Open Estuaries Research Articles

Effects of Coastal Reclamation on the Topographic Changes of an Open Estuary: A Case Study in Taizhou Bay, East China

Effects of Coastal Reclamation on the Topographic Changes of an Open Estuary: A Case Study in Taizhou Bay, East China

Tidal Impoundment and Mangrove Dieback at Cabbage Tree Basin, NSW: Drivers of Change and Tailored Management for the Future

Major storms can cause significant changes to coastal and wetland environments. A series of storm events in 2020 resulted in closure of the historically open estuary at Cabbage Tree Basin, Port Hacking, New South Wales (NSW), Australia. Prolonged tidal impoundment (3 months) led to substantial changes in hydrological and sedimentological processes, resulting in widespread mangrove dieback. This study aimed to quantify the degree of impact and recovery for mangroves, identify factors contributing to dieback, and consider the implications for carbon sequestration. This was achieved using remotely piloted aircraft structure-from-motion approaches, aerial photography, and field-based assessments of vegetation health and above-ground biomass (AGB). Mangroves were classified as ‘dead’, ‘partially dead’, and ‘live’. In October 2019, there was 10.8 ha of live mangroves, with this reduced to 6.6 ha by August 2022. Digital surface models (DSMs) were intersected with classified mangroves to assess the vertical distribution of each zone. All mangroves classified as ‘dead’ were distributed at elevations < 0.4 m with respect to the Australian Height Datum (AHD), suggesting these regions were persistently inundated, which was confirmed by water level loggers (inundated during logger deployment). Field data confirmed substrate elevation related to dieback with the proportion of ‘live’ mangroves greatest at elevations > 0.6 m AHD. Substrate elevation and distance to the estuary mouth were significantly correlated with species, with Avicennia marina located at lower tidal positions and closer to the entrance compared to Aegiceras corniculatum. The dieback event equated to a loss of 81.5 ± 48 Mg of above-ground biomass, 38.1 ± 22.5 Mg C, or 140 ± 82 Mg CO2 equivalence (CO2e). This study provides an important baseline for monitoring dieback events. Continued monitoring is crucial to assess recovery and to tailor management strategies.

Emigration of Juvenile Tarpon Megalops atlanticus from Ephemerally Connected Coastal Ponds

Worldwide, coastal wetlands are threatened by disrupted hydrology, urbanization, and sea-level rise. In southwest Florida, coastal wetlands include tidal creeks and coastal ponds, which are the primary habitats used by juvenile Tarpon, Megalops atlanticus, an important sport fish. Coastal ponds can occur near uplands and are ephemerally connected to the open estuary, creating conditions of variable dissolved oxygen and salinity. Juveniles can tolerate wide-ranging abiotic conditions, but little is known about how they egress from their remote nursery habitats, which often requires them to cross > 1 km of mangrove forest to reach the open estuary. The objective of this study was to (1) compare Tarpon body condition among ponds close to the open estuary versus those ponds farther away on the Cape Haze peninsula of Charlotte Harbor, Florida, and (2) using acoustic telemetry determine what factors contribute to Tarpon emigration from the ponds to open estuarine waters. We tested the hypothesis that distinct groups of Tarpon occur in isolated ponds, leading to variation in fish length and body condition, and that opportunities for emigration from these ponds hinge on high water events. No pond stood out as having Tarpon of low body condition. Factors contributing to increased probabilities of Tarpon emigration were low barometric pressure, high-water level, and Tarpon body length. Tarpon emigrated from ponds near tidal creeks during summer king tides, while tropical cyclone conditions were needed to allow for movement from ponds farther in the landscape. The juvenile Tarpon were later detected at the mouths of large rivers 30 km up-estuary. The characterizations of water levels and event criteria needed for successful Tarpon nurseries should aid in habitat conservation and the creation of Tarpon nursery habitat in restoration designs.

