Estuary Mouth Research Articles

River plume source-front connectivity

River plume source-front connectivity, or the relationship between processes at the source, plume body and front, is investigated with numerical simulations of the Merrimack River plume using the Regional Ocean modeling System (ROMS). Source water, i.e. water discharged from the estuary during an ebb pulse, overtakes the front as it propagates offshore, resulting in the strong convergence that exists there. Model results show that much of the water released over an ebb pulse does not interact with the propagating tidal plume front, suggesting that the front decouples from the source and sustains itself as a distinct water mass. When a gate is closed at the estuary mouth during early ebb, the overall plume structure changes; gates closed late in ebb do not significantly effect the along- or cross-shore scale of the plume. The change in plume extent is only evident hours after the early-ebb gate closing, when source water is no longer supplied to the front. These experiments suggest that the growing distance between the highly energetic liftoff region and the radial expansion of the plume front results in a flattening of the surface gradient driving flow in the interior, diminishing source-front connectivity.

THE CHARACTERISTICS OF SEDIMENTATION PATTERN AND MORPHOLOGICAL CHANGE OF SIGELENG RIVER ESTUARY IN RANDUSANGA KULON VILLAGE BREBES DISTRICT

Sigeleng River Estuary is located in Brebes Regency. The mouth of Sigeleng River Estuary suffering morphological change that caused by the sand spit that close the mouth of river estuary and creating deflection.
 The results of characteristics test of the sediment from the estuary of Sigeleng River shows that the average density is 2.228 kg/l and the average particle diameter (D60) is 0.466 mm. On the other hand the largest result of total suspended solid test of the sediment samples is 1.58 mg/l. According to USDA, the soil texture in B location is Silt Loam and in A and C location are Sandy Loam. SED2D modeling that performed for 720 hours that assumed as 1 year shows a base elevation change that is 11,79%-30,20% in the river estuary mouth at the existing condition 
 Sediment cumulation that occurred around the mouth of the Sigeleng River Estuary within 5 years is 0.007 m - 2,560 m, while the depth of the Sigeleng River Estuary is only -3.00 m. Therefore, a treatment by dredging the sediment aroud the mouth of Sigeleng River Estuary is needed so that the river estuary is not closed.

Abiotic and biotic responses to the 2016/2017 restoration project at the St Lucia Estuary mouth, South Africa

The St Lucia Estuary is the largest estuarine lake in Africa and has been heavily impacted by anthropogenic activities, which include a history of human-induced sediment loading and separation in the 1950s of the originally common St Lucia Estuary-uMfolozi River mouth. Over the years a number of projects have been implemented aimed at countering the adverse effects that resulted from isolation of the St Lucia mouth. The most recent (2016/2017) project is the Global Environment Facility (GEF) funded initiative to restore the common St Lucia-uMfolozi River mouth. During, but especially after completion of the rehabilitation project, distinct changes in the sediment grain size composition of the estuarine substratum were observed, resulting primarily from deposition of uMfolozi River sediment in the area. Before commencement of the project (i.e. prior to June 2016) coarse and medium sediment (<1 mm >500 µm and <500 µm >250 µm, respectively) dominated. After project completion (July 2017 onwards) the substrate became muddy as a shift from coarse and medium grains to silt (<63 µm) occurred. Preliminary analyses show a corresponding shift in zooplankton and macrofauna composition. Impacts on other animals, including hippopotami and crocodiles, as well as on various flora have also been observed.

Organic pollutants in sedimentary microplastics from eastern Guangdong: Spatial distribution and source identification.

