Inverse Estuary Research Articles

Recognizing Low-Inflow Estuaries as a Common Estuary Paradigm

It has become clear that estuaries with low rates of freshwater inflow are an important but overlooked sphere of estuarine science. Low-inflow estuaries (LIEs) represent a major class of estuary long downplayed because observations do not fit well in the dominant estuary paradigm, which was developed in perennially wet climates. Rather than being rare and unusual, it is now evident that LIEs are common globally and an alternate estuary paradigm within the idea of an estuary as the place where a river meets the sea. They are found mostly in areas with arid, semi-arid, or seasonally arid climates, but LIE phenomena are also found in estuaries along mountainous coasts with small watersheds and short-tailed hydrographs. Inflows can be defined as “low” relative to basin volume, tidal mixing, evaporative losses, or wave forcing at the mouth. The focus here is on common physical phenomena that emerge in low-inflow estuaries—how low river flow is expressed in estuaries. The most common is hypersalinity (and the associated potential for inverse conditions), which develops where there is a net negative water balance. However, in small microtidal estuaries, low inflow results in mouth closure even as a positive water balance may persist, accounting for extreme stratification. Attention is also given to the longitudinal density gradient and the occurrence of thermal estuaries and inverse estuaries. Finally, ocean-driven estuaries are highlighted where marine subsidies (nutrients, particulates) dominate watershed subsidies. While climate change is altering freshwater inflow to estuaries, locally driven changes are generally more important and this presents an opportunity to restore estuaries through restoring estuarine hydrology.

Effects of seawater and freshwater challenges on the Gh/Igf system in the saline-tolerant blackchin tilapia (Sarotherodon melanotheron).

Prolactin (Prl) and growth hormone (Gh) as well as insulin-like growth factor 1 (Igf1) are involved in the physiological adaptation of fish to varying salinities. The Igfs have been also ascribed other physiological roles during development, growth, reproduction and immune regulation. However, the main emphasis in the investigation of osmoregulatory responses has been the endocrine, liver-derived Igf1 route and local regulation within the liver and osmoregulatory organs. Few studies have focused on the impact of salinity alterations on the Gh/Igf-system within the neuroendocrine and immune systems and particularly in a salinity-tolerant species, such as the blackchin tilapia Sarotherodon melanotheron. This species is tolerant to hypersalinity and saline variations, but it is confronted by severe climate changes in the Saloum inverse estuary. Here we investigated bidirectional effects of increased salinity followed by its decrease on the gene regulation of prl, gh, igf1, igf2, Gh receptor and the tumor-necrosis factor a. A mixed population of sexually mature 14-month old blackchin tilapia adapted to freshwater were first exposed to seawater for one week and then to fresh water for another week. Brain, pituitary, head kidney and spleen were excised at 4 h, 1, 2, 3 and 7 days after both exposures and revealed differential expression patterns. This investigation should give us a better understanding of the role of the Gh/Igf system within the neuroendocrine and immune organs and the impact of bidirectional saline challenges on fish osmoregulation in non-osmoregulatory organs, notably the complex orchestration of growth factors and cytokines.

Impact of Evaporation and Precipitation on Estuarine Mixing

AbstractRecent studies could link the quantities of estuarine exchange flows to the volume-integrated mixing inside an estuary, where mixing is defined as the destruction of salinity variance. The existing mixing relations quantify mixing inside an estuary by the net boundary fluxes of volume, salinity, and salinity variance, which are quantified as Knudsen or total exchange flow bulk values. So far, river runoff is the only freshwater flux included, and the freshwater exchange due to precipitation and evaporation is neglected. Yet, the latter is the driving force of inverse estuaries, which could not be described by the existing relations. To close this gap, this study considers evaporation and precipitation to complete the existing mixing relations by including cross-surface salinity variance transport. This allows decomposing the mixing into a riverine and a surface transport contribution. The improved relations are tested against idealized two-dimensional numerical simulations of different combinations of freshwater forcing. The mixing diagnosed from the model results agrees exactly with the derived mixing relation. An annual hindcast simulation of the Persian Gulf is then used to test the mixing relations, both exact and approximated, e.g., long-term averaged, for a realistic inverse estuary. The results show that the annual mean mixing contributions of river discharge and evaporation are almost equal, although the freshwater transport due to evaporation is about one order of magnitude larger than the river runoff.