Spatial patterns and environmental drivers of demersal fish assemblages in the Swartkops and Sundays estuarine to marine seascapes, South Africa

Estuaries are often observed as the boundary between the ocean and the river, separating coastal environments, rather than being seen as a continuum. Owing to insufficient research on demersal fish communities across nearshore marine to estuarine habitats, particularly in South Africa, this study employed a spatial seascape approach, with seasonal sampling of soft-bottom benthic habitats in two permanently open estuaries and the adjacent marine nearshore environment of Algoa Bay on the south coast of South Africa. A total of 790 fish representing 29 species were collected in the marine nearshore, and 6 414 fish (28 species) were caught in the estuarine zone. Two discrete demersal fish assemblages were identified as representing the marine nearshore and estuarine environments. The marine nearshore fish assemblages were dominated by marine species (e.g. Argyrosomus inodorus and Cynoglossus zanzibarensis) that were not recorded in the estuarine environment, whereas marine estuarine-opportunist species (e.g. Pomadasys olivaceus and Galeichthys feliceps) were more abundant in the marine nearshore than in the estuarine environment. The estuaries were dominated by benthic marine estuarine-dependent and marine and estuarine species (e.g. Rhabdosargus holubi, Solea turbynei, Heteromycteris capensis, Psammogobius knysnaensis and Glossogobius callidus). Differences observed in abundance and species richness along the estuarine to marine continuum were primarily related to the salinity gradient. These findings are significant as altered freshwater flows and consequent changes in salinity and productivity can have an impact on the nursery use of both estuaries and the marine nearshore by fish species.

Short food chains, highly diverse and complex food web networks in coastal lagoons

Shallow semi-enclosed coastal lagoons are recognized as important biodiversity hotspots and nursery areas for many organisms. However, the topology of the complex food web networks therein has never been studied. Highly defined food web networks were assembled for Ria de Aveiro, Ria de Alvor and Ria Formosa. Their structural network properties were analysed and compared to those of large open estuaries, small intermittent estuaries, as well as other marine ecosystems. The main conclusion was that these coastal lagoons are dominated by intermediate species like other estuarine systems, however they present more complex trophic networks (higher connectance) than large open estuaries, even though having shorter food chains. They also have lower mean path length between pairs of species. Shorter chain length means that disturbance is more likely to travel from basal to top species and likewise from the top to the bottom of the food web, while low path between species pairs implies a higher likelihood that disruption of one species affects any other species. These fragilities may be somewhat counterbalanced by the high connectance of these networks. The most connected species (with the highest degree = highest number of links at node) in the networks are crab and shrimp species. The non-indigenous blue crab appears as the most connected species in Ria de Alvor and among the top 5 most connected species in Ria Formosa. Highly commercial Sparid fishes play particularly important roles in the network as both highly connected and generalist predators. Top species are mostly birds. The top 10 species with more predators are all amphipods in Ria de Aveiro, while in Ria de Alvor they encompass insects, crabs, and gastropods, and in Ria Formosa they are all gastropods, with the exception of a non-indigenous polychaete. The particular network topology and inherent potential fragility of coastal lagoon food webs should be taken into account when designing environmental management plans.

Ecological impacts of dam construction on epilithic diatom communities in estuaries: A case study of the Korean peninsula

This study, spanning 2016 to 2018, investigated diatom distribution in 324 Korean Peninsula estuaries, emphasizing the ecological impact of artificial dams. Examining environmental and biological variables, we focused on dam construction. OPEN estuaries exhibited higher urban/forest land cover and had more alkaline, saline, conductive, and oxygen-rich water. In CLOSE estuaries, agricultural farmland and nitrogen compound concentrations were higher. Dams in CLOSE estuaries correlated with lower pH, salinity, electrical conductivity, and dissolved oxygen. Regional variations were evident, with WEST OPEN estuaries showing higher salinity, turbidity, chemical oxygen demand, and total phosphorus. SOUTH estuaries displayed intermediate characteristics, while EAST OPEN estuaries had higher dissolved oxygen but lower temperature and turbidity.Diverse epilithic diatoms were identified, with OPEN estuaries displaying greater species richness, primarily dominated by Nitzschia inconspicua Grunow. CLOSE estuaries, conversely, exhibited higher richness with a different subdominant species. Indicator species analysis revealed taxa specific to estuaries and sea areas, with CLOSE ones exhibiting lower salinity and higher nitrogen/phosphorus concentrations. Nitzschia inconspicua dominated all estuaries, but subdominant species varied; N. fonticola (Grunow) Grunow in OPEN, N. paleaceae (Grunow) Grunow in CLOSE. CLOSE estuaries had higher species richness. In the West Sea, turbidity and TP levels affected diatom growth, with dams exacerbating turbidity and potentially disrupting the food chain.Results underscored intricate interactions between dams, land use, and diatom communities, offering foundational data for ecosystem management and biodiversity conservation in Korean Peninsula estuaries. Understanding the ecological consequences of artificial dams on diatom distribution is crucial for preserving estuarine health. Future research is recommended to explore additional factors influencing these ecosystems, particularly focusing on various organisms in West Sea estuaries for effective ecosystem management and conservation.