Microplastics have a strong affinity for potentially toxic organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs). Since 2005, the International Pellet Watch used plastic pellets to monitor hydrophobic organic contaminants in marine environments. We examined the spatial distribution and sources of 16 PAHs and eight OCPs on microplastics (pellets, fragments, and foam) collected from eastern Guangdong beaches with the goal of evaluating the feasibility of exclusively using pellets in global monitoring of hydrophobic organic contaminants. The ∑PAH and ∑OCP concentrations ranged from 11.2 to 7710ngg-1 and 2.2-1970ngg-1, respectively. Although inter-site and regional differences were insignificant in fragments and foam, regional differences were observed in ∑OCP concentrations on pellets samples collected at the estuary mouth and the distributary (p<0.05). No regional difference in ∑PAH concentration on microplastics was observed, except between fragments from the remote beach and those from the distributary (p=0.015) and the estuary mouth (p=0.015). The compositional profiles of PAHs revealed that PAHs with 2-4 rings were more dominant than those with >4 rings. Considering low molecular weight PAHs are less hydrophobic and more toxic; the results suggest low molecular weight PAHs may pose a significant risk to marine organisms. This study shows that the International Pellet Watch could offer critical data on source and transport of microplastic-associated OCPs but may overlook critical vehicles of PAHs in coastal environments such as foam.

Hurricanes fertilize mangrove forests in the Gulf of Mexico (Florida Everglades, USA)

Hurricanes are recurring high-energy disturbances in coastal regions that change community structure and function of mangrove wetlands. However, most of the studies assessing hurricane impacts on mangroves have focused on negative effects without considering the positive influence of hurricane-induced sediment deposition and associated nutrient fertilization on mangrove productivity and resilience. Here, we quantified how Hurricane Irma influenced soil nutrient pools, vertical accretion, and plant phosphorus (P) uptake after its passage across the Florida Coastal Everglades in September 2017. Vertical accretion from Irma's deposits was 6.7 to 14.4 times greater than the long-term (100 y) annual accretion rate (0.27 ± 0.04 cm y-1). Storm deposits extended up to 10-km inland from the Gulf of Mexico. Total P (TP) inputs were highest at the mouth of estuaries, with P concentration double that of underlying surface (top 10 cm) soils (0.19 ± 0.02 mg cm-3). This P deposition contributed 49 to 98% to the soil nutrient pool. As a result, all mangrove species showed a significant increase in litter foliar TP and soil porewater inorganic P concentrations in early 2018, 3 mo after Irma's impact, thus underscoring the interspecies differences in nutrient uptake. Mean TP loading rates were five times greater in southwestern (94 ± 13 kg ha-1 d-1) mangrove-dominated estuaries compared to the southeastern region, highlighting the positive role of hurricanes as a natural fertilization mechanism influencing forest productivity. P-rich, mineral sediments deposited by hurricanes create legacies that facilitate rapid forest recovery, stimulation of peat soil development, and resilience to sea-level rise.

The calculation of estuarine flushing times in convergent estuaries using fresh-water fraction method

ABSTRACT The flushing time in an estuary is important for water quality analysis, and it is one of the major transport time scales used in estuaries to quantify the hydrodynamic processes and for water resources management strategies. The purpose of this paper is to investigate the influence of tidal amplitude, river discharge, and salinity stratification on the spatially varying flushing time of the two selected Moroccan Atlantic semi-closed estuaries, i.e. Bouregreg and Loukkos. A freshwater fraction method (FFM) was used to calculate flushing time for both estuaries using a spatially varying freshwater fraction. The results showed that the flushing time varies from 2.7 to 6.3 days for a river discharge ranges from 17 to 22 m3s−1 during spring tide, and from 3.4 to 11.2 days for a river discharge ranges from 17 to 31 m3s−1 during neap tide in Bouregreg estuary, and varies from 10 to 13.6 days for a river discharge ranges from 3.47 to 5.12 m3s−1 during spring tide, and from 9.11 to 13.1 days for a river discharge ranges from 3.47 to 5.48 m3s−1 at neap tide in Loukkos estuary. In addition, the flushing time near the mouth for both estuaries is shorter during spring tide than during neap tide because of the larger amplitude of the spring tide. At the estuary mouth (e.g. Bouregreg estuary), the potential energy anomaly of the water column is negligible (<4 J m−3).