Evaluation of a popular spatial cumulative impact assessment method for marine systems: A seagrass case study.

Human activities put stress on our oceans and with a growing global population, the impact is increasing. Stressors rarely act in isolation, with the majority of marine areas being impacted by multiple, concurrent stressors. Marine spatial cumulative impact assessments attempt to estimate the collective impact of multiple stressors on marine environments. However, this is difficult given how stressors interact with one another, and the variable response of ecosystems. As a result, assumptions and generalisations are required when attempting to model cumulative impacts. One fundamental assumption of the most commonly applied, semi-quantitative cumulative impact assessment method is that a change in modelled cumulative impact is correlated with a change in ecosystem condition. However, this assumption has rarely been validated with empirical data. We tested this assumption using a case study of seagrass in a large, inverse estuary in South Australia (Spencer Gulf). We compared three different seagrass condition indices, based on survey data collected in the field, to scores from a spatial cumulative impact model for the study area. One condition index showed no relationship with cumulative impact, whilst the other two indices had very small, negative relationships with cumulative impact. These results suggest that one of the most commonly used methods for assessing cumulative impacts on marine systems is not robust enough to accurately reflect the effect of multiple stressors on seagrasses; possibly due to the number and generality of assumptions involved in the approach. Future methods should acknowledge the complex relationships between stressors, and the impact these relationships can have on ecosystems. This outcome highlights the need for greater evaluation of cumulative impact assessment outputs and the need for data-driven approaches. Our results are a caution for marine scientists and resource managers who may rely on spatial cumulative impact assessment outputs for informing policy and decision-making.

Temperate southern Australian coastal waters are characterised by surprisingly high rates of nitrogen fixation and diversity of diazotrophs.

Biological dinitrogen (N2) fixation is one mechanism by which specific microorganisms (diazotrophs) can ameliorate nitrogen (N) limitation. Historically, rates of N2 fixation were believed to be limited outside of the low nutrient tropical and subtropical open ocean; however, emerging evidence suggests that N2 fixation is also a significant process within temperate coastal waters. Using a combination of amplicon sequencing, targeting the nitrogenase reductase gene (nifH), quantitative nifH PCR, and 15N2 stable isotope tracer experiments, we investigated spatial patterns of diazotroph assemblage structure and N2 fixation rates within the temperate coastal waters of southern Australia during Austral autumn and summer. Relative to previous studies in open ocean environments, including tropical northern Australia, and tropical and temperate estuaries, our results indicate that high rates of N2 fixation (10–64 nmol L−1 d−1) can occur within the large inverse estuary Spencer Gulf, while comparatively low rates of N2 fixation (2 nmol L−1 d−1) were observed in the adjacent continental shelf waters. Across the dataset, low concentrations of NO3/NO2 were significantly correlated with the highest N2 fixation rates, suggesting that N2 fixation could be an important source of new N in the region as dissolved inorganic N concentrations are typically limiting. Overall, the underlying diazotrophic community was dominated by nifH sequences from Cluster 1 unicellular cyanobacteria of the UCYN-A clade, as well as non-cyanobacterial diazotrophs related to Pseudomonas stutzeri, and Cluster 3 sulfate-reducing deltaproteobacteria. Diazotroph community composition was significantly influenced by salinity and SiO4 concentrations, reflecting the transition from UCYN-A-dominated assemblages in the continental shelf waters, to Cluster 3-dominated assemblages in the hypersaline waters of the inverse estuary. Diverse, transitional diazotrophic communities, comprised of a mixture of UCYN-A and putative heterotrophic bacteria, were observed at the mouth and southern edge of Spencer Gulf, where the highest N2 fixation rates were observed. In contrast to observations in other environments, no seasonal patterns in N2 fixation rates and diazotroph community structure were apparent. Collectively, our findings are consistent with the emerging view that N2 fixation within temperate coastal waters is a previously overlooked dynamic and potentially important component of the marine N cycle.

