is required to prevent saturation of nutrient uptake processes.•Flooding of an abandoned saltpan to expand the current CW is a remediation option.
Mobile metazoans in the Phanerozoic transformed shallow coastal habitats covered in biofilms into bioturbated mixed grounds. Benthic biofilms during this time rarely preserved into the fossil record ...as microbialites, and more specifically their layered form, stromatolites. Living microbialites are rare for this reason, in addition to other factors such as unfavourable modern geochemical conditions. Counterintuitively, macroinvertebrate grazers and laminated stromatolites coexist along the Nelson Mandela Bay coastline, South Africa. Here we aimed to determine if nutritional qualities of algae in these microbialite pools are linked to the diet of a dominant amphipod grazer, Melita zeylanica Stebbing, 1904. We assessed this in 3 microbialite pools using stable isotopes coupled to seasonal shifts in abundant food resources of microbialite biofilms and exposed (bleached and rimstone) or submerged (pool) macroalgae, Ulva spp. Amphipod diet was proportional to the quantity available (biomass) and nutritional properties of available algal food (energy, protein, total phosphorus and C:N ratios). Macroalgae in all states were the most nutritious, and the diet of M. zeylanica was dominated by submerged macroalgae. Although microbialite material had the worst nutritional profile, it was consumed in larger quantities compared to exposed bleached and rimstone macroalgae. Therefore, factors unrelated to nutritional profiles, such as predation, allelopathy or exposure risks, likely drive food choices. High levels of grazing on nutritious pool macroalgae likely releases most of the grazing pressure on the microbialite biofilm, therefore partially explaining why the microbialites can persist in this environment instead of being bioturbated through foraging.
