Tidal Mixing Research Articles

Spurious numerical mixing under strong tidal forcing: a case study in the south-east Asian seas using the Symphonie model (v3.1.2)

Abstract. The role of mixing between layers of different densities is key to how the ocean works and interacts with other components of the Earth's system. Correctly accounting for its effect in numerical simulations is therefore of utmost importance. However, numerical models are still plagued with spurious sources of mixing, originating mostly from the vertical advection schemes in the case of fixed-coordinate models. As the number of phenomena explicitly resolved by models increases, so does the amplitude of resolved vertical motions and the amount of spurious numerical mixing, and regional models are no exception to this. This paper provides a clear illustration of this phenomenon in the context of simulating the south-east Asian (SEA) seas along with a simple way to reduce its impact. This region is known for its particularly strong internal tides and the fundamental role they play in the dynamic of the region. Using the Symphonie ocean model, simulations including and excluding tides and using a pseudo-third-order upwind advection scheme on the vertical are compared to several reference datasets, and the impact on water masses is assessed. The high diffusivity of this advection scheme is demonstrated along with the importance of accounting for tidal mixing for a correct representation of water masses. Simultaneously, we present an improvement in this advection scheme to make it more suitable for use in the vertical. Simulations with the new formulation are added for comparison. We conclude that the use of a higher-order numerical diffusion operator greatly improves the overall performance of the model.

Spatial Variation of Asymmetry in Velocity and Sediment Flux along the Artificial Aam Tidal Channel

Tidal flats, crucial for biodiversity and ecosystem services, are facing significant alterations due to human activities such as reclamation. In South Korea, over 65% of tidal flats have been reclaimed since the 1970s, resulting in morphological changes and altered sediment transport dynamics. This study investigates sediment transport processes in the artificial Aam tidal channel, created as part of the megacity development project in Incheon, Korea. Using data from Acoustic Doppler Current Profiler and Vector instruments deployed in 2019 and 2021, we analyzed tidal asymmetry, current velocities, shear stress, and suspended sediment concentration. Our results reveal a pronounced tidal asymmetry influencing sediment transport, with ebb-dominant currents near the channel entrance and flood-dominant currents in the interior. We observed significant sediment deposition in the landward section of the channel, driven by tidal mixing asymmetry and rainfall events. These findings highlight the complex interactions between artificial structures and natural sediment dynamics, informing future coastal development and management strategies.

Characteristics and Mechanisms of Spring Tidal Mixing and Sediment Transport in a Microtidal Funnel-Shaped Estuary

Information about estuarine mixing and its control of sediment transport is crucial to elucidating the dynamics and evolution of estuaries. Here, the microtidal and funnel-shaped Zhenhai Estuary, located in the southwestern Pearl River Delta of China, is used to investigate the characteristics and mechanisms of water mixing and sediment transport based on observations from three spring tides. The results reveal that the studied estuary remains well mixed during spring tides from 2013–2022 despite its microtidal regime. Tidal stirring, which is enhanced by tidal energy convergence and benefits from the funnel-shaped geometry and shallow bathymetry, favors vertical mixing, contributing to the formation of strong mixing in the estuary. Due to the well-mixed regime, sediment transport in the estuary is dominated by the advective term, followed by a moderate tidal pumping term and minor estuarine circulation term. Accordingly, sediments within the estuary tend to be transported landward owing to the regulation of the funnel-shaped geometry, and a gradual but slow infilling trend is predictable. This paper deepens our understanding of hydrodynamics and sediment transport in microtidal estuaries.

Lateral Transport Controls the Tidally Averaged Gravitationally Driven Estuarine Circulation: Tidal Mixing Effects

Abstract In classic models of the tidally averaged gravitationally driven estuarine circulation, denser salty oceanic water moves up the estuary near the bottom, while less dense riverine water flows toward the ocean near the surface. Traditionally, it is assumed that the associated pressure gradient forces and salt advection are balanced by vertical mixing. This study, however, demonstrates that lateral (across the estuary width) transport processes are essential for maintaining the estuarine circulation. This is because for realistic estuarine bathymetry, the depth-integrated salt transport up the estuary is enhanced in the deeper estuary channel. A closed salt budget then requires the lateral transport of this excess salt in the deeper channel toward the estuarine flanks. To understand how such lateral transport affects the estuarine salt and momentum balances, we devise an idealized model with explicit lateral transport focusing on tidally averaged lateral mixing effects. Solutions for the along-estuary velocity and salinity are nondimensionalized to depend only on one single nondimensional parameter, referred to as the Fischer number, which describes the relative importance of lateral to vertical tidal mixing. For relatively strong lateral tidal mixing (greater Fischer number), salinity and velocity variations are predominantly vertical. For relatively weak lateral tidal mixing (smaller Fischer number), salinity and velocity variations are predominantly lateral. Overall, lateral transport greatly affects the estuarine circulation and controls the estuarine salinity intrusion length, which is demonstrated to scale inversely with the Fischer number.