Concentrations of some legacy pollutants have increased in South Australian bottlenose dolphins from 1989 to 2014

Information about pollution and its potential impact in Australian marine wildlife is scarce. To fill this knowledge gap, our study investigated concentrations of legacy pollutants as well as naturally produced methoxylated polybrominated diphenyl ethers (MeO-PBDEs) in blubber, liver, kidney and muscle of Indo-Pacific bottlenose dolphins (Tursiops aduncus) from two large inverse estuaries in South Australia from 1989 to 1995 and 2009–2014. Our results show that concentrations of most pollutant classes are relatively low compared to the literature but at the higher end of the ranges reported for marine mammals in Australia. Results for some individuals exceed toxicity thresholds indicative of immunotoxicity in marine mammals. It is important to note that concentrations of some compound classes, particularly PBDEs and polychlorinated biphenyls (PCBs), increased over a time interval of 20 years thereby placing more individuals at risk in recent years. Some of the highest concentrations of persistent organic pollutants (POPs) were measured in juveniles, which may jeopardize their development and the success of future generations. These results indicate that legacy pollutants may play a role in the long-term health of T. aduncus and should be included in biomonitoring efforts.

Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery?

In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968–1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1–10 km downwards from the upstream estuary entry, during the dry season, to a location 40–70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 and 26,500 km² respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary.

Seasonal occurrence and site fidelity of juvenile bronze whalers (Carcharhinus brachyurus) in a temperate inverse estuary

Assessing the movements and habitat use of wide-ranging predators is critical for understanding their role within an ecosystem and vulnerability to anthropogenic threats. The bronze whaler (Carcharhinus brachyurus) is a large-bodied, slow-growing shark species that is widely distributed throughout temperate Australian waters. Currently, habitat use, spatio-temporal presence and fine-scale movement information is unknown for Southern Australian waters. This study investigated the spatio-temporal patterns of occurrence, habitat use, and residency of bronze whalers in Gulf St. Vincent, South Australia. Forty-seven predominately small juvenile bronze whalers (83% of tagged sharks < 150 cm TL) were tagged with acoustic transmitters from 2009 to 2014 and monitored for periods of up to 55 months (range 516–1634 days; mean ± standard error 825 ± 47). The seasonal presence of bronze whalers peaked in spring–early autumn (September–April). Site fidelity was high, with 77% of tagged sharks detected over multiple years and 36% detected over 3 years. Low estimates of mean residency index (Ri = 0.05 ± 0.01) indicated that time spent within the array was limited. Shark occurrence was driven by a combination of factors, which included water temperature, season, moon illumination, and wind speed. Manly–Chesson’s Electivity Index showed bronze whalers exhibited the highest affinity to seagrass (Posidonia spp.), suggesting that this habitat plays an important role in the early life history stages. The slow growth and low productivity of bronze whalers combined with its predictable nearshore occurrence result in this species having an increased susceptibility to anthropogenic threats, which highlights the need for continued monitoring and vulnerability assessments.

Mollusks associated with the Avicennia schaueriana (Magnoliophyta: Acanthaceae) roots of a hypersaline estuary in northeastern Brazil

Abstract Aim Avicennia schaeuriana is one of the six species that composes the mangrove forests of Brazil and form mangrove fringes along waterways. The mangrove-roots of the A. schaeuriana act as one of the microhabitats used by the invertebrate biodiversity of the estuarine ecosystem. The present note aimed to inventory the malacofauna associated with roots of Avicennia schaueriana of the hypersaline and inverse estuary, highlighting variations in abundance and diversity along a salinity gradient and different seasonal periods. Methods Three sampling stations were delineated along the course of the estuary. At each station 3 transects of 10 meters were placed, and for each of them 3 sample points were selected, and at each point 3 samples of roots of A. schaueriana were obtained. A total of 108 samples were stored in plastic bags containing 4% formaldehyde and subsequently passed through a screening process, with identification of the associated specimens with the lowest possible taxonomic level. Results A total of 171 specimens was recorded, belonging to 10 species, 7 families, and 9 genera of the Bivalvia and Gastropoda classes. The abundance of the specimens differed between the seasonal periods analyzed. Conclusions The highest species richness and abundance of organisms in the rainy period allows inferences about the importance of the estuarine water cycle in regulating the biodiversity of these environments. Thus, the importance of the maintenance and protection of the mangrove forests composed of A. schaueriana, as well as the need for more in-depth studies becomes evident.

