Estuary Mouth Research Articles

Human activity intensity controls the relative importance of denitrification and anaerobic ammonium oxidation across subtropical estuaries

Human activities are dramatically increasing estuarine nitrogen (N) loads, further altering N processes. However, relative importance of denitrification and anaerobic ammonium oxidation (anammox) in response to human activities intensity gradient remains poorly understood for estuaries. In this study, we used a 15N isotope tracer approach to characterize the variations in sediment denitrification and anammox rates and determined the crucial factors controlling the partitioning of N2 production and regulating N2O production across five subtropical estuaries in southeast China. Denitrification rates increased significantly from 8.82 ± 3.89 nmol N g−1h−1 (low human activity intensity) to 41.2 ± 11.5 nmol N g−1h−1 (high human activity intensity) across the studied estuaries. Anammox rates not varied significantly between low (2.37 ± 0.66 nmol N g−1h−1), moderate (3.96 ± 0.91 nmol N g−1h−1), and high (4.04 ± 1.09 nmol N g−1h−1) human activity intensity estuaries. Relative contribution of anammox to total N2 production (Ra) decreased toward estuary mouth within each estuary. The Ra was also significantly lower in the estuaries characterized by high N loads and low DO. N2O production rates were in a range of 0.192–1.92 nmol N2O g−1h−1 across the estuaries and controlled by water NO2–, salinity and TOC. DO, NH4+, and NO3– were the best predictors of the partitioning of N2 production between denitrification and anammox. The high human activities intensity increased NH4+ and NO3– loads and further enhanced denitrification, leading to the decrease in Ra and increase in N2O production. These findings suggest that low DO and high N loads estuaries can increase denitrification and N2O emissions, whereas not affect anammox substantially under the higher intensity of human activities.

Nitrogen loads alter the N2 production between denitrification and anammox in Min River Estuary, a highly impacted estuary in southeast China.

Estuarine sediment denitrification and anammox in response to increased nitrogen (N) loads remain poorly understood. In this study, we used N isotope tracer approach to investigate the spatial distribution of denitrification and anammox and identified the crucial controls on the partitioning of dinitrogen gas (N2) production along the Min River Estuary (MRE), a highly impacted estuary in southeast China. The results indicated that denitrification and anammox rates ranged from 10.5 to 70.7nmolg-1 h-1 and from 0.44 to 4.31nmolg-1 h-1, respectively. Relative contribution of anammox to N2 production (Ra) was in a range of 1.04-15.1%, tending to increase toward estuary mouth. Denitrification rates were significantly higher in upper (high N loads) than in lower estuary (low N loads), while anammox rates and Ra showed inverse distributions along the MRE. Wastewater discharge caused the N point pollution triggering denitrification but inhibiting anammox. The best predictor of the variations in denitrification rates was total nitrogen, whereas pH and NH4+ could explained the variations in anammox rates across the estuary. The crucial predictors for the partitioning of N2 production between denitrification and anammox were NH4+ and NOx-. These results suggest that the increase in human activities intensity can alter the partitioning of N2 production between denitrification and anammox, and the magnitude of this switch can be predicted by N loads in MRE and other highly impacted estuaries.

Seascape heterogeneity modifies estuarine fish assemblages in mangrove forests

AbstractMangroves are a dominant structural habitat within tropical and subtropical estuaries that provide a number of ecosystem services, including habitat for a range of crustaceans and fish. However, mangroves are one of the most threatened estuarine habitats globally, having been severely reduced in extent, and replaced by urban structures. Here, we test for the effects of both natural (e.g. seagrass, rock and mangroves habitat extent, and connectivity) and human (e.g. extent of urban area) landscape variables on the number and type of fish inhabiting mangroves forests. We used remote underwater video stations to quantify fish assemblages within mangroves at 150 sites in 30 estuaries across Queensland, Australia. Fish community structure was best explained by the extent of mangroves and seagrasses within an estuary, the distance to the estuary mouth, and the size of the estuary and catchment. Moderate catchment size and proximity to the estuary mouth increased species richness and abundance of harvestable fish at individual mangrove sites. In order to maintain mangrove fish assemblages and the functions they provide, management initiatives should focus on maintaining natural estuarine seascapes that are located closer to the mouth of estuaries, in particular, focusing on estuaries that have lower levels of catchment urbanization.