A box model to represent estuarine dynamics in mesoscale resolution ocean models

Representing the net freshwater flux at river mouths is challenging for global and regional scale ocean modelling. Although rivers are well known to affect both the coastal and basin-wide circulation and dynamics, coarse resolution ocean models cannot resolve the estuarine dynamics and are usually forced at river outlets in a simplistic way, with climatological runoff and zero or constant salinity values. The aim of this study is to provide a more realistic representation of the estuarine water inputs to a coarse but eddy-resolving regional model.First, the river volume transport and salinity values at the outlets are modelled with three different Estuary Box Models (EBMs) for stratified estuaries: the Knudsen relations model, a published EBM, called UCONN-NCAR EBM, which parameterizes the tidal inflow and mixing inside the estuary, and a new model, called CMCC-EBM. The CMCC EBM has been conceived to represent the estuarine processes coupled to a mesoscale resolving hydrodynamic model that resolves the entering flow field at the estuary mouth and it offers a new representation of the tidal inflow and a new salinity tidal mixing parameterization via horizontal diffusive processes.The Ofanto and Po rivers flowing into the Adriatic Sea (northern part of the central Mediterranean Sea) are selected as case studies. The coupling of the eddy resolving ocean model to the CMCC EBM is found to outperform the one with the UCONN-NCAR EBM in the region of freshwater influence on the shelf areas.

Fate and spatial distribution of Pb, Cd, Cu and Zn in the water column and in the surface sediment of Indonesian Estuary (Citarum River Estuary)

The Citarum River Estuary is an area where mixing among seawater and freshwater occurs since directly connected with the Java Sea. The different condition in estuary will affect heavy metal concentrations in the dissolved phase and suspended/deposited in sediment phase. In this study, fates and distributions from Pb, Cd, Cu and Zn were analyzed along 2 km from estuary mouth. Samples were analyzed by Flame AAS. The study began with water/sediment quality assessments, and fate and distribution analyses with Adsorption Capacity Indices (ACI), Dissolved Transport Indice (DTI), Koc, Kow, spatial distribution in water (3D) and sediment (2D) using MATLAB, and statistical analysis (ANOVA/Kruskal-Wallis). In conclusion, heavy metals sequence from the highest pollution level in water were: Zn &gt; Pb &gt; Cu &gt; Cd, and in sediment were: Cd &gt; Pb &gt; Zn &gt; Cu. Pb, Cd, Cu, and Zn had a tendency to be in sediment compared to water (ACI = 99.69% – 99.89%; DTI = 0.109% – 0.309%), slightly mobile (log Koc= 3.11 – 3.58), and hydrophobic (log Kow= 3.35 – 3.87). Pb and Cd showed significantly different concentrations in water columns (surface/midst/base) (p&lt;0.05), in contrast to Cu and Zn. Pb, Cd, Cu, and Zn had insignificant concentration difference based on zone division from estuary mouth, both in water and sediment.

Freshwater intrusion during ebb and flood tide in the Balikpapan Bay

The Balikpapan Bay waters is influenced by freshwater discharge from many rivers and its distribution is affected by tidal current. This study aims to estimate seawater intrusion during flood and ebb tides, using the box model and freshwater fraction methods. The data used in this study for calculating freshwater fraction and its transport volume were 45 conductivity- temperature-depth CTD casts and single moored ADCP current measurement, carried out between 17 and 25 May 2018. The budget of the box model system is calculated using evaporation and precipitation data. The results show that estimated freshwater fraction and transport volume during flood tide vary between 16.92 and 36.26%, and between 2.048 and 9.661x109 m3day−1, respectively. Near the entrance of the bay, seawater flux urges freshwater westward with surface salinity budget of -95.352x106 psu.m3day−1. Furthermore, during ebb tide, freshwater fraction and transport volume are estimated to be 19.33 – 34.5% and 2.048–9.661x109 m3day−1, respectively. Near the mouth of estuary, freshwater pushes seawater toward east side of the bay with salinity budget of -186.187x106 psu.m3day−1. During flood tide (ebb tide) estimated surface salinity budget in transect 5 that came out from the system is about -135.512x106 psu.m3day−1 (-143.794x106 psu.m3day−1) toward the southern bay.