Temperature, salinity and water-age variations in a tidal creek network, Bushehr Port, Iran

The spatio-temporal variability of temperature and salinity was studied for a creek network and its adjacent coastal waters along the northwestern coast of the Persian Gulf during warm and cold months. Salinity variations and tidal fluctuations were found to be out of phase throughout the creek. Temperature variations at the creek were exhibit a direct correlation with tidal fluctuations during cold months and were inversely related during warm months. The creek water was colder (warmer) than offshore water during the cold (warm) season. The salinity values observed inside this inverse estuary were higher during the warm season than the corresponding values during the cold season due to a change in evaporation rates; while the open water salinity had an opposite pattern. Using salinity as a tracer, the water-age (WA) was calculated, which shows almost linear increase from the mouth to the head. The maximum WA increases from ~10 days in winter to ~30 days in summer due to the corresponding increase in longitudinal salinity gradient. Based on the calculated non-dimensional Peclet number, the diffusion process is more rapid than the advection process in this water body, especially during the cold season.

Introgression between ecologically distinct species following increased salinity in the Colorado Delta- Worldwide implications for impacted estuary diversity.

We investigate hybridization and introgression between ecologically distinct sister species of silverside fish in the Gulf of California through combined analysis of morphological, sequence, and genotypic data. Water diversions in the past century turned the Colorado River Delta from a normal estuary to a hypersaline inverse estuary, raising concerns for the local fauna, much of which is endangered. Salinity differences are known to generate ecological species pairs and we anticipated that loss of the fresher-water historic salinity regime could alter the adaptive factors maintaining distinction between the broadly distributed Gulf-endemic Colpichthys regis and the narrowly restricted Delta-endemic Colpichthys hubbsi, the species that experienced dramatic environmental change. In this altered environmental context, these long-isolated species (as revealed by Cytochrome b sequences) show genotypic (RAG1, microsatellites) evidence of active hybridization where the species ranges abut, as well as directional introgression from C. regis into the range center of C. hubbsi. Bayesian group assignment (STRUCTURE) on six microsatellite loci and multivariate analyses (DAPC) on both microsatellites and phenotypic data further support substantial recent admixture between the sister species. Although we find no evidence for recent population decline in C. hubbsi based on mitochondrial sequence, introgression may be placing an ancient ecological species at risk of extinction. Such introgressive extinction risk should also pertain to other ecological species historically sustained by the now changing Delta environment. More broadly, salinity gradient associated ecological speciation is evident in silverside species pairs in many estuarine systems around the world. Ecological species pairs among other taxa in such systems are likely poorly understood or cryptic. As water extraction accelerates in river systems worldwide, salinity gradients will necessarily be altered, impacting many more estuary and delta systems. Such alteration of habitats will place biodiversity at risk not only from direct effects of habitat destruction, but also from the potential for the breakdown of ecological species. Thus, evolutionary response to the anthropogenic alteration of salinity gradients in estuaries merits investigation as the number of impacted systems increases around the globe, permitting parallel study of multiple systems, while also permitting a conservation management response to help preserve this little championed component of biodiversity.

Using oocyte essential fatty acid composition to assess spawner reproductive potential under hypersaline conditions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 584:199-212 (2017) - DOI: https://doi.org/10.3354/meps12366 Using oocyte essential fatty acid composition to assess spawner reproductive potential under hypersaline conditions Julian Döring*, Werner Ekau Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstraße 6, 28359 Bremen, Germany *Corresponding author: julian.doering@me.com ABSTRACT: Inversion of estuaries in the dry tropics (i.e. salinity increases upstream) against the backdrop of a globally changing climate is forcing migratory clupeid fish species to adapt quickly to modifications in their spawning habitats to ensure reproductive success. Hatching success and survival probabilities of marine fish eggs and early life stages are prone to impairment under high temperature and elevated salinity conditions. Essential fatty acids affect osmoregulation in marine fish early life stages, and deficiencies can therefore be expected to cause elevated offspring mortality under hyperosmotic conditions, ultimately modifying a stock’s reproductive potential. Here we show that oocytes spawned by bonga shad Ethmalosa fimbriata inside a hypersaline estuary exhibited significantly higher 20:4 (n-6) proportions (1.6 ± 0.7% of total fatty acids) than oocytes spawned under marine conditions (0.6 ± 0.2%). Further, oocyte proportions in 20:4 (n-6), 20:5 (n‑3), and 22:6 (n-3) were positively correlated with water temperature, water salinity, and female reproductive investment (relative batch fecundity, gonado-somatic index, oocyte dry weight). Oocytes spawned during high temperature/high salinity conditions inside the estuary are therefore likely to develop normally under the prevailing conditions. Reproductive potential was determined to be highest in females spawning inside the estuary at the end of the wet season, when temperatures are high and salinities are less stressful due to freshwater input. Results lead to the conclusion that migratory clupeid fishes can maintain their reproductive potential in hypersaline estuaries and lagoons, which potentially allows for sustainable stock sizes. KEY WORDS: Arachidonic acid · Clupeidae · Fatty acids · Maternal effects · Salinity · Temperature · Inverse estuary · Reproductive investment Full text in pdf format Correction PreviousNextCite this article as: Döring J, Ekau W (2017) Using oocyte essential fatty acid composition to assess spawner reproductive potential under hypersaline conditions. Mar Ecol Prog Ser 584:199-212. https://doi.org/10.3354/meps12366 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 584. Online publication date: December 07, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