How drought affects range and variability of environmental conditions in two subtropical South African estuaries

Globally estuaries are under increasing pressure from human development impacts including the growing effects of climate change that already cause major changes in freshwater inflow. One of these are drought events. Although they are regular occurrences and are projected to become progressively more significant in many parts of the world, their realised impact on the estuarine biome requires better understanding and quantification. In this study, we analysed the environmental niche variability along horizontal and vertical estuarine axes during the dry and wet season of a drought and non-drought period, in two contrasting subtropical estuary types in South Africa: the temporarily closed uMdloti Estuary and the predominantly open uMlalazi Estuary. We found quantitative support for the hypothesis that extended periods of drought lead to a decreased niche availability for estuarine organisms over both space and time, expressed as narrower salinity ranges, less habitat with amicable dissolved oxygen (DO) concentrations, and more frequent higher temperature. Drought caused a more homogenous salinity environment in both estuaries, more specifically extremely low salinity throughout the temporarily closed estuary, but increased salinity in the predominantly open estuary. DO levels were more variable during drought conditions and included periods of hypoxia in both estuaries. They were more stable at elevated levels (>5 mg/L) during non-drought conditions. The opposite was true for pH, for which we measured lower values and larger vertical ranges during non-drought conditions when rainfall and tidal exchange were more pronounced. Higher temperature value density peaks were apparent from the drought period, especially pronounced in the temporarily closed uMdloti Estuary. The physico-chemical environment was typically more estuarine during the non-drought period, and drought caused both similar (temperature, DO, pH) and different (salinity) responses in the two estuarine types. Drought-induced conditions can span several generations for estuarine and estuarine-associated species. It is important to quantify niche variability for a range of estuary types to subsequently understand drought effects on estuarine biodiversity.

A multi-metric fish index to measure the ecological quality of tropical predominantly open estuaries along the western coast of India

Defining the ecological quality of estuaries is challenging because of the inadequacy of available methods and indices to characterise the ecosystem. In Indian estuaries, there are no scientific attempts to establish multi-metric fish index to determine the ecological status. A multi-metric fish index (EMFI) was customised for twelve predominantly open estuaries on India's western coast. The index was established at the individual estuary level to ensure uniformity and contrast from sixteen metrics that designate the fish community and diversity, composition and abundance, estuarine use and trophic integrity from 2016 to 2019. A sensitivity study was followed to measure the responses of the EMFI under different metric varying scenarios. There were seven metrics identified as the most prominent for the EMFI in metric alteration scenarios. We also deduced a composite pressure index (CPI) based on the anthropogenic pressures described for the estuaries. The ecological quality ratios (EQR) defined based on EMFI (EQRE) and the CPI (EQRP) for all the estuaries were positively correlated. The EQRE values calculated based on the regression relationship (EQRE on EQRP) showed the divisions from 0.43 (bad) to 0.71 (high) for the Indian west coast estuaries. Similarly, the standardised CPI (EQRP) values for different estuaries indicated that the values range from 0.37 to 0.61. Our results based on the EMFI suggest four estuarine systems (33%) as 'good', seven (58%) as 'moderate' and one (9%) as 'poor'. Generalised Linear Mixed Model for EQRE showed that EQRP and estuary influenced EQRE; however, the effect of year was not significant. This comprehensive study based on the EMFI is the first record for predominantly open estuaries along the Indian coast. Therefore, the EMFI established in this study could be reliably advocated as a sound, effective and composite tool of ecological quality for tropical open transitional waters.