The role of estuarine convergence on the salinity distribution and the estuary response to short river discharge pulses

Estuarine behaviour is primarily controlled by the interaction of river discharge and tidal mixing. Transient increases in river discharge, which occur naturally during the wet season or can be managed in regulated basins, alter estuarine dynamics and reduce salt intrusion. Two characteristic times have been identified in the response to these river pulses: the adjustment time (the time required to adapt to the peak discharge) and the recovery time (the time required to return to steady state conditions). Previous work has analysed the effect of peak discharge and pulse duration in the adjustment process, focusing on long discharges (i.e. longer than the intrinsic estuarine timescale) and straight channels. This paper presents an idealised 3-dimensional model (Delft3D) to assess the effect of estuarine convergence on the response to short river pulses in tide dominated, well-mixed estuaries. The similarity between the tidal period, the estuarine timescale and the duration of the pulse led to the introduction of the time between tidal and river conditions as an additional parameter in the analysis. The results show that salt intrusion in convergent estuaries have a lower sensitivity to variations in river discharge compared to the behaviour of prismatic estuaries. Conversely, the influence of convergence on changes in salt intrusion due to the increased river discharge gains relevance in convergent estuaries. Seaward transport of salt is significantly enhanced by freshwater releases that occur between early and mid ebb, especially during spring tides. Increasing convergence reduces the asymmetry of the estuarine response, shortens the recovery time, and shifts the onset of maximum salt displacement and the minimum adjustment time. The results are particularly relevant for water management and salt intrusion control in regulated basins with limited resources, as the volume of fresh water used to reduce salt intrusion can be optimised considering the timing of the pulse release, which depends on the tidal conditions at the estuary.

Role of tidal mixing on ocean exchange through the Strait of Hormuz

We investigate the influence of tides on the exchange of water between the Arabian Gulf and the Sea of Oman through the Strait of Hormuz using a high-resolution numerical model. Two numerical simulations are contrasted, one with and one without tidal forcing. We find that tides suppress exchange through the Strait, by ∼20% in the annual mean, being largest in the summer (∼30%) and diminishing in the winter (∼13%). Tides enhance the parameterised shear-driven vertical mixing inside the Gulf and Strait, mixing warm, relatively fresh surface waters downward thus reducing the density of bottom waters flowing outwards. This reduces the lateral difference of density between Gulf and Sea of Oman and hence the exchange through the Strait. Maximum reductions occur in summer when both the vertical stratification and mixing is the largest.

The Role of Tidal Mixing in Shaping Early Eocene Deep Ocean Circulation and Oxygenation

AbstractDiapycnal mixing in the ocean interior is largely fueled by internal tides. Mixing schemes that represent the breaking of internal tides are now routinely included in ocean and earth system models applied to the modern and future. However, this is more rarely the case in climate simulations of deep‐time intervals of the Earth, for which estimates of the energy dissipated by the tides are not always available. Here, we present and analyze two IPSL‐CM5A2 earth system model simulations of the Early Eocene made under the framework of DeepMIP. One simulation includes mixing by locally dissipating internal tides, while the other does not. We show how the inclusion of tidal mixing alters the shape of the deep ocean circulation, and thereby of large‐scale biogeochemical patterns, in particular oxygen distributions. In our simulations, the absence of tidal mixing leads to a relatively stagnant and poorly ventilated deep ocean in the North Atlantic, which promotes the development of a basin‐scale pool of oxygen‐deficient waters, at the limit of complete anoxia. The absence of large‐scale anoxic records in the deep ocean after the Cretaceous anoxic events suggests that such an ocean state most likely did not occur at any time across the Paleogene. This highlights how crucial it is for climate models applied to the deep‐time to integrate the spatial variability of tidally driven mixing as well as the potential of using biogeochemical models to exclude aberrant dynamical model states.

