Major Estuaries Research Articles

Long-Term Changes in the Land–Ocean Ecological Environment in Small Island Countries in the South Pacific: A Fiji Vision

Small island countries in the South Pacific are ecologically fragile areas, vulnerable to climate change, and the long-term ecological changes in the sea and land have an important impact on their sustainable development. This study takes Fiji, a typical small island country in the South Pacific, as an example, to analyze the change and connection of marine and terrestrial ecosystem environments based on 30 years of multi-source, satellite, remote-sensing data. From 1991 to 2019, according to the change in forest area in Fiji, three stages were delineated: first was a period of stability, then a decrease, and then a recovery in recent years. From 1991 to 2002, Fiji’s vegetation accounted for 73% of the total area; sea environment surrounding the islands, such as sea level height and sea surface temperature, were relatively low, with high water transparency. From 2002 to 2014, with the development of forestry and tourism, vegetation decreased by 6.89% and bare land increased, which changes the runoff erosion in the drainage basin; correspondingly, the chlorophyll a concentration in three major estuaries was found to be slightly increased with low water transparency. Meanwhile, coupled with the rising sea temperature, the area of Fiji’s coral reefs shrank significantly, with 51.13% of the total loss of coral reefs occurring in the Vanua Levu, where bare land and runoff were more distributed in its drainage basin. From 2014 to 2019, Fiji’s vegetation and coral reef areas recovered from the former stage; affected by short-term climate oscillations such as El Niño-Southern Oscillation (ENSO), the sea surface temperature showed a significant abnormal drop and the water transparency decreased. In the past 30 years (1993–2018), the sea level rise rate around Fiji reached 4 mm/year, and the temperature increased by 0.3 °C, which threatens the coastal ecosystem environment, including coral reefs and mangrove; inappropriate land-use change would worsen the situation in these ecologically fragile areas.

Diversity and Abundance of Phytoplankton in the Coastal Waters of South Sulawesi

Phytoplankton are primary producers that can be used as seawater condition indicators. Certain phytoplankton can proliferate, causing harmful algal blooms (HABs). The coastal waters of South Sulawesi, Indonesia are under pressure from land-based processes and activities resulting in inputs of organic and inorganic materials. This study analysed phytoplankton diversity and abundance in coastal waters around South Sulawesi. Phytoplankton were sampled and seawater parameters (salinity, temperature, turbidity, pH, nitrate concentration) measured in-situ at six stations around seven major river estuaries in three seaways (Makassar Strait, Flores Sea, Gulf of Bone). Phytoplankton taxonomic composition, abundance and indices of diversity (H’), evenness (E), and dominance (D) were analysed. Phytoplankton from 31 species and three classes (Bacillariophyceae, Cyanophyceae, Dinophyceae) were identified. Phytoplankton abundance and community structure differed significantly between sites and seaways but were not significantly correlated with water quality parameters although Dinophyceae abundance correlated significantly with observed pollution levels. Phytoplankton abundance was strongly influenced by the Dinophyceae, especially Ceratium furca, a potential HAB species; Cyanophyceae had the strongest influence on species richness but least on community structure. C. furca abundance was strongly correlated negatively with species richness, H’ and E, and positively with D, indicating negative impacts of this species on phytoplankton communities.

Long‐term study reveals top‐down effect of crabs on a California salt marsh

AbstractConsumers can structure plant communities and may function as keystone species or ecosystem engineers. In salt marshes, the prevailing paradigm has shifted in recent decades from nearly complete focus on bottom‐up processes to inclusion of top‐down effects. Although the number of studies investigating top‐down control continues to climb, few experiments span multiple years, so temporal variability in or long‐term impacts of consumers have not been well characterized. In addition, while top‐down control has been found to be common in Western and Eastern Atlantic and Western Pacific salt marshes, our study is one of the first to experimentally consider top‐down control of salt marsh plants in the Eastern Pacific. We conducted a five‐year field experiment along eroding creekbank edges of a California salt marsh in which we manipulated densities of the shore crab, Pachygrapsus crassipes, and tracked marsh responses over time. Our results demonstrate that, through both consumption and engineering activities, this superabundant crab is regulating marsh vegetation and soil structure. Experimentally reducing crab abundance enhanced vegetation biomass and sediment bulk density. Moreover, root biomass and bulk density—factors known to increase marsh resilience to erosion and sea‐level rise—decreased linearly with increasing burrow density. Our long‐term study uniquely revealed that burrows can persist for years after crab abundances are reduced and that plant responses from grazer exclusions gradually strengthen over time, likely due to the relatively slow growth of woody perennial foundational plants. Since shore crabs are abundant throughout the marsh in most major estuaries within the range of the species (from Baja California, Mexico to Oregon, USA), we hypothesize that this species is exerting significant, yet underappreciated top‐down control and modifying the sediment properties of many West Coast salt marshes.