The nursery function of shallow nearshore and estuarine benthic habitats for demersal fishes

Estuaries and shallow marine nearshore areas are highly productive and valuable ecosystems, which provide numerous habitats for fish and support fundamental ecological links with other environments. Yet only a few studies have focussed on fish community patterns across an estuary and marine nearshore gradient concurrently to determine the nursery value of both environments. This research gap is particularly apparent in southern Africa. The main aim of this study was to assess the relative roles of two permanently open estuaries and the adjacent marine nearshore areas of Algoa Bay, on the south coast of temperate South Africa, as settlement and nursery areas for demersal fishes. Soft-bottom habitats in both the estuaries and the nearshore were dominated by early-life stages (postflexion larvae, settlement stage and young-of-the-year fishes), but a significantly greater density of early-life stage fishes was found in the estuarine environment, highlighting the importance of estuarine habitats for demersal fishes within the Algoa Bay nursery seascape. Settlement and nursery habitats also differed by estuarine-association, with the marine nearshore environment dominated by marine and marine estuarine-opportunists, with core settlement and nursery areas for these species located close to estuary mouths, freshwater outlets and harbours. In contrast, the estuarine environment was dominated by marine estuarine-dependent and estuarine and marine species. This highlights the importance of concurrently sampling habitats in marine nearshore and estuarine environments, which allows for a better understanding of the degree of estuarine dependency of marine fish as well as an understanding of the nursery function of coastal habitats.

A preliminary fish survey of the estuaries on the east coast of South Africa, Mpande to Mtentwana: a comparative study

A preliminary ichthyofaunal and physico-chemical survey of estuaries on the east coast of South Africa from the Mpande Estuary to the Mtentwana Estuary was undertaken between November 1997 and January 1998. Sixteen estuaries were surveyed along this stretch of coastline and these were grouped into three estuary types: small (&lt; 10 ha) predominantly closed estuaries, moderate to large (&gt; 10 ha) predominantly closed estuaries and predominantly open estuaries. Multivariate analyses revealed significant differences between the three groups in terms of both their physico-chemical characteristics (small predominantly closed estuaries were different from predominantly open estuaries) and their fish communities (all three estuary types were significantly different). The estuaries in the study area fall within the subtropical/warm-temperate transition-zone and north of the subtropical/warm-temperate biogeographic break; tropical species dominated the fish communities of all the estuaries in terms of numbers of species and biomass. This survey represents one of the few fish surveys undertaken along this little-studied section of the coastline.

The Application of a Fish-Based Multi-Metric Index for the Assessment of Ecological Qualities of Estuaries in the Korean Peninsula

Brackish water zones are areas with high ecological conservation value. In this study, 325 river estuaries in the Korean peninsula in individual sea areas (West Sea, South Sea, and East Sea) were divided into types of estuaries (upstream and downstream of open estuaries, closed estuaries) through the assessment of the health of the estuary aquatic ecosystems and fish communities were identified. An ecological assessment was carried out using the Korea Estuary Fish Assessment Index (KEFAI). The number of species increased as the size of the river increased in the case of small estuaries but gradually decreased in the case of large estuaries. In the closed estuaries, the relative abundances (RAs) of primary freshwater fish were the highest; however, in the open estuaries, the RAs of estuary fish were the highest. Non-metric dimensional analysis results suggested that there was a clear difference between the fish assemblages in the closed and open estuaries. The overall results of this study were that the RA of tolerant species was higher, and KEFAI was lower in closed estuaries than in open estuaries, indicating the negative effects of the construction of transverse structures on fish assemblages. The health of these estuarine ecosystems can be improved by addressing these negative effects.

