Sensitivity of Estuaries to Compound Flooding

Fluvial and surge-tide extremes can occur synchronously resulting in compound flooding in estuaries, greatly intensifying the hazard. This flood risk has the potential to increase in the future as the frequency, phasing and/or intensity of these drivers change. Improved understanding of how extreme fluvial discharge and surge-tides interact will help inform future flood mitigation methodology. In this paper, therefore, we resolve for the first time intra-estuary sensitivities to fluvial and surge-tide extremes, for two contrasting UK estuaries (Humber and Dyfi). Model simulations at hyper-spatial resolution (< 50 m) using a 2D hydrodynamic model predicted compound flooding hazards based on: (1) present-day extreme events (worst on record); (2) present-day extreme events with shifted timings of the drivers to maximise flooding; and (3) modified drivers representing projected climate change. We found that in a small estuary with short-duration, high-intensity fluvial inputs (Dyfi), flood extent is sensitive to the relative timing of the fluvial and surge-tide drivers. In contrast, the relative timing of these drivers did not affect flooding in a larger estuary with a slower fluvial response to rainfall (Humber). In the Humber, extreme fluvial inputs during a compound hazard actually reduced maximum water depths in the outer estuary, compared with a surge-tide-only event. Projected future changes in these drivers by 2100 will increase compound flooding hazards: simulated sea-level rise scenarios predicted substantial and widespread flooding in both estuaries. However, projected increases in surge-tide behaved differently to sea-level rise of the same magnitude, resulting in a greater seawater influx and more flooding. Increased fluvial volumes were the weakest driver of estuarine flooding. In this paper we show how these interactions are complex and how the hydrodynamics vary considerably between different estuaries and sites within estuaries, making it difficult to generalise, use probabilistic or use 1D approaches for assessing compound flooding hazards. Hence, we contribute new knowledge and methods for catchment-to-coast impact modelling used for flood mitigation strategies.

Does a Trawl Ban Benefit Commercially Important Decapoda and Stomatopoda in Hong Kong?

Crustaceans were among the most valuable fishery resources in Hong Kong. However, the unrestricted and intensive use of different fishing gears, especially bottom trawling, has led to the depletion of commercially important crustaceans in Hong Kong since the 1980s. This study investigated whether commercial crustaceans recovered after the implementation of a permanent Hong Kong-wide trawl ban that began on December 31, 2012. Standardized field surveys were conducted using a commercial shrimp trawler at two sites in eastern and western waters of Hong Kong before (2004) and after the trawl ban (2013–2014 and 2015–2016) and two sites in southern waters after the trawl ban. Diversity, mean size, abundance, biomass and level of disturbance of commercial crustaceans from the three periods were investigated. The eastern waters exhibited an increased diversity of crustacean assemblages in Inner Tolo, and a higher abundance and biomass of crabs were detected in Outer Tolo after the trawl ban. Reduced disturbance, higher diversity in crustacean assemblages and greater abundance and biomass of predatory crabs were observed after the trawl ban in the outer estuary of western waters, and increased abundance and biomass of shrimp were detected in the inner estuary of western waters. No temporal or negative changes were detected in the southeast and southern waters of Lamma Island. The various responses of crustacean assemblages in Hong Kong waters revealed the critical role of complex interactions among multiple stresses, such as ongoing reclamation works, illegal trawling activities and increased fishing efforts using other (legal) fishing methods.

Bank Erosion Processes, Trends and Impacts in a Hypertidal Estuarine System

&lt;p&gt;Estuarine systems represent the dynamic transition zone between fluvial and marine systems and as such they are sensitive to changes in both domains resulting from impacts of climate change and human activities related to coastal and water-flow management especially in densely inhabited areas. Further, these tidally influenced systems are subject to a unique set of driving conditions linked to bidirectional flow processes. The potential growing risks of shoreline erosion in coastal, estuarine and inter-tidal environments have been identified by a number of studies in recent years. However, bank erosion processes in tidal settings remain poorly understood, especially when compared to the large volume of research concerning fluvial bank erosion. In general, the well-established fluvial bank erosion literature suggests that bankline erosion involves two main sets of processes: hydraulic erosion and gravitational collapse. Given the additional complexity of the process mechanics involved in tidal settings, arising mainly from the presence of bi-directional flows, process insights gained from studies of fluvial bank erosion might not be appropriately applied in a tidal context.&lt;/p&gt;&lt;p&gt;The present study aims to improve our understanding of estuarine bank mobility dynamics through investigation of the evolution and rates of bank retreat/accretion acting in the Severn Estuary (UK). The Severn Estuary has one of the highest semidiurnal tidal ranges in the world (about 15 m in the outer estuary, up to 8-9 m in the middle parts of the system, and 2 to 3 m in the inner river-dominated sector). Here we estimate bank mobility throughout the estuary from the river-dominated to the tidal-dominated zones during the last 119 years, via analysis of historical maps and recent satellite images. We use the findings from this analysis coupled with recent data collection to propose an empirical model of bank mobility throughout the entire estuary, highlighting the characteristics and the differences between riverine and coastal erosive processes. The model indicates that (i) the highest bank mobility (both in term of erosion and deposition) is located in the mid part of the estuary, close to the bedload convergence zone (BLZ), with other &amp;#8216;hot spots&amp;#8217; of change linked to major anthropogenic disturbances either in the outer and inner estuary, and (ii) that the erosive mechanics associated to severe lateral land losses in the estuary are mainly driven by impulses in energy delivery to the bank surface in occasion of very high tidal oscillations (particularly in spring overbank tides) and severe storms triggering mass wasting in form of toppling and rotational failures.&amp;#160;&amp;#160;&lt;/p&gt;