Human settlement and landscape dynamics on the coastline south of the Gironde estuary (SW France): A multi-proxy approach

The Gironde estuary in SW France is the largest in Western Europe and has attracted human populations since prehistoric times. From the 1970s to the 1990s, intense archaeological research was undertaken on the long and highly dynamic coastline just south of the estuary mouth. In recent years, the combined action of increased coastal erosion and human pressure has proved a serious threat to the integrity of archaeological sites in the area. As a consequence, a whole array of previously unrecorded archaeological remains across the intertidal zone and coastal strip is being exposed. In this context, innovative interdisciplinary research since 2014 is yielding new information about the settlement and landscape dynamics and about the long-term interaction between human societies and the environment. The sedimentary context and the exceptional preservation conditions of organic remains have made possible a multi-proxy approach combining archaeological, geomorphological, palaeobiological, and archaeoentomological methods. In this paper we discuss the different approaches and the way they jointly contribute to the project. The results obtained so far from this multi-proxy approach challenge the traditional view of the historic occupation and the landscape dynamics around the Gironde estuary from prehistoric times to antiquity. They show that the intense occupation of this area during certain periods of human history is related to the development of marshy environments, which can now be analyzed at higher temporal resolution owing to this approach.

The impacts of runoff decrease and shoreline change on the salinity distribution in the wetlands of Liao River estuary, China

Abstract. The wetland of Liao River estuary (LRE) in northeastern China is one of the best-preserved wetlands across the globe. However, it is extremely vulnerable to hydrological changes as well as other disturbances, particularly upstream river discharges and the reclamation from anthropogenic activities. In this study, a 3D hydrodynamic model was used to reproduce the flow patterns of the LRE and to explore the variation in salinity under different scenarios. Furthermore, the impact of river discharge and shoreline changes on the salinity distribution in the LRE was quantitatively analyzed and discussed through several simulation experiments. The model reasonably reconstructed the spatiotemporal variability and distribution of salinity in the Liao River estuary and the wetlands across intertidal areas. Increases in runoff were demonstrated to significantly decrease the mean salinity values of the estuary, with changes in salinity negatively correlated to the longitudinal distance from the estuary mouth. Moreover, the shoreline change caused by the construction of Panjin Port was observed to have an obvious influence on the salinity distribution of the LRE, particularly in the lower reaches of the Pink Beach wetland. Comparisons of the Liao River estuary residual flow fields under different shorelines revealed that the establishment of the port resulted in the diffusion of the runoff from the Daliao River due to the obstruction of the port body, which enhanced the tidal mixing effect and consequently weakened the dilution effect of freshwater entering the Pink Beach wetland. Overall, the current study should be helpful for offering a greater understanding of Suaeda heteroptera vegetation degradation in the LRE, China, and also provides a new perspective in investigating the degradation mechanism in other estuarine wetlands.

Turbulence in forced stratified shear flows

Abstract

Estimation of tidal excursion length along the Shatt Al-Arab estuary, Southern Iraq

The tidal excursion length along the Shatt Al-Arab estuary was estimated based on the mathematical relation proposed by Parsa and Shahidi (2010). The field measurements of water level, bathymetry, and discharges were conducted to fulfill the objective of the study. The results revealed that the tidal excursion length is site-specific and depends on the characteristics of location which include tidal phases, bathymetry, and geometry. However, the results indicated that there are pronounced differences in tidal excursions lengths between the spring and neap tide phases in all studies stations. The spring tide coincided with the maximum tidal excursion lengths in Shatt Al-Arab estuary with 16.537, 16.187, 11.122, and 9.139 km in the estuary mouth, Faw, Siba, and Abo Flous stations respectively. While the neap tidal excursion lengths were 12.298, 9.254, and 7.269 km in Faw, Siba, and Abo Flous stations respectively.