High-resolution bathymetry estimates via X-band marine radar: 2. Effects of currents at tidal inlets

In this second of two papers, the high spatial resolution cBathy depth estimate algorithm is extended to account for the existence of currents aligned with the wave directions. The extended algorithm is applied using X-band radar image time series collected at two hydrodynamically and morphologically complex environments: an ebb-tidal shoal (New River Inlet, NC) and an estuarine river mouth (Columbia River, OR/WA). Even without accounting for Doppler effects, the depth estimates are capable of discerning the location of channels and shoals over domain sizes of 4km2 at the ebb-tidal shoal and 25km2 at the estuary mouth. However, the tidal currents at these sites are shown to strongly modulate depth estimate accuracy when only the intrinsic linear wave dispersion relation (neglecting Doppler shift effects) is inverted. Low-pass filtering of the depth estimate time series, via an uncertainty-weighted tidal average, explicitly attenuates the error oscillations beyond that of the uncertainty-weighted Kalman filter. This post-hoc method of compensating for the unmodeled Doppler term achieves 0.35m RMSE and 0.02m shallow bias at the shallow ebb tidal shoal as compared to a concurrent sounding survey, but substantial bias remains at the estuary mouth due to asymmetric Doppler effects for large following or opposing currents at large relative water depths. Simultaneous estimation of depth and currents via inversion of the dispersion relation including Doppler shifts is more effective at the deep estuary mouth, where currents more strongly affect the incident wave field. In the estuary channel with locations as deep as 27m, Kalman-filtered depths achieve 0.92m RMSE and 0.38m deep bias as compared to a concurrent sounding survey (95% of estimates are within 10% of those surveyed). Independent of the Doppler compensation technique, Kalman filtered depths can be used to more tightly constrain high-resolution current velocity estimates.

Bathymetric flow rectification in a tropical micro-tidal estuary

Temperature, salinity and current velocity data were recorded during the two principal atmospheric seasons affecting the tropical micro-tidal estuaries in the western Gulf of Mexico. Tropical micro-tidal estuaries in the western Gulf of Mexico share many characteristics, among them a narrow connection between the estuary and the adjacent continental shelf. Therefore, samples were taken during the dry (April 2009) and the rainy (July 2009) seasons, in order to elucidate the effect of the narrow connection between the estuaries and the continental shelf on flow dynamics. Due to the similarities between the tropical micro-tidal estuaries located in the western Gulf of Mexico, the Jamapa River estuary, a shallow estuary, with a narrow connection with the continental shelf and a ~5 m wide navigational channel, was used as a case study. During the dry season (April 15–17th, 2009) there was a surface horizontal displacement of the salinity and the temperature front of ~2 km, while during the rainy season (July 22–24th, 2009) the salinity and temperature gradients were mainly observed in the vertical at ~1 m depth. In this particular case, there was a marked difference between the northern and southern part of the estuary, due to the presence of a second river discharge (Arroyo Moreno), which discharges always stays in the northern part of the estuary (shallow area) owing to the influence of the navigational channel on fluid dynamics. Finally, a cyclonic recirculation was observed at the estuary mouth area. According to model outputs, the recirculation was observed when the ratio between the mouth and the estuary width were below 0.4, otherwise (>0.4) the recirculation was not observed. This should be a general behavior for all tropical micro-tidal estuaries located in the western Gulf of Mexico.