Recent Spatio-Temporal Variations of Suspended Sediment Concentrations in the Yangtze Estuary

Water and sediment are two of the most essential elements in estuaries. Their product, suspended sediment concentration (SSC), is involved in hydrology, geomorphology and ecology. This study was focused on the spatial and temporal variations of SSC in the Yangtze Estuary under new situations after the closure of ~50,000 dams in the Yangtze basin, including the Three Gorges Dam (TGD) in 2003. It was found that the SSC first exhibited an increasing and then a decreasing trend longitudinally from Xuliujing Station to the outer estuary with the Turbidity Maximum Zone located in the mouth bar area. Vertically, the SSC in the bottom layers averaged 0.96 kg/m3, about 2.4 times larger than the surface layers (0.40 kg/m3). During spring tides, the SSCs were always higher than those in neap tides, which was fit for the cognition law. As for the seasonal variations in the North Branch and mouth bar area, the SSCs in the dry season were higher than those in the flood season, while in the upper reach of the South Branch and outer estuary, the seasonal variation of SSCs reversed. This phenomenon primarily reflected the competition of riverine sediment flux and local resuspended sediment flux by wind-induced waves. As for the interannual changes, the SSCs demonstrated overall fluctuant downward trends, determined by riverine sediment flux and influenced by waves. This study revealed the new situation of SSC and can be a reference for other related researches in the Yangtze Estuary.

A comparative analysis of the spatial distribution of salinity and biomass of nekton and nektobenthos in the estuaries of the large plain river Razdolnaya and the mountain river Sukhodol (the Sea of Japan)

This paper discusses the two estuaries of the large flat river Razdolnaya and the mountain river Sukhodol (the Sea of Japan). The data of hydrological surveys on these estuaries are presented and the distribution zones of waters of different salinity are shown. The graphs of changes in the biomass of some hydrobionts along the river are given. The studies have shown that water with a salinity of 5% penetrates the Razdolnaya River up to the main channel for 15 km. In the Sukhodol River, it happens that almost the entire estuary is occupied by salt water, and fresh water is traced in a layer of water no more than 0,5 m thick, no closer than 500 m to the mouth of the river. In the Razdolnaya River, there is a tendency for the growth of hydrobiont catches from the outer estuary to the inner one (from the sea to the river) to maximum values at a distance of 5 km from the mouth, followed by a gradual descent upstream. In the Sukhodol River, the specific biomass gradually decreases from the top of the river to a minimum of 0,4 km from the mouth with a sharp increase in the area of the mouth itself (the mouth of the lagoon and the sea coast) and the subsequent smooth decrease. The specific biomass of hydrobionts in the inner estuaries of the Razdolnaya and Sukhodol rivers was 6,7 and 4,0 g/m respectively.

Nitrogen cycling in the Elbe estuary from a joint 3D-modelling and observational perspective

The study addresses the nitrogen cycling in Elbe estuary. Observations of salinity, nutrients and oxygen from moored stations, ship casts and helicopter surveys are presented. Observations are complemented by simulations obtained from a coupled physical-biogeochemical 3D unstructured model, applied for the first time to the estuarine environment. Model simulations reproduce the temporal variability of nutrients and oxygen along the estuarine salinity gradient. Both, observations and model results, demonstrate mostly conservative mixing of nitrate and non-conservative behavior of ammonium. Model hind-casts of the years 2012 and 2013 provide a detailed reconstruction of nitrogen recycling with ammonium appearing as the key species of the remineralisation process. Estuarine turnover processes are fueled by inputs of diatoms and organic nitrogen at the tidal weir with intense primary production manifest in the shallow river section downstream of the weir. The harbor area is the hot spot of heterotrophic decay associated with growth of meso-zooplankton, sedimentation of degradable material, remineralisation, oxygen depletion, denitrification and ammonium production. In the harbor, biochemistry shows strong vertical gradients while hydrodynamics demonstrate connectivity between the main channel and the harbor. At the estuary bed nitrogen is deposited during spring and early summer. Resuspension leads to nearly closed budget by the end of the year. During the Elbe flood in June 2013, estuarine biogeochemistry is significantly disturbed with the harbor being deactivated as hot spot of heterotrophic decay. Plankton and organic matter are flushed towards the outer estuary which in consequence sees high abundance of grazers, oxygen depletion and elevated release of ammonium.