Hydrodynamics of a hyper-tidal estuary influenced by the world's second largest tidal power station (Rance estuary, France)

The Rance estuary is a relatively small low-discharge steep-sided ria, located along the Brittany coast in northern France, with a maximum spring tidal range of 13.5 m. Taking advantage of this hyper-tidal regime, the first and currently the second largest operational tidal power station in the world was built at the estuary's mouth and has been in operation since the 1960s. Despite the well-known effect of damping of estuarine water levels, little attention has been given to quantifying the influence of the plant on the propagation and asymmetry of the tidal wave inside the estuary. In this study, hydrodynamics and tidal wave patterns were analyzed in this anthropogenically influenced estuarine system. A two-dimensional depth-averaged numerical model of the Rance estuary was developed. Two scenarios without the tidal power plant involving the dam's pre- and post-construction bathymetry (1957 and 2018 respectively) and present-day conditions scenarios were designed, to highlight the impact of bed evolution and the tidal power station on hydrodynamics and tidal asymmetry. Numerical results showed that, without the structure, bathymetric evolution did not substantially influence estuarine hydrodynamics. Nevertheless, on the estuary-side of the dam, the presence of the tidal power plant induced (i) a decrease in both tidal range and tidal prism, (ii) an increase of low water levels, and (iii) a decrease in both flood and ebb currents. Contrastingly, the region close to the structure reacted differently to plant operating modes, with an increase in flood currents (ebb currents) upstream of the sluice gates (downstream of the turbines). For both the natural condition and the artificially-induced hydrodynamic forcing due to the presence of the plant, numerical results showed that the Rance estuary mainly exhibits flood-dominant behavior, with a longer duration of falling than rising water and stronger peak flood currents than ebb currents. Spanning a period of approximately 60 years, this study presents a quantitative analysis of the influence of the tidal power station on the hydrodynamics in the Rance estuary, and its possible consequences for sediment dynamics. This approach is novel for this particular enclosed water body, characterized by the presence of a dam at its mouth and a lock at its uppermost limit.

Influence of River Runoff and Change in Salinity on the Composition of Coastal Fouling Community of the Northwestern Part of the Black Sea

The paper deals with the study of the influence of the transformed river waters incoming into the northwestern part of the Black Sea from the Dnieper-Bug estuary on the composition and quantitative parameters of coastal fouling community located on the path of their distribution. It has been found that the species richness of the community and relative biomass of its species, with the exception of Bivalvia, increased with distance from the mouth of the Dnieper-Bug estuary toward Odesa Bay. The taxonomic structure of the community changed along salinity gradient. A degree of community stability was assessed at different sections of the studied area of water using the index approach and the ABC-method. It has been shown that the intensity of stress influence of the transformed river waters on the coastal fouling community decreased with distance from the mouth of the Dnieper-Bug estuary.

Structural and morphological characteristics of the bottom settlements of mussels in the coastal ecosystems in the river runoff influence zone of the Ukranian shelf of Black Sea

The formation of the density, biomass, size-mass and phenotypic structure of the bottom populations of mussels Mytilus galloprovincialis (Lamarck, 1819) in three areas of the Ukrainian shelf of North-Western part of the Black Sea – near Adjiyask Cape (the water area near the Dnieper-South Bug region), near mouth of the Grigorievsky estuary and on the Odessa coast, with different gradients of salinity of sea waters due to influence of the transformation of the river runoff, were studied. The samples of mollusks were collected from surface of the seabed soil at a depth of 3.0 meters in the spring (March–April), summer (June–July) and autumn (October–November) 2016–2019. In addition to the number, biomass, size and phenotypic structure, the ratios of the individual components of the mollusk mass (wet and dry mass of the animal’s soft body and mass of its valves) were also studied. It is noted that the greatest salinity stratification of the water column, caused by the influence of fresh river runoff, was observed near Adjiyask Cape, where the salinity of the surface water layer (depth 0.5 m) in spring and early summer did not exceed 8.4±2.4 and 11.0±2.5 ppm, respectively. Nevertheless, bottom settlements of mussels in this and in two other seawater areas were relatively in favorable conditions, as the salinity at the depth of its habitat (depth 3.0 m) remained within 12–15 ppm, which corresponds to the “river–sea” ecotone. Based on the data obtained, it is concluded that such halo-tolerant mollusks as the Black Sea mussels in coastal areas affected by fresh river runoff can only slightly differ in their number and biomass indices, as well as in the phenotypic structure of settlements. As a rule, differences may be related exclusively to the individual morphological characteristics of animals, especially for its soft body weight (wet and dry mass) and mass of its valves.