Elevated dissolved heavy metal discharge following rainfall downstream of intensive horticulture

Heavy metals may be released from soils during rain events. Observations during the first flush (rain event preceded by dry period) are difficult, but may account for a large fraction of annual loads. Here, we report detailed time series measurements of dissolved heavy metals and the groundwater tracer radon (222Rn) over multiple rain events from a creek draining a catchment dominated by intensive horticulture on the subtropical east coast of Australia. The creek drains to an intermittently closed and open lake or lagoon (ICOLL), a typical estuary type along the east coast of Australia. A sandbank prevents the estuary mouth from connecting with the ocean during dry conditions. Following a 109 mm rain event, the ICOLL began to drain to the coastal ocean. Mean export of Hg, Cu, and Zn was 0.23 ± 0.05, 1.06 ± 0.25, and 15.70 ± 2.69 g m−2 day−1 for the entire time series and 0.72 ± 0.17, 1.27 ± 0.12, 24.14 ± 3.82 g m−2 day−1 averaged from high resolution sampling over 72 h of the first flush event. Trends of Hg export differed from the other heavy metals. Over 79% of the estimated Hg export occurred within 72 h of the ICOLL opening, compared to 30 and 38% of Cu and Zn. The first flush and subsequent major rain events (> 50 mm day-1) drove concentrations of Hg, Cu and Zn to exceed the Australia and New Zealand Environment and Conservation Council (ANZECC) water quality guidelines (WQG) for both fresh and marine water. Comparisons of heavy metal export to downstream estuary sediment burial rates revealed that the estuary is likely a source of dissolved Hg and Cd to the coastal ocean when the ICOLL is open.

Physico-chemical and microalgal gradients change rapidly in response to mouth closure in a predominantly open estuary

A third of South Africa’s freshwater flow no longer reaches the coast, yet few systems have shifted from being permanently open to temporarily closed. For the first time since 1969, the Gamtoos Estuary mouth closed in June 2018, because of extensive drought conditions and freshwater abstraction. Accordingly, physico-chemical gradients, microalgal biomass and species composition were sampled during low (closed mouth), normal (open mouth) and high (breaching event) flow conditions. The closed phase was characterised by polyhaline and homogenous salinity structures and a rapid shift towards eutrophic conditions, as evidenced by bottom water hypoxia (<2 mg l−1), elevated phytoplankton (>20 μg l−1) and microphytobenthos (>100 mg m−2) biomass, and proliferation of a harmful algal bloom (HAB) species (Prorocentrum cordatum, Dinophyceae). An artificial breaching event in September 2018 restored connectivity with the marine environment and within a month re-established the usual physico-chemical gradients that characterise the open phase. Although the current study demonstrates ecosystem recovery from stochastic events, it also highlights the potential impact (e.g. HABs, hypoxia) of shifting an estuary into a new stable state. This is a concern given that mouth closure events in South African estuaries are likely to increase in the future in response to global change pressures.

Estuarine salt-plug induced by freshwater pulses from the inner shelf

Estuarine salt plugs can cause water quality problems and are usually attributed to hypersaline conditions in low-inflow estuaries, where evaporation rates exceed or match freshwater input rates. However, salt plugs may also develop in typical estuaries, in brackish waters, by an additional freshwater influence from the estuary's mouth. In this study, month-long and years-long data showed the formation of a salt plug by a freshwater pulse from the ocean into Barataria Bay, in the Mississippi River Delta. Month-long currents showed an inverse estuarine circulation at Barataria Pass with inflow in the surface and outflow underneath. A dimensionless analysis showed that the dynamics governing the inverse circulation was related to a density gradient reversal, which was related to a salt-plug. Empirical Orthogonal Functions applied to nine years of salinity values along the estuary showed that Mode 1 (EOF1) represented the salt-plug structure and explained 86% of the variability. The salt-plug structure was most pronounced during winter, under minimum river discharge, and least evident in summer. Wavelet coherence analysis showed an annual relationship between EOF1 weights and both, river discharge and along-channel wind-stress. This study shows evidence of a salt-plug development by buoyant water pulses from outside an estuary.