High site fidelity and restricted ranging patterns in southern Australian bottlenose dolphins.

Information on site fidelity and ranging patterns of wild animals is critical to understand how they use their environment and guide conservation and management strategies. Delphinids show a wide variety of site fidelity and ranging patterns. Between September 2013 and October 2015, we used boat‐based surveys, photographic identification, biopsy sampling, clustering analysis, and geographic information systems to determine the site‐fidelity patterns and representative ranges of southern Australian bottlenose dolphins (Tursiops cf. australis) inhabiting the inner area of Coffin Bay, a highly productive inverse estuary located within Thorny Passage Marine Park, South Australia. Agglomerative hierarchical clustering (AHC) of individuals’ site‐fidelity index and sighting rates indicated that the majority of dolphins within the inner area of Coffin Bay are “regular residents” (n = 125), followed by “occasional residents” (n = 28), and “occasional visitors” (n = 26). The low standard distance deviation indicated that resident dolphins remained close to their main center of use (range = 0.7–4.7 km, X ± SD = 2.3 ± 0.9 km). Representative ranges of resident dolphins were small (range = 3.9–33.5 km2, X ± SD = 15.2 ± 6.8 km2), with no significant differences between males and females (Kruskal–Wallis, χ2 = 0.426, p = .808). The representative range of 56% of the resident dolphins was restricted to a particular bay within the study area. The strong site fidelity and restricted ranging patterns among individuals could be linked to the high population density of this species in the inner area of Coffin Bay, coupled with differences in social structure and feeding habits. Our results emphasize the importance of productive habitats as a major factor driving site fidelity and restricted movement patterns in highly mobile marine mammals and the high conservation value of the inner area of Coffin Bay for southern Australian bottlenose dolphins.

Spatiotemporal variation of Van der Burgh's coefficient in a salt plug estuary

Abstract. Salt water intrusion in estuaries is expected to become a serious global issue due to climate change. Van der Burgh's coefficient, K, is a good proxy for describing the relative contribution of tide-driven and gravitational (discharge-driven and density-driven) components of salt transport in estuaries. However, debate continues over the use of the K value for an estuary where K should be a constant, spatially varying, or time-independent factor for different river discharge conditions. In this study, we determined K during spring and neap tides in the dry (&lt; 30 m−3 s−1) and wet (&gt; 750 m−3 s−1) seasons in a salt plug estuary with an exponentially varying width and depth, to examine the relative contributions of tidal versus density-driven salt transport mechanisms. High-resolution salinity data were used to determine K. Discharge-driven gravitational circulation (K ∼ 0.8) was entirely dominant over tidal dispersion during spring and neap tides in the wet season, to the extent that salt transport upstream was effectively reduced, resulting in the estuary remaining in a relatively fresh state. In contrast, K increased gradually seaward (K ∼ 0.74) and landward (K ∼ 0.74) from the salt plug area (K ∼ 0.65) during the dry season, similar to an inverse and positive estuary, respectively. As a result, density-driven inverse gravitational circulation between the salt plug and the sea facilitates inverse estuarine circulation. On the other hand, positive estuarine circulation between the salt plug and the river arose due to density-driven positive gravitational circulation during the dry season, causing the upstream intrusion of high-salinity bottom water. Our results explicitly show that K varies spatially and depends on the river discharge. This result provides a better understanding of the distribution of hydrographic properties.