Spatio-temporal variation in length-frequency and salinity tolerance of dominant fishes utilizing the Orange River-Estuary Continuum

The length-frequency distribution of the dominant species of euryhaline marine Chelon richardsonii, estuarine Gilchristella aestuaria and freshwater species Labeobarbus aeneus, Mesobola brevianalis and Pseudocrenilabrus philander were investigated seasonally along the Orange River Estuary Continuum (OREC) during high-flow and low-flow periods, conducted in eight different years, within a 15-year period (2004-2018). Fish were sampled using a seine net at 18 sites spanning from the mouth of the estuary up to 35 km upstream. The length-frequency distributions of all species did not differ between the high-flow and low-flow season. Spatial differentiation in size frequency distribution of all freshwater taxon was recorded along the river-estuarine continuum but not for C. richardsonii and G. aestuaria. Total catch-per-unit-effort (catch per haul, CPUE) was low for both M. brevianalis and P. philander and their abundance declined downstream into the estuarine region as expected for these two freshwater species. A negative correlation between salinity and fish length of all five species was found, demonstrating that juvenile fishes, in particular, freshwater species are less salt-tolerant than the adults. It is concluded that the lower OREC provides an important fish nursery habitat suitable for growth and development, particularly for marine C. richardsonii and estuarine G. aestuaria.

Nutrients Load Estimation in a Regulated Streamflow Estuary: The São Francisco Estuary (NE/Brazil)

Abstract This study aimed to estimate the nutrients concentration variability in the São Francisco estuary from observed data during the summer and winter seasons of 2014 and to assess the influence of outflow discharges, circulation and precipitation on nutrients fluxes exchange. The marginal difference in the streamflow (1,160 m3/s - summer; 1,260 m3/s - winter) reflected the small role of the river discharge on the variability of the nutrients load. The increase in the sediments load from February (13189.70 T/month) to July (36088.56 T/month) revealed that high precipitation (153.6 mm - winter; 37.6 mm - summer) and internal circulation might have contributed to enhancing the sediment budge towards the estuary mouth. The strong current velocity in July (~0.9 m/s) revealed more potential to export estuarine waters towards the coast when compared to ~0.7 m/s (February). The concentrations and nutrients load were higher during the winter season led by phosphate (3.70 μgL-1) and NID (83.64 μg L-1), against (1.38 μgL-1 and 30.70 μgL-1 - summer), except for silicates with 4.20 mgL-1 (summer) and 3.59 mgL-1 (winter). Despite the active control of outflows, the internal circulation, followed by local precipitation, are considered the main mechanisms behind the increased nutrients load within the estuary.

Impact of Shelf Valleys on the Spread of Surface-Trapped River Plumes

AbstractThis study focuses on mechanisms of shelf valley bathymetry affecting the spread of riverine freshwater in the nearshore region. In the context of Changjiang River, a numerical model is used with different no-tide idealized configurations to simulate development of unforced river plumes over a sloping bottom, with and without a shelf valley off the estuary mouth. All simulated freshwater plumes are surface-trapped with continuously growing bulges near the estuary mouth and narrow coastal currents downstream. The simulations indicate that a shelf valley tends to compress the bulge along the direction of the valley long axis and modify the incident angle of the bulge flow impinging toward the coast, which then affects the strength of the coastal current. The bulge compression results from geostrophic adjustment and isobath-following tendency of the depth-averaged flow in the bulge region. Generally, the resulting change in the direction of the bulge impinging flow enhances down-shelf momentum advection and freshwater delivery into the coastal current. Sensitivity simulations with altered river discharges Q, Coriolis parameter, shelf bottom slope, valley geometry, and ambient stratification show that enhancement of down-shelf freshwater transport in the coastal current, ΔQc, increases with increasing valley depth within the bulge region and decreasing slope Burger number of the ambient shelf. Assuming potential vorticity conservation, a scaling formula of ΔQc/Q is developed, and it agrees well with results of the sensitivity simulations. Mechanisms of valley influences on unforced river plumes revealed here will help future studies of topographic influence on river plumes under more realistic conditions.