Tidally resolved observations of organic carbon exchange through Eastern Long Island Sound

Long Island Sound (LIS), is an urban estuary on the US east coast that undergoes seasonal hypoxia in its western and central regions. Approximately 90% of the water exchange between LIS and the adjacent continental shelf, the Mid Atlantic Bight, occurs through its eastern boundary. Recent estimates of organic carbon (OC) export from LIS have shown that this value varies appreciably, both seasonally and inter-annually. In this study, the spatial, tidal, and seasonal variability were resolved by measuring dissolved and particulate organic carbon (DOC and POC) concentrations and currents at a cross-section in eastern LIS near the estuary mouth. Fluxes were extrapolated from these high-resolution observations collected in May, August, and November 2016, and August 2017. In August of 2016 (a low flow year), an OC flux of 41 × 106 kg C month−1 resulted from a net import of DOC and an export of POC. In August of 2017 (an average flow year), there was an OC flux of 30 × 106 kg C month −1 due to both DOC and POC export, indicating significant seasonal and inter-annual variability. The seasonally weighted-average total OC flux for 2016 was −41 × 106 ± 9 kg C yr−1 (net import). Results show that spot sampling in tidal estuaries can lead to spatial and tidal bias in concentrations of DOC (6–15%) and POC (31–18%) under high and low river flow conditions, respectively. This uncertainty is recommended for spot sampling in future OC studies and flux estimates in LIS.

Identifying restoration hotspots that deliver multiple ecological benefits

Ecological restoration principally seeks to restore lost or degraded ecosystems. Restoration can, however, also deliver a suite of wider ecological, social, and economic benefits. To optimize performance it is, therefore, important to plan the design and placement of restoration initiatives with a view to maximizing joint effects on ecosystems, animal populations, ecological functions, and ecosystem services. We measured the effects of multiple (13) restored oyster reefs on a suite of restoration benefits (oyster settlement and growth, fish diversity and abundance, the ecological functions of scavenging and predation) in the Noosa River estuary, Australia, and used distribution models to identify potential restoration sites with the greatest overall benefits. Oysters recruited to reefs, and reefs enhanced the diversity and abundance of fishes and had higher rates of ecological functions than control sites. However, the growth of oyster reefs was most correlated with the proximity of restoration sites to urbanized shorelines and the estuary mouth, and the area of mangroves around the site. By contrast, fish diversity and abundance, and the rates of ecological functions, were typically negatively correlated with the proximity of reefs to both mangroves and seagrasses. This complex spatial mosaic resulted in distinct areas predicted to achieve all restoration benefits that were significantly smaller than the total area that could be restored. Applying a systematic and defensible method to identify potential restoration sites that maximize multiple benefits while lowering costs is a sensible social, economic, and ecological strategy.

Sea-level-rise-induced threats depend on the size of tide-influenced estuaries worldwide

The effects of sea-level rise on the future morphological functioning of estuaries are largely unknown because tidal amplitudes will change due to combined deepening of the estuary mouth and shifting amphidromic points at sea. Fluvial sediment supply is also globally decreasing, which hampers infilling necessary to maintain elevation relative to sea level. Here we model 36 estuaries worldwide with varying sizes, shapes and hydrodynamic characteristics, and find that small shallow estuaries and large deep estuaries respond in opposite ways to sea-level rise. Large estuaries are threatened by sediment starvation and therefore loss of intertidal area, particularly if tidal amplitude decreases at the mouth. In contrast, small estuaries face enhanced flood risks and are more sensitive to tidal amplification on sea-level-rise-induced deepening. Estuary widening can partly mitigate adverse effects. In large estuaries, expanded intertidal areas increase tidal prism and available erodible sediment for adaptation, whereas it slightly reduces tidal amplification in small estuaries. Small shallow estuaries face enhanced flood risk under climate change because of sea-level-rise-induced tidal amplification. In contrast, large deep estuaries are threatened by sediment starvation and therefore a loss of intertidal area. Both cases can potentially be mitigated by estuary widening.

Assessing Agricultural Livelihood Vulnerability to Climate Change in Coastal Bangladesh.