Spatio-temporal Variation of Fish Communities in Open Estuary, Seomjin River Estuary and Gwangyang Bay Coast

Spatio-temporal Variation of Fish Communities in Open Estuary, Seomjin River Estuary and Gwangyang Bay Coast

Effects of environmental factors on ichthyoplankton in a permanently open estuary under the influence of a semi-arid climate, north-eastern Brazil

AbstractEstuarine ecosystem conditions actively influence the early life stage of fishes. This study reports how environmental factors influenced the ichthyoplankton in a tropical estuary within an Environmental Protection Area by comparing the structure and composition of fish eggs and larval assemblages. A total of 1672 fish larvae and 486 fish eggs were collected. Higher densities of larvae were recorded for Engraulidae, Characidae, Clupeidae, Gerreidae, Mugilidae and Atherinopsidae, and higher egg densities of the families Mugilidae, Clupeidae and Engraulidae were found. The spatio-temporal variations were determined by the environmental predictors salinity, pH, dissolved oxygen and temperature, with salinity influenced by precipitation as one of the main predictors of the distribution of ichthyoplankton. During the rainy season, greater densities of eggs were recorded in the upper and intermediate zones, mainly Characidae and Engraulidae; in the dry season, in the lower zone, there was a greater density of larvae, particularly Atherinopsidae and Mugilidae. The information provided in the present study contributes to our knowledge of nursery habitat requirements for the initial development of marine migrant and resident species in tropical estuaries.

The impacts of long-term flow reductions and an extreme drought on a large, permanently open estuary, and implications for setting the ecological reserve

Environmental water requirements (EWRs) are set for South Africa’s estuaries to ensure that they are maintained in a state that is both achievable and commensurate with their level of conservation and economic importance. However, these EWRs are typically determined on the basis of models and scenario analyses that require extrapolation beyond existing data and experience, especially if climate change is considered. In the case of the Berg Estuary, South Africa, available data on changes in freshwater flow and water quality span a period of at least five decades (1970s–present) during which significant reduction in flows has been observed. Monitoring data also cover an extreme 3-year drought, from 2015−2017, which provided a unique opportunity to study the effects of severe freshwater starvation (zero-flow for an extended period) on this large, permanently open system. Our analyses show that mean annual runoff (MAR) under present-day conditions has been reduced to around 50% of that under reference (natural) conditions and that reduction in runoff during the low-flow season (summer) has been more severe (80–86% reduction) than for the high-flow season (39–42% reduction). The salinity gradient now extends much further upstream than under reference conditions. Hypersaline conditions along with a reverse salinity gradient were recorded in the estuary for the first time ever during the drought of 2015/17. Levels of dissolved inorganic nitrogen (NOx) reaching the estuary from the catchment have increased dramatically (6–7 fold) over the past five decades, dissolved reactive phosphate (PO4) slightly less so (2–3 fold), but ammonia (NH4) hardly at all. Increases in nutrient input from the catchment in the high-flow season are also much more dramatic than in the low-flow season. The estuary is no longer compliant with gazetted EWRs and requires urgent interventions to restore the quantity and quality of freshwater it receives.&#x0D;

Is the largest African penguin colony in South Africa influencing local ocean productivity?

In coastal waters, nutrient supplies originate principally from allochthonous sources, such as inputs from rivers, estuaries or oceanic waters. Recently, it has been suggested that marine life contributes to the nutrient load, with penguin colonies being the largest contributor worldwide. This study aimed to quantify the nitrogen (N) and phosphorus (P) produced by the largest colony of African penguins Spheniscus demersus, which is located in South Africa, using a bioenergetics model (GUANO). We modelled the fate of N excreted and measured potential effects on the physicochemical parameters of the waters surrounding St Croix Island, Algoa Bay, as well as on local primary and secondary productivity. A total of 58.66 (SD 14.99) tonnes (t) of N and 4.35 (SD 1.20) t of P were estimated to be produced by penguins annually, a magnitude comparable to that exported by permanently open estuaries in Algoa Bay. However, only 8% of penguin-produced N washed into the ocean (after moderate to heavy precipitation) as total ammoniacal N, while 58% of N was excreted as volatilised ammonia. Consequently, penguin excreta appeared to have limited influence on productivity in the waters surrounding St Croix Island, at both proximate (200 m) and distant (2 km) locations. Additional experiments should confirm whether the input of N excretion in coastal waters was too ephemeral to be detected at the monthly scale, or whether the effects were restricted to intertidal communities. Regardless, sources of nutrients other than penguins, such as coastal upwelling and runoff, are likely to have a greater influence on the physicochemical dynamics and productivity of Algoa Bay nearshore waters.