Ethmalosa fimbriata (Bowdich 1825), a Clupeid Fish That Exhibits Elevated Batch Fecundity in Hypersaline Waters

Little is known about the concerted influence of temperature and salinity on the fecundity of clupeid fishes. Due to a globally changing climate, both physical parameters might act as stressors, severely affecting the reproductive potential of clupeid fish populations inhabiting tropical estuaries. Differences in relative batch fecundities, the gonado-somatic index, and the condition index of bonga shad (Ethmalosa fimbriata) were analysed in individual females sampled at the Senegalese coast and inside the inverse Sine Saloum estuary, where salinity increases upstream in all seasons. Multiple linear regression models on fecundity and gonadal energy storage show that clupeids can adapt towards increasing their reproductive investment at temperatures (26–30 °C) and salinities (42–51), which by far exceed marine conditions, in an effort to maximize recruitment success. This reproductive strategy, however, is accompanied by a trade-off between reproductive effort and somatic growth, which ultimately limits the species’ reproductive potential inside the estuary. The observed high variability in batch fecundities might be a viable mechanism to adjust to fluctuating and rather extreme environmental conditions. Understanding the spawning biology of exploited clupeid fishes in drastically changing environments is crucial for evaluating the reproductive potential of stocks at the outer reach of their physiological performance curve.

Composition and structure of the larval fish community related to environmental parameters in a tropical estuary impacted by climate change

Mangrove ecosystems have long been considered essential habitats and are commonly viewed and referred to as “nursery areas”. They are highly sensitive to climate change, and environmental transformations in these ecosystems are expected. The Sine Saloum estuary is a case of a system affected by global climate change where reduced precipitation and temperature increase have resulted in an inversion of the salinity gradient. Within the estuary, the composition and structure of the larval fish community related to environmental parameters were investigated using neuston and ring trawl nets. Larval fishes were sampled at 16 stations distributed along a salinity and distance-to-the-sea gradient during four field campaigns (November 2013, February, June, and August 2014) covering an annual cycle. This is the first study documenting the spatial and temporal assemblages of fish larvae in an inverse estuary. The total of 41 taxa representing 24 families and 34 genus identified in this study was lower than that of other tropical estuaries. Clupeidae spp. was the dominant taxon, accounting for 28.9% of the total number of fish larvae caught, followed by Gerreidae spp. (21.1%), Hyporamphus picarti (18.8%), Diplodus bellottii (8.9%), Hypleurochilus langi (4.8%), Mugilidae spp. (4.4%), and Gobiidae sp.1 (3.5%). A total of 20 taxa were recorded within the upper estuary region, whereas 29 and 37 taxa were observed in the middle and lower reaches, respectively. While larval fish were captured at all sites and during all seasons, abundances and richness decreased with increasing salinity. Larval fish assemblages also showed a clear vertical structure corresponding to three distinct water strata. Salinity, water temperature, and dissolved oxygen were the variables that best explained the spatial and temporal differences in larval fish assemblages. It is difficult to forecast the future situation for this system but so far, compared to other mangrove estuarine systems, we have observed the loss of freshwater species in favour of species of marine origin. The information provided in the present study is a contribution to the knowledge of tropical biodiversity and modifications of the ichthyoplankton communities in the context of climate change and future green fund action.

Demography of southern Australian bottlenose dolphins living in a protected inverse estuary

Abstract Assessments of demographic parameters are essential to understand the dynamics of wild populations, and for their efficient conservation and management. Here, sex‐specific abundance, apparent survival and temporary emigration of southern Australian bottlenose dolphins (Tursiops cf. australis) in Coffin Bay (CB), South Australia, is investigated. Results are based on capture–recapture modelling of photo‐identification data and molecular analyses of biopsy samples collected during boat‐based surveys between September 2013 and October 2015 in the inner and outer areas of CB. The total super‐population of dolphins (including calves) using the entire study area (263 km2) was estimated with POPAN models at 306 (95% CI: 291–323), which included 71 (68–73) marked females and 57 (55–60) marked males. Seasonal estimates of abundance for the inner area of CB (123 km2) obtained with Pollock's Closed Robust Design models remained relatively constant over the two years (marked females: 52–60, marked males: 46–52, and total: 193–209). The high density of dolphins inhabiting the inner area (seasonal range: 1.57–1.70 individuals km−2), high apparent survival rates estimated for both sexes (females: 0.99; 95% CI: 0.96–1.0; males: 0.95; 0.82–0.99), and low temporary emigration rates (0.02; 95% CI: 0.01–0.11) indicate that the inner area of CB offers highly favourable habitat for these dolphins. High biological productivity and low predation risk may promote these demographic patterns in the inner area of CB. This study provides a robust baseline of sex‐specific population demographics of southern Australian bottlenose dolphins with important implications for future research and their management and conservation in South Australia.