Dynamics of green macroalgal micro-propagules and the influencing factors in the southern Yellow Sea, China

Micro-propagules (banks of microscopic forms) play important roles in the expansion of green tides, which are spreading on eutrophic coasts worldwide. In particular, large-scale green tides (Yellow Sea Green Tide, YSGTs) have persisted in the Yellow Sea for over 15 years, but the dynamics and functions of micro-propagules in their development remain unclear. In the present study, year-round field surveys were conducted to identify the reservoirs and investigate the persistence mechanisms and associated biotic and abiotic factors driving the temporal and spatial variations of micro-propagules. Micro-propagules in the southern Yellow Sea (SYS) showed evident spatial heterogeneity in terms of seasonal patterns and major influencing factors. Offshore of the SYS, the micro-propagule population underwent ephemeral expansion along with a large-scale bloom of floating Ulva algae in late spring and early summer. The Subei Shoal, particularly the sediments in the central raft region, had the highest micro-propagule abundance (MA) and was a major reservoir. The pronounced seasonal variation of MA in the Subei Shoal was primarily associated with the attached Ulva algae on Neopyropia aquaculture rafts. Vast aquaculture rafts provided essential substrates for micro-propagules to complete their life cycle and replenish the seed bank, thereby sustaining persistent YSGTs. It implied that habitat modification has pronounced ecological impacts on this intertidal muddy flat. The unique environmental conditions (enriched nutrients, esp. nitrate, favourable seawater temperatures in spring, and strong tidal mixing) facilitated the abundance, seasonal variation and recruitment of micro-propagules in the Subei Shoal. Given the current mitigation measures implemented in the raft region, further research is required to monitor and investigate the physiological and ecological responses of micro-propagule populations to the complex hydrobiological, geochemical, and physical matrices.

The deepwater oxygen deficit in stratified shallow seas is mediated by diapycnal mixing

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. Here we integrate turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum and associated mid-water oxygen maximum. We show that the oxygen maximum supports both upward and downwards O2 fluxes. The upward flux is into the surface mixed layer, whilst the downward flux into the deep water will partially off-set the seasonal O2 deficit. The results indicate the fluxes are sensitive to both the water column structure and mixing rates implying the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that the downward flux is supported by tidal mixing, whilst the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in the development of the seasonal deep water O2 deficit.

Frontal slope: A new measure of ocean fronts

Ocean fronts, as typical marine phenomena, significantly impact the marine environment and the distribution of organisms. However, the research on their tilted structure is limited. This study considered the frontal slope as a key parameter for describing the tilting characteristics of ocean fronts. A novel method, based on high-resolution in situ observations and reanalysis data, was proposed to extract global frontal slopes using contour lines and surfaces. To validate this approach, a mathematical model was developed for the oceanic front slope and verified through in situ observations. The frontal slope equation produced reliable results, indicating that the front tilting primarily arises from shear flow and density differences. Furthermore, a modified oceanic front gradient parameter was proposed based on the frontal slope, and its significance in understanding the impacts of ocean fronts on underwater acoustic propagation was validated through numerical simulation and statistical analysis. The distribution characteristics of frontal slope for ocean fronts on a global scale were analyzed for the first time. Marginal ice zone fronts had the highest slope (>1.5 m/km), followed by tidal mixing fronts, western boundary current fronts, and water mass convergence fronts (1–1.5 m/km). In contrast, upwelling fronts and estuarine plume fronts generally had slopes below 1 m/km. This parameter provides a new perspective on ocean front systems and their influence on dynamic structures and ecological resource distributions. This study helps in a comprehensive understanding of the tilting characteristics of the ocean front and their implications.

Significant CO2 emission in the shallow inshore waters of the southeastern Yellow Sea in 2020

This study investigated the carbonate system and air-sea CO2 exchange in the inshore waters along South Korea's western coastline in 2020. Overlooking these waters might introduce significant errors in estimating air-sea CO2 fluxes of the southeastern Yellow Sea, given their interaction with land, offshore regions, and sediments. During periods other than summer, seasonal variations in seawater CO2 partial pressure (pCO2) could be generally explained by thermal effects. Tidal mixing and shallow depths resulted in weaker stratification-induced carbon export compared to offshore regions. However, during summer, inshore waters exhibited high spatial variability in pCO2, ranging from approximately 185 to 1000 μatm. In contrast to offshore waters that modestly absorbed CO2, inshore waters shallower than 20 m emitted ∼100 Gg C yr−1 to the atmosphere. However, considering the high heterogeneity of the study area, additional observations with high spatial and temporal resolution are required to refine estimates of air-sea CO2 exchange.