Wetlands in intermittently closed estuaries can build elevations to keep pace with sea-level rise

Sea-level rise is a threat to coastal ecosystems, which have important conservation and economic value. While marsh response to sea-level rise has been well characterized for perennially open estuaries, bar-built intermittently-closed estuaries and their sea-level rise response are seldom addressed in the literature – despite being common globally. We seek to advance the conceptual understanding of sea-level rise response of marshes by incorporating the unique nature of intermittently-closed estuaries in a marsh model. We hypothesize that intermittently-closed-estuary marshes may be more resilient to sea-level rise than open-estuary marshes due to greater initial elevation capital and higher accretion rates due to closure events. Using California, USA as a case study, spatial analysis shows that marshes in intermittently-closed-estuaries had significantly greater elevations (x̄ = 1.93 m ± 0.2 standard error, n = 14) than marshes in permanently open estuaries (x̄ = 0.94 m ± 0.1 standard error, n = 8; P = 0.003). We then used a process-based model to determine marsh elevation change under 840 simulated responses to sea-level rise to 2100. Our modeling shows that regular annual mouth closure can promote accretion rates and increase marsh elevations fast enough to match even high rates of sea-level rise, as fluvial sediment pulses can be captured in the estuary. Modeled suspended sediment concentration had the strongest effect on accretion, followed by probability of annual mouth closure. Intermittently closed estuaries are critical environments where marshes may be sustained under high rates of sea-level rise, thus reducing the anticipated global loss of these important ecosystems. Our results begin to fill an important gap in the knowledge about marsh accretion and identify research needs to inform coastal management.

Possible impacts of non-native plant, pathogen, invertebrate and fish taxa on the indigenous ichthyofauna in South African estuaries: a preliminary review

We review the possible impacts of non-native biota on the indigenous fishes of South African estuaries, including macrophytes, algae, pathogens, invertebrates, and fishes. Freshwater macrophytes are one of the primary non-native groups in the oligohaline reaches of some predominantly open estuaries, lake and river mouth type estuaries, as well as the entire area of certain low salinity, temporarily closed estuaries. Anoxia and hypoxia in the water column below Salvinia molesta and Pontederia crassipes floating mats have caused fish kills in certain temporarily closed estuaries. Mass mortalities of fish in estuaries have arisen from harmful algal blooms (HABs) and a catchment-derived pathogenic water mould, Aphanomyces invadans. Non-native invertebrate species in local estuaries are derived from freshwater, estuarine and marine sources. The freshwater gastropod Tarebia granifera has invaded many subtropical estuaries and may be negatively impacting their food webs, with estuarine zoobenthivorous fishes not appearing to consume this mollusc. The marine polychaete Ficopomatus enigmaticus has invaded many South African estuaries and, in some of them, changed the zoobenthic food web by encrusting on hard surfaces and filtering particulate matter from the water column. This species also does not appear to be eaten by zoobenthivorous fishes within these systems. No non-native marine or estuarine fish species have been recorded in South African estuaries but non-native freshwater fish species now occur in 25% of estuaries in the region. Degraded estuaries in particular are more vulnerable to colonisation by non-native and translocated fish species than unimpacted systems. During the 1990s, a fish survey of 191 estuaries revealed that only 0.04% of the catch comprised non-native or translocated species but this percentage has increased in many estuaries in recent decades. Non-native and translocated freshwater fish species have successfully colonised the oligohaline and mesohaline reaches of many systems and there may be an impact due to predation on the eggs and larvae of resident estuarine taxa and the recruitment success of catadromous and some estuarine-associated fish species. However, most non-native fishes have a limited tolerance for the salinity regimes found in the lower and middle reaches of many South African estuaries, with an even larger threat to the indigenous estuarine ichthyofauna coming from non-native plant, invertebrate and pathogen invaders. Based on this review, and other similar global studies, there is a developing paradigm that non-native invasions by fishes and other organisms into South African and global estuaries are driven primarily from freshwater taxa and not estuarine or marine species.