The Río de la Plata plume dynamics over the Southwestern Atlantic Continental Shelf and its link with the large scale atmospheric variability on interannual timescales

The interannual variability of the Río de la Plata plume over the Southwestern Atlantic Continental Shelf is analyzed based on the global ocean reanalysis ORAP5.0 and the global atmospheric reanalysis Era-Interim for the 35-years period 1979–2013. A regional validation of the dataset is performed to evaluate the ocean reanalysis ability to solve the main features associated with the plume dynamics. Following the satisfactory evaluation, an Empirical Orthogonal Function analysis is performed to extract characteristic modes of variability of the sea surface salinity. Three leading modes explain 35%, 30% and 15% of the total variance and are associated with large scale atmospheric variability patterns characteristic of the Southern Hemisphere circulation. Mode 1 is characterized by marked salinity anomalies to the south and north of the RdlP Estuary mouth. Its particular imprint is associated with an organized alongshelf wind structure modulated by the Southern Annular Mode (SAM). Mode 2 presents salinity anomalies that extend from the RdlP Estuary to the Uruguayan and southern Brazilian coasts and is related to interannual variability of the RdlP discharge and to regional wind forcing over the plume. Both the river outflow and the atmospheric forcing over the shelf are strongly associated with the Pacific South American pattern (PSA1). Finally, mode 3 presents large salinity anomalies at 38°S, south of the mouth of the RdlP Estuary and is associated with displacements of the freshwater plume due to local wind stress anomalies over the estuary, which are modulated by the PSA2.

Microplastic Depuration on Asaphis Detlorata

Depuration is an effort to reduce/eliminate contamination including microplastics, which one is using a water circulation system. This study aims to determine the effective depuration time to reduce the microplastic content in Asaphis detlorata This study used a quantitative approach with experimental research design with a completely randomized design. There are 450 shells used as an experimental animal where is the treatment consisted of four depuration times, namely 1;2;3; and 4 days with 3 repetitions of each treatment, while the control shells were without depuration. Analysis of variance (ANOVA) was used to see the effect of depuration treatment on the microplastic content. If the effect of the treatment was significantly different, then the post hoc test was continued to determine the differences between treatments. The results showed that Asaphis detlorata obtained from the mouth of the Lakatong river estuary were contaminated with microplastics ranging from 0.6 to 8.1 MPs/shellfish and an average of 3.96 MPs/shellfish. Depuration time significantly affected the microplastic content in shellfish depuration effectiveness. There is a tendency that the longer depuration time is decreased microplastic content in shellfish. The effective depuration time to reduce the microplastic content in Asaphis detlorata was 3 and 4 days. Further research is needed for a more effective depuration for cleaning microplastics in shellfish.

The Mouths of Estuaries Are Key Transition Zones that Concentrate the Ecological Effects of Predators

Whether and how landscape context and habitat traits combine to shape animal assemblages and the rate and distribution of ecological functions remains unresolved in many aquatic settings. Saltmarshes are one such ecosystem in which these considerations are frequently acknowledged as important, but quantitative studies of these effects are rare, especially for ecological functions. In this study, the influence of landscape configuration and habitat traits on the composition of fish assemblages and rates of predation were quantified around 30 saltmarshes in three estuaries (i.e., 10 per estuary) in eastern Australia. Fish assemblages were surveyed using unbaited underwater video cameras, and predation was quantified using videoed “Squidpop” predation assays at 10 sites at each saltmarsh. The structure of fish assemblages was best explained by the estuary in which saltmarsh was located, the proximity of sites to estuary mouth, and the area of nearby saltmarsh and mangroves. Predation was dominated (90% of total predation events) by yellowfin bream Acanthopagrus australis (Sparidae), and so rates of predation correlated positively with yellowfin bream abundance. Predation peaked in the lower reaches of estuaries at saltmarshes with lower vegetation cover. These findings suggest that the mouths of estuaries might function as key transition zones that concentrate prey, the products of trophic relay, and the ecological effects of predators near the estuarine-sea interface.

Multiple Estuarine Gradients Influencing Tidal Flat Benthic Algal Biomass and Community Structure in the Yaquina Estuary, OR, USA