Mixing of the Connecticut River Plume During Ambient Flood Tides: Spatial Heterogeneity and Contributions of Bottom‐Generated and Interfacial Mixing

AbstractThe Connecticut River plume is influenced by energetic ambient tides in the Long Island Sound receiving waters. The objectives of this modeling study are (a) characterizing the spatial heterogeneity of turbulent buoyancy fluxes, (b) partitioning turbulent buoyancy fluxes into bottom‐generated and interfacial shear contributions, and (c) quantifying contributions to plume‐integrated mixing within the tidal plume. The plume formed during ambient flood tides under low river discharge, spring tides, and no winds is analyzed. Turbulent buoyancy fluxes (B) and depth‐integrated B through the plume (Bd) are characterized by pronounced spatial heterogeneity. Strong mixing (Bd ∼ 10−5‐10−4 m3/s3) occurs near the mouth, in the nearfield plume turning region, over shoals, and nearshore shallow areas. Low to moderate mixing (Bd ∼ 10−8‐10−6 m3/s3) occupies half the plume. Buoyancy fluxes are first partitioned based on the depth of the shear stress minimum between plume‐generated and bottom‐generated shear maxima. Four other tested partitioning methods are based on open channel flow and stratified shear flow parameterizations. Interfacial and bottom‐generated shear contribute to different areas of intense and moderate mixing. All methods indicate a significant plume mixing role for bottom‐generated mixing, but interfacial mixing is a bigger contributor. Plume‐integrated total and interfacial mixing peak at max ambient flood and the timing of peak bottom‐generated mixing varies among partitioning methods. Two‐thirds of the mixing occurs in concentrated intense mixing areas. A parameter space with the ambient tidal Froude number and plume thickness to depth ratio as axes indicates many tidally modulated plumes are moderately to dominantly influenced by bottom‐generated tidal mixing.

Semidiurnal Internal Tides in a Shelf Sea South of Japan: Characteristics, Energetics, and Temporal variations

Tidal mixing in a shelf sea south of Japan (Bungo Channel) plays an important role in modulating the water exchange between the Seto Inland Sea and Pacific Ocean. In this study, based on moored observations and model results from the Japan Coastal Ocean Predictability Experiment—Tides (JCOPE-T), the generation, propagation, and dissipation of semidiurnal internal tides in the Bungo Channel are investigated. Observational results indicate that semidiurnal internal tides induce strong baroclinic currents reaching 0.3 m/s. Their energy shows obvious spring-neap tidal cycles, generally coinciding with the local barotropic tidal forcing. By conducting the empirical orthogonal function analysis, we find that the observed semidiurnal internal tides are mainly dominated by the first two baroclinic modes. The JCOPE-T results suggest two main generation sites for semidiurnal internal tides in the region: one is located at a narrow strait north of the Bungo Channel, while the other is at the shelf break south of the Bungo Channel. The latter makes a major contribution to the observed semidiurnal internal tides. Northward internal tides generated at the shelf break are superposed with those generated at the narrow strait, causing a complex interference pattern in the channel. The temporal variation of semidiurnal internal tides in the Bungo Channel is affected by several factors. The intraseasonal variation of semidiurnal internal tides can be modulated by the Kuroshio warm water intrusion (Kyucho) because the occurrence of Kyucho changes the stratification in the channel and hence affects the energy conversion. The seasonal variation of semidiurnal internal tides in the Bungo Channel is determined mainly by the seasonally varying stratification; while those generated at the shelf break are under the combined influence of seasonal stratification and background currents. Southward internal tides from the shelf break are refracted due to the spatially varying stratification and background currents. The varying Kuroshio path and strength modulate the refraction of internal tides.