Diet overlap and resource partitioning among three forage fish species in Coorong, the largest inverse estuary in Australia

Protracted drought in southern Australia has degraded the system function of the Coorong estuary and intensified resource competition among forage fish species. The present study investigates the gut content, prey composition and dietary overlap of three forage fishes: smallmouth hardyhead (Atherinosoma microstoma), Tamar River goby (Afurcagobius tamarensis) and sandy sprat (Hyperlophus vittatus) influenced by environmental variation in the Murray Estuary and Coorong. The prey species identified in fish stomachs were dominated by crustaceans (amphipods, ostracods and harpacticoids), but nematodes and acanthocephalans were also common in all forage fishes. The diet of the sandy sprat and Tamar River goby highly overlapped (α = 0.8) in the Murray Estuary and all three forage fishes showed potential diet overlap (α ≥ 0.6) in the North Lagoon. Spatiotemporal variation of prey diversity was observed in smallmouth hardyhead while temporal variation of prey diversity was observed in sandy sprat and Tamar River goby. Overall, the prey abundance was temporally variable and predominantly regulated by salinity, pH, dissolved oxygen, water transparency and chlorophyll a in the Murray Estuary and Coorong. This study adds to our knowledge on dietary overlap and resource partitioning among small-bodied forage fishes mediated by environmental factors in the Murray Estuary and Coorong.

Estuary-type circulation as a factor sustaining horizontal nutrient gradients in freshwater-influenced coastal systems

Estuary-type circulation is a residual circulation in coastal systems with horizontal density gradients. It drives the accumulation of suspended particulate matter in coastal embayments where density gradients are sustained by some freshwater inflow from rivers. Ebenhoh et al. (Ecol Model 174(3):241–252, 2004) found that shallow water depth can explain nutrient gradients becoming established towards the coast even in the absence of river inflow. The present study follows their concept and investigates the characteristic transport of organic matter towards the coast based on idealised scenarios whereby an estuary-type circulation is maintained by surface freshwater fluxes and pronounced shoaling towards the coast. A coupled hydrodynamical and biogeochemical model is used to simulate the dynamics of nutrient gradients and to derive budgets of organic matter flux for a coastal transect. Horizontal nutrient gradients are considered only in terms of tidal asymmetries of suspended matter transport. The results show that the accumulation of organic matter near the coast is not only highly sensitive to variations in the sinking velocity of suspended matter but is also noticeably enhanced by an increase in precipitation. This scenario is comparable with North Sea conditions. By contrast, horizontal nutrient gradients would be reversed in the case of evaporation-dominated inverse estuaries (cf. reverse gradients of nutrient and organic matter concentrations). Credible coastal nutrient budget calculations are required for resolving trends in eutrophication. For tidal systems, the present results suggest that these calculations require an explicit consideration of freshwater flux and asymmetries in tidal mixing. In the present case, the nutrient budget for the vertically mixed zone also indicates carbon pumping from the shelf sea towards the coast from as far offshore as 25 km.

Tidal asymmetry in a tidal creek with mixed mainly semidiurnal tide, Bushehr Port, Persian Gulf

This study investigated the tidal asymmetry imposed by both the interaction of principal tides and the higher harmonics generated by distortions within a tidal creek network with mixed mainly semidiurnal tide in the Bushehr Port, Persian Gulf. Since velocity and water-level imposed by principal triad tides K1-O1-M2 are in quadrature, duration asymmetries during a tidal period in this short, shallow inverse estuary should be manifest as skewed velocities. The principal tides produce periodic asymmetries including a strong ebb-dominance and a weak flood-dominance condition during spring and neap tides respectively. The higher harmonics induced by nonlinearities engender a flood-dominance condition where the convergence effects are higher than frictional effects, and an ebbdominance condition where intertidal storage are extended. Since the triad K1-O1-M2 driven asymmetry is not overcome by higher harmonics close to the mouth, the periodic asymmetry dominates within the creek in which higher harmonics reinforce the weak flood-dominance (strong ebb-dominance) condition in the convergent channel (divergent area). Also, the maximum flood and the maximum ebb from all harmonic constituents occurred close to high water slack time during both spring and neap tides in this short creek. Since occational wetting of intertidal areas happened close to the high water (HW) time during spring tide, the water level flooded slowly close to the HW time of the spring tide.