Considerations for metabarcoding-based port biological baseline surveys aimed at marine non-indigenous species monitoring and risk-assessments

Monitoring introduction and spread of non-indigenous species via maritime transport and performing risk assessments require port biological baseline surveys. Yet, the comprehensiveness of these surveys is often compromised by the large number of habitats present in a port, the seasonal variability and the time-consuming morphological approach used for taxonomic identification. Metabarcoding represents a promising alternative for rapid comprehensive port biological baseline surveys, but its application in this context requires further assessments.We applied metabarcoding (based on barcodes of the Cytochrome c oxidase subunit I and of the 18S ribosomal RNA gene) to 192 port samples collected i) from diverse habitats (water column – including environmental DNA and zooplankton, sediment and fouling structures), ii) at different sites (from inner to outer estuary), and iii) during the four seasons of the year.By comparing the biodiversity metrics derived from each sample group, we show that each sampling method resulted in a distinct community profile and that environmental DNA alone cannot substitute for organismal sampling, and that, although sampling at different seasons and locations resulted in higher observed biodiversity, operational results can be obtained by sampling selected locations and seasons.By assessing the taxonomic composition of the samples, we show that metabarcoding data allowed the detection of previously recorded non-indigenous species as well as to reveal presence of new ones, even if in low abundance.Synthesis and application. Our comprehensive assessment of metabarcoding for port biological baseline surveys sets the basics for cost-effective, standardized and comprehensive monitoring of non-indigenous species and for performing risk assessments in ports. This development will contribute to the implementation of the recently entered into force International Convention for the Control and Management of Ships’ Ballast Water and Sediments.

Genetic divergence and fine scale population structure of the common bottlenose dolphin (Tursiops truncatus, Montagu) found in the Gulf of Guayaquil, Ecuador

The common bottlenose dolphin, Tursiops truncatus, is widely distributed along the western coast of South America. In Ecuador, a resident population of bottlenose dolphins inhabits the inner estuarine area of the Gulf of Guayaquil located in the southwestern part of the country and is under threat from different human activities in the area. Only one genetic study on South American common bottlenose dolphins has been carried out to date, and understanding genetic variation of wildlife populations, especially species that are identified as threatened, is crucial for defining conservation units and developing appropriate conservation strategies. In order to evaluate the evolutionary link of this population, we assessed the phylogenetic relationships, phylogeographic patterns, and population structure using mitochondrial DNA (mtDNA). The sampling comprised: (i) 31 skin samples collected from free-ranging dolphins at three locations in the Gulf of Guayaquil inner estuary, (ii) 38 samples from stranded dolphins available at the collection of the “Museo de Ballenas de Salinas,” (iii) 549 mtDNA control region (mtDNA CR) sequences from GenBank, and (iv) 66 concatenated sequences from 7-mtDNA regions (12S rRNA, 16S rRNA, NADH dehydrogenase subunit I–II, cytochrome oxidase I and II, cytochrome b, and CR) obtained from mitogenomes available in GenBank. Our analyses indicated population structure between both inner and outer estuary dolphin populations as well as with distinct populations of T. truncatus using mtDNA CR. Moreover, the inner estuary bottlenose dolphin (estuarine bottlenose dolphin) population exhibited lower levels of genetic diversity than the outer estuary dolphin population according to the mtDNA CR. Finally, the estuarine bottlenose dolphin population was genetically distinct from other T. truncatus populations based on mtDNA CR and 7-mtDNA regions. From these results, we suggest that the estuarine bottlenose dolphin population should be considered a distinct lineage. This dolphin population faces a variety of anthropogenic threats in this area; thus, we highlight its fragility and urge authorities to issue prompt management and conservation measures.

Cyanobacterial community diversity in the sediments of the Pearl River Estuary in China

Cyanobacterial community diversity in the sediment of the Pearl River Estuary in China was evaluated in this study by denaturing gradient gel electrophoresis (DGGE) during the wet and dry seasons. Nucleotide sequences obtained from DGGE bands were classified into five cyanobacterial clusters, including Synechococcus, Cyanobium, Chroococcus, Prochlorales and Tolypothrix. Synechococcus was identified as the dominant cyanobacterial group in the sediment samples; its distribution varied from the inner estuary to the outer estuary, with a wide range of salinity adaptation. Observed patterns of cyanobacterial communities changed markedly between sampling sites and seasons, suggesting that most cyanobacteria were not delivered via fresh water. Canonical correspondence analysis was conducted to determine the relationship between environmental variables and bacterial community structures during the dry season. The results suggested that the cyanobacterial community was significantly influenced by pH, salinity, PO4-P and NO3-N in sediments.