Influence of tidal mixing on bottom circulation in the Caroline Sea

Bottom circulation in the abyssal Caroline Sea is an important component of the global meridional overturning circulation. By use of a high-resolution regional ocean model, the influence of tidal mixing processes on bottom water and circulation in the Caroline Sea is investigated. Based on different configurations for diapycnal diffusivities of tidal mixing, three numerical experiments are performed: one completely without tidal mixing, one only with local tidal mixing due to the locally dissipated tidal energy, and one considering tidal mixing processes induced by the total dissipated tidal energy. The results show that tidal mixing processes in the abyssal Caroline Sea could sustain a relatively high horizontal density gradient and hence baroclinic pressure gradient not only across the two deep-water passages connecting to the open ocean, but also within the abyssal West Caroline Basin (WCB) and East Caroline Basin (ECB). Therefore, tidal mixing processes could maintain the large amounts of bottom water inflow, intensify the bottom basin/subbasin-scale horizontal circulation, and drive a more vigorous meridional overturning circulation in the abyssal WCB and ECB. Moreover, simulations of bottom water transport in the experiment with tidal mixing processes are more consistent with previous observations and estimates. These results suggest that tidal mixing processes play a crucial dynamic role in the bottom circulation, and is essential for ocean modelling.

Coastal trapped waves and tidal mixing control primary production in the tropical Angolan upwelling system.

Eastern boundary upwelling systems are hotspots of marine life and primary production. The strength and seasonality of upwelling in these systems are usually related to local wind forcing. However, in some tropical upwelling systems, seasonal maxima of productivity occur when upwelling favorable winds are weak. Here, we show that in the tropical Angolan upwelling system (tAUS), the seasonal productivity maximum is due to the combined effect of coastal trapped waves (CTWs) and elevated tidal mixing on the shelf. During austral winter, the passage of an upwelling CTW displaces the nitracline upward by more than 50 m. Thereby, nitrate-rich waters spread onto the shelf, where elevated vertical mixing causes a nitrate flux into the surface mixed layer. Interannual variability of the productivity maximum is strongly correlated to the amplitude of the upwelling CTW as seen in sea level data. Given that CTWs are connected to equatorial forcing, a predictability of the strength of the productivity maximum is suggested.

Use of mesocosm and field studies to assess the effects of nutrient levels on phytoplankton population dynamics in Korean coastal waters

We integrated data from field observations during April and March with data from a 2-week mesocosm experiment to investigate changes in phytoplankton populations in southern Korean coastal waters (KCWs) following nutrient enrichment during early spring of 2021. The mesocosm experiments used 1000 L cylindrical plastic containers that had natural seawater (control), a low nutrient (LN) treatment, or a high nutrient (HN) treatment. The field observations showed that increased freshwater runoff following spring rainfall led to elevated levels of dissolved inorganic nitrogen and silicate and a significant increase in total phytoplankton abundance. In March, nutrient enrichment from water mixing and terrestrial runoff led to dominance of cryptophyte Cryptomonas spp. In April, higher nutrient levels than March (p< 0.05) resulting from increased terrestrial runoff after rainfall and dominant species were Skeletonema spp., and Cryptomonas spp. In the mesocosm experiment, a succession from E. zodiacus initially to Chaetoceros spp. in the middle stage, and then to Cylindrotheca closterium and Pseudo-nitzschia spp. finally was observed, depending on the species-specific nutrient availability after nutrient addition. In principal component analysis, the negative correlation between C. closterium and nutrient levels supports their nutrient availability, which is an adaptation to low-nutrient conditions. The combined data from the field observations and mesocosm experiments indicated that nutrient supplementation from terrestrial runoff and tidal mixing played a crucial role in determining the dynamics of phytoplankton populations during early spring in the KCWs.

Influences of tides on shelf circulation in the northeastern South China Sea during summer

Tides have significant effects on material and energy transports over the continental shelf. A mooring system located in the shelf region of the northeastern South China Sea observed strong and weak mean currents during the spring and neap tides in summer, respectively. The mechanism leading to this difference is investigated by using a three-dimensional circulation model. We find that the impact of tides (TI) on currents exhibited as current components opposing wind-driven circulation play an important role. The TI current components are stronger than the tidal residual currents in summer when vertical stratification enhances the tidal rectification. Tides can affect pressure gradient by modifying sea surface height on the shelf, while in shallow and shelf break regions, the non-linear advection and vertical diffusion become significant in momentum equations attributable to the interactions of tidal currents, bottom topography and water column stratification. Consequently, the along-shelf volume transport is reduced by about 21.6% due to the TI current components and only by about 6.1% due to the tidal residual currents. This study highlights the importance of both tidal mixing and tidal rectification in regulating shelf circulation in the northeastern South China Sea.