Holocene infill of the Anglesea Estuary, Victoria: a keep-up estuary in a geologically constrained environment

Holocene coastal infill was examined in Anglesea Estuary, an intermittently open barrier-type estuary in Victoria. Based on coring and radiocarbon dating, it was found that accommodation space in the estuary was almost entirely occupied in the early–mid Holocene corresponding to periods of higher sea-level. The pattern of infill is in stark contrast to established models of estuarine sedimentation developed on the east Australian coast. The contemporary estuary is composed of two distinct sedimentary basins. The upper basin is infilled by autochthonous peaty-marsh sediments, whereas the lower basin is infilled by saltmarsh material. The traditional tripartite flood-tide delta–central mud basin–fluvial delta sequence is absent. The contemporary estuary appears to have evolved as two distinct basins, which are connected through hydrodynamic rather than sedimentological processes. This pattern of connected basins occurs from the top of the catchment to the wave base, with the contemporary estuary infilling the two basins coincident with modern sea-level. Low sediment supply from both terrestrial and marine sources and bedrock-limited accommodation space means that Anglesea can be considered to be geologically constrained barrier estuary. KEY POINTS Geological-control limits mud basin development. Low allochthonous sediment supply significantly affected estuarine facies in Anglesea. Anglesea Estuary infilled to higher mean sea-level elevations in the mid Holocene. Anglesea Estuary is composed of two distinct sedimentary basins.

Historic dispersal barriers determine genetic structure and connectivity in a supratidal sandy-beach brooder

The supralittoral zones of sandy beaches are particularly vulnerable to coastal development and other activities that cause localised habitat destruction. Supratidal species, such as peracarid crustaceans, which lack a pelagic larval phase and tend to avoid direct contact with the swash, are therefore expected to be distributed as isolated populations, with implications for their long-term survival. Genetic population structure of one such species, the pill bug Tylos capensis, was investigated based on mitochondrial cytochrome c oxidase subunit I (COI) haplotype sequences, to assess demographic history and regional population connectivity in the presence of potential dispersal barriers (an estuary, coastal cliffs) along the Indian Ocean coast of South Africa. Here, pill bugs demonstrated strong and significant genetic structure at the regional scale, with 3 distinct clades across the species’ geographic distribution. At a localised spatial scale, coastal cliffs intersecting the high shore appeared to be a strong barrier to gene flow between adjacent populations, while a permanently open estuary did not limit gene flow. Estimates of historic gene flow and patterns of COI differentiation coincided with greater habitat continuity during the Pleistocene glaciations at sea levels between -75 and -120 m, when the African south coast was probably dominated by sandy beaches. While gene flow among low-dispersing pill bug populations is unlikely to benefit from a network of closely spaced coastal protected areas, the isolated nature of this species, coupled with the cryptic diversity inherent in this taxon, emphasizes the need for their protection. The importance of protecting the intact littoral active zone of beaches is highlighted.

Life histories of two hyperbenthic mysids (Mysidacea: Mysidae) in an open estuary

ABSTRACT Mysidae are widely distributed in aquatic ecosystems and form a large proportion of the hyperbenthic biomass in estuaries. The seasonality, spatial distribution and reproductive biology of the two hyperbenthic mysid species, Tenagomysis chiltoni and Tenagomysis novaezealandiae, were studied in an open estuary. Sampling was conducted bi-monthly at night between February 2008 and April 2009. These two mysid species are spatially segregated along a salinity gradient, with T. chiltoni upstream and T. novaezealandiae downstream in the estuary. The life cycle and breeding dynamics of T. chiltoni (bivoltine life cycle) and T. novaezealandiae (multivoltine life cycle) were similar to those described in intermittently closed estuaries, but their breeding period (December to April) was relatively shorter and densities were lower. Life-stage specific segregation in summer in both mysid species likely provided favourable conditions for maximum growth rates, short maturation times, and sheltering, enhancing reproductive success of both mysid species.