Tidal flat is often the dominant habitat type in US Pacific Northwest (PNW) estuaries. This research examined environmental factors that explain the spatial patterns of microphytobenthos (MPB) biomass and community structure in a PNW estuary with complex nutrient dynamics. MPB biomass, individual species abundances, diatom community metrics, and diatom community structure were strongly correlated with distance from the estuary mouth, salinity, substrate composition, and pore water-soluble reactive phosphorus (SRP). The microphytobenthos was dominated by diatoms and diatom diversity and species richness were negatively associated with salinity, substrate sand content, and pore water SRP. Diatom community structure varied with estuary position, with the epipsammic taxa Catenula adhaerens, Opephora spp. 1, and Planothidium delicatulum dominating the assemblage at sites near the estuary mouth and epipelic taxa, such as Nitzschia frustulum and Nitzschia palea, being more abundant at sites in the middle and upper estuary. Distance from the estuary mouth and salinity were the most important predictors of diatom assemblage structure, based on CCA analysis. MPB biomass was highest at sites in the lower estuary, characterized by higher salinities, SRP, and substrate sand composition. Pore water nitrogen and surrounding land use were not important predictors of MPB biomass or community structure. We attribute our findings to the nutrient dynamics of Yaquina estuary, which is high in nutrients due to both coastal upwelling and watershed-derived nitrate and exhibits an increasing gradient of phosphorus towards the estuary mouth.

Influence of Seasonal River Discharge on Tidal Propagation in the Ganges‐Brahmaputra‐Meghna Delta, Bangladesh

AbstractSignificant research efforts have been devoted to understanding river‐tide interactions in estuaries. However, studies on the impact of monsoon‐driven fluctuations of river discharge are limited. Here, the role of varying river discharge on the tidal propagation and tidal limit along the Ganges‐Brahmaputra‐Meghna Delta (GBMD), a macrotidal estuary subject to seasonal and annual river discharge variations, is investigated. The Delft3D hydrodynamic model is validated and applied to an average flood year condition and nine idealized scenarios covering the typical hydrological conditions. Results reveal that the upper limit of the tidal propagation shifts 75 km upstream during the dry season. The residual water level slope and tidal damping rate increase with river discharge beyond 100 km from the estuary mouth. The balance between the generation and dissipation of quarterdiurnal tides shifts spatially as a result of changes in channel convergence and friction and temporally as a function of river discharge, which controls the total friction in the upper tidal river. The balance between tidal dissipation and generation depends on the residual velocity generated by river discharge and the velocity of the principal tides. The maximal generation of quarterdiurnal tides in the upper GBMD depends on the friction generated from the river‐tide interaction. Critical river discharge thresholds produce an optimal condition of dissipation of semidiurnal tides and generation of quarterdiurnal tides through friction at the upper and middle estuary. River discharge above the critical river discharge amount more rapidly dissipates both semidiurnal and quarterdiurnal tides than generates quarterdiurnal tides from nonlinear interactions.

Rhizosediments of Salicornia tegetaria Indicate Metal Contamination in the Intertidal Estuary Zone

Metal pollution is a well-known anthropogenic impact of highly developed estuaries, with dire consequences to the ecosystem. This study investigated the metal concentrations (Al, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in the sediment colonised by Salicornia tegetaria, a dominant salt marsh plant in the lower intertidal zone of the Swartkops Estuary. The samples were collected at five sites along the banks of the middle and lower reaches of the estuary, and analysed using an Inductively Coupled Plasma - Optical Emission Spectrometer. Metal contamination was determined using established normalised baseline models. It was found that all the sites contained metal enrichment, with the estuary mouth experiencing the least enrichment. Salicornia tegetaria holds a substantial amount of anomalous metals within its rhizosediment — providing a valuable ecosystem service to a highly developed Swartkops Estuary.

Diel and tidal periodicity in the responses of early life stages of marine fishes to an estuary opening event in temperate South Africa

Early life stages of fish were monitored in a breach channel during an opening event of the temporarily open/closed East Kleinemonde Estuary. For comparison, early life stages were also collected in the surf zone and estuary mouth region before, during and after the breaching event on a weekly basis. The breach channel was sampled day and night at each midtide state for five consecutive days using a larval fish seine net (mesh 500 µm). Catch per unit effort (CPUE) of mostly postflexion stage larvae increased in the surf zone during opening events with a concomitant peak in abundance in the breach channel. Catch composition included many estuarine associated species typically found in temporarily open/closed estuaries. Predominantly postflexion stage larvae and newly settled juveniles were caught. No overall patterns of diel and tidal use across species were found in the breach channel suggesting some degree of plasticity in recruitment that could be opportunistically driven for most species. However, for the abundant Rhabdosargus holubi (Sparidae), nocturnal flood tides were favoured for movement into the estuary. Natural estuary opening events, especially during peak rainfall in the recruitment season are critical to the early life history strategy of estuary-associated marine fish species.