The adverse impacts of climate change exert mounting pressure on agriculture-dependent livelihoods of many developing and developed nations. However, integrated and spatially specific vulnerability assessments in less-developed countries like Bangladesh are rare, and insufficient to support the decision-making needed for climate-change resilience. Here, we develop an agricultural livelihood vulnerability index (ALVI) and an integrated approach, allowing for (i) mapping out the hot spots of vulnerability distribution; (ii) identifying key factors of spatially heterogeneous vulnerability; and (iii) supporting intervention planning for adaptation. This study conceptualized vulnerability as a function of exposure, sensitivity, and adaptive capacity by developing a composite index from a reliable dataset of 64 indicators comprising biophysical, agro-ecological, and socioeconomic variables. The empirical studies of coastal Bangladesh revealed that Bhola, Patuakhali, and Lakshmipur districts, around the mouth of the deltaic Meghna estuaries, are the hot spot of vulnerability distribution. Furthermore, the spatially heterogeneous vulnerability was triggered by spatial variation of erosion, cyclones, drought, rain-fed agriculture, land degradation, soil phosphorus, crop productivity, sanitation and housing condition, infant mortality, emergency shelters, adoption of agro-technology. The integrated approach could be useful for monitoring and evaluating the effectiveness of adaptation intervention by substituting various hypothetical scenarios into the ALVI framework for baseline comparison.

The geomorphology and evolution of intermittently open and closed estuaries in large embayments in Victoria, Australia

Intermittently open and closed estuaries (IOCE) are common landforms on open-ocean microtidal coasts. They are often viewed as evolutionary end members of open estuaries where all the accommodation space has been infilled by sediment. While IOCE are typically recognised to form in wave-dominated, microtidal environments, they can also form within embayments where the relative balance between waves and tides is highly spatially, and temporally, variable. This study examines the geomorphology and infill of two such embayed IOCE in Victoria, southern Australia, in order to assess how these systems compare to their open ocean counterparts in order to gain an understanding of the boundary conditions of such landforms. It is found that IOCE’s can occur in embayments where there is sufficient energy to form a beach berm. The infill of these systems broadly follows the transgressive – stillstand pathways of open-coast systems, but the facies are modulated by the size of the catchment and open-ocean wave exposure. These boundary conditions also affect the evolution of the coastal plain in which the embayed IOCE may form and in turn the estuary planform. The entrance dynamics of these estuaries are highly variable, and it appears that they may switch between permanently open or closed states in relation to modification of boundary conditions such as roughness. It is inferred that the relative tidal range (RTR) of the marine environment at the estuary mouth is an important determination of entrance condition with the observed systems being close to the maximum RTR value on the open coast at which these landforms can exist.

Biogeography and fish community structure in Irish estuaries

Estuaries represent important transitional waters where marine and freshwater ecosystems meet and mix. Estuaries are dynamic systems due to their tidal nature and their ecology is expected to shift with climate change and the arrival of new species. Spatially extensive descriptive studies that provide temporal baselines for species distribution and abundance, and associated environmental variables deliver a comprehensive reference point against which to monitor change. To provide this biogeographic context for Ireland, which is surrounded by temperate Atlantic waters, the biogeography and fish community structure of Irish estuaries was examined using a large dataset comprised of 208,313 individual fish, 80 different species sampled from 37 estuaries from 2008–2017.Species richness was strongly correlated with the area of shallow littoral and subtidal habitats. Estuaries at higher latitudes tended to have lower species richness in shallow littoral areas. Estuary mouth width and proportion of subtidal area were both positively related to species richness in subtidal habitats. The main driver of estuary clustering is the changing proportion of estuarine species versus marine migrants. The results show a dominant group of larger, more open estuaries where marine migrants consistently dominate the fish population.This study confirms the role of marine migrants in contributing to fish community composition in estuaries and highlights the vital nursery function that Irish estuaries perform for estuarine-dependent marine fish species. The associations between fish community structure and broad-scale environmental parameters provide a time stamped baseline for monitoring future climate change impacts on estuarine fish assemblages in Ireland.