Dissolved carbon dynamics and exchange in a high permeability beach aquifer

The biogeochemical transformation of dissolved carbon in tidal beaches affects the coastal carbon cycle and budget. Yet, the influence of groundwater flow on the behavior of dissolved carbon and CO2 flux across the different interfaces in these dynamic beach systems remains poorly understood, especially in deep beach aquifers. In this study, we investigated the biogeochemical reactions and origins of dissolved carbon and quantitatively evaluated CO2 outgassing driven by submarine groundwater discharge (SGD) in a high permeability beach aquifer (9–13 m depth) located in the South China Sea shelf. Results revealed that both dissolved inorganic carbon (DIC) and total alkalinity (TA) exhibited non-conservative additions across the salinity gradient. The excess DIC and TA most likely resulted from organic carbon decomposition in the brackish and saline zones. Aerobic respiration (upper saline plume zone), anaerobic reactions, and tidal mixing were identified as the dominant processes controlling DIC and TA dynamics. The mean CO2 outgassing flux across the beach sediment-air interface was estimated to be 1.05 g m−2 d−1, significantly higher than the CO2 flux across the sea-air interface. The outgassing pattern of groundwater CO2 from beach aquifer appeared to be driven by tidal pumping. Considering the combined effects of SGD-derived dissolved carbon and nutrients, we suggest that SGD yielded a net CO2 flux, contributing to 45 % of the CO2 flux across the sea-air interface. SGD directly and indirectly delivered substantial CO2 from the beach aquifer to the atmosphere, approximately (4.89–9.21) × 103 tons of CO2 per year along the 76.2 km long sandy coast. These findings demonstrate that intertidal beach aquifers, as active biogeochemical reactors have the strong potential to intensify CO2 emissions to the atmosphere through both beach sediment-air and sea-air interfaces. This study provides a better understanding of the links between carbon biogeochemical cycles and hydrologic processes in the coastal zones.

Sensitivity of Internal-Tide Generation to Stratification and Its Implication for Deep Overturning Circulations

Abstract The simple scaling relation for internal-tide generation proposed by Jayne and St. Laurent is widely used for parameterizing turbulent mixing induced by breaking of internal tides. Based on the internal-tide generation derived from a 0.1° ocean general circulation model, we show that depending on which stratification is used, this relation produces different vertical distributions of internal-tide generation. When using the buoyancy frequency at the seafloor, which is a common practice, the scaling relation produces, relative to the model, too-strong internal-tide generation in the upper 2000 m and too-weak internal-tide generation in the lower 2000 m. Moreover, the different vertical distributions in the different ocean basins, characterized by a generally decreasing internal tide generation with increasing depth in the Indo-Pacific but not-decreasing or even increasing internal tide generation with increasing depth in the upper 3000 m of the Atlantic, cannot be captured when using bottom stratification. These unsatisfactory features can be easily removed by replacing the buoyancy frequency at the seafloor by a buoyancy frequency averaged over a large part of the water column. To our knowledge, this sensitivity to stratification has not been explicitly quantified for the global ocean. Because of this sensitivity, the scaling relation of Jayne and St. Laurent should be used with an averaged stratification to ensure a more adequate representation of turbulent diffusivity due to tidal mixing and water mass transformation in the deep oceans.

Benthic macrofauna communities reflect eutrophic condition in a low-inflow estuary

Nutrient enrichment drives potentially severe changes in low-inflow and temporarily closed estuaries where tidal mixing and freshwater flushing is limited, and bottom-water anoxia occurs frequently. Our research assesses the influence of nutrient enrichment by comparing macrobenthos community metrics in a persistently hypereutrophic South African estuary. This is then compared to published literature for an estuary with a similar catchment size and mean annual runoff, but which experiences limited anthropogenic nutrient input. Macroinvertebrate abundance and richness were higher in an estuary with limited anthropogenic nutrient loading, and this varied seasonally for both estuaries. Oxygen saturation had a significant influence on taxonomic level abundances, with lower oxygen levels in the polluted estuary reducing macroinvertebrate abundances. Macroinvertebrates tolerant of organic pollution (e.g., Oligochaeta) were also more abundant in the polluted estuary. This study highlights the effects of increased nutrient loading in low-inflow estuaries and confirms the supposition from several previous studies that macrozoobenthos could be used as a responsive tool from a bioindicator perspective to monitor eutrophication.