Genome-wide analysis reveals regional patterns of drift, structure, and gene flow in longfin smelt (Spirinchus thaleichthys) in the northeastern Pacific

The southernmost stock of longfin smelt (Spirinchus thaleichthys) is approaching extirpation in the San Francisco Estuary (SFE); however, patterns of genetic structure, diversity and gene flow which are vital for management are poorly understood in this species. Here, we use genome-wide data to evaluate population structure of longfin smelt across a broad latitudinal scale across estuaries ranging from the SFE to Yakutat Bay and Lake Washington, and fine scale within the Fraser River and the SFE. Results indicate high genetic structure between major estuaries, fine-scale structure within the Fraser River, and low levels of structure within the SFE. Genetic structure was more pronounced between northern estuaries whereas southern estuaries showed shared ancestry and ongoing gene flow, most notably unidirectional northward migration out of the SFE. Furthermore, we detected signatures of local adaptation within the Fraser River and the Skeena River estuaries. Taken together, our results identify broad patterns of genetic diversity in longfin smelt shaped by co-ancestry, unidirectional migration and local adaptation. Results also suggest that the SFE population is genetically distinct from northernmost populations and an important source for maintaining nearby populations.

Morphological Changes along the shoreline of the arcuate Niger Delta from Parts of Delta State to Akwa Ibom State between 1986 and 2016

This study analyzes the morphological changes occurring at the arcuate Niger Delta shoreline by identifying factors that contribute to erosion-induced shoreline changes and the rate of shoreline change from Forcados (Delta State) to Ibeno (Akwa Ibom State). Shoreline changes were compared from 1986 to 2016 as well as among western, central and eastern sections of the arcuate Niger Delta using Landsat satellite imagery within Geographic Information System (GIS) environment. In the assessment of the entire shoreline, accretion was 5,477.814Ha (1.2%) in 1986 and 19181.53Ha (4.1%) in 2016. For erosion, it was 48,400.03 (10.3%) in 1986 and 68398.84Ha (14.5%) in 2016. This indicates that accretion increased by 2.91% and erosion also increased by 4.25% in 30 years. The major accretion was detected within Brass (Bayelsa State), Bonny (River State), Andoni (River State) and Ibeno (Akwa Ibom State). Forcados South Point (Delta State), Sombreiro River mouths and eastern Obolo recorded the highest level of shoreline erosion. For land use/land cover (LULC), vegetation was 27.4% in 1986 but reduced to 22.9% in 2016; a change I attributed to massive conversion of vegetative cover for anthropogenic activities in the area. The study concluded that major erosion cases are due to the changing climate and the velocity of flow of major estuaries into the ocean. The activities that contribute to the shoreline change aside the challenging natural factors are sand mining, hard and engineered structures on the shore and other commercial and social activities. The occurrences of shoreline erosion have contributed to the degradation of these coastal ecosystems, loss of human settlements and livelihood. It is therefore essential to implement precautionary measures to mitigate the risk of shoreline erosion within this sensitive zone

The Estuarine Ecological Knowledge Network: Future Prospects

Abstract Estuaries are profoundly rich, diverse, and complex ecosystems, and crucial to the overall health of Earth's oceans. Estuarine ecological complexity is matched by tremendous human cultural diversity. In the United States, millions of people live in estuarine environments from the Gulf of Mexico to the Arctic—many of whom directly depend on the productivity of marine resources in both commercial and subsistence fishing activities. Yet, estuaries are also among Earth's most threatened landscapes against the backdrop of global warming, sea-level rise, agricultural and industrial pollution, habitat loss, overfishing, and so on. This represents a looming disaster for our oceans at a global scale. The Estuarine Ecological Knowledge Network (EEKN) is based on the idea that fishing communities living within major estuaries are the key to ensuring the health of global oceans. Coastal fishing communities have vast accumulations of ecological knowledge about the functioning of estuarine ecosystems and interact with those ecosystems in intimate ways on a daily basis. This network is designed to connect coastal communities in monitoring the health of estuarine ecosystems and in using traditional ecological knowledge to develop strategies for enhancing ecosystem health and resilience.

Hydrodynamic variability and nutrient status structuring the mesozooplankton community of the estuaries along the west coast of India.

The influence of distinct tidal characteristics and nutrient status on mesozooplankton community was studied in six major estuaries along the west coast of India during the late-monsoon (MS) and post-monsoon (PM) periods. The macro-tidal estuaries in the north (Amba and Thane) exhibited higher nutrient concentration compared to the micro- and meso-tidal estuaries located in the south (Cochin and Nethravati) and central (Zuari and Mandovi) west coast of India. The markedly higher nitrate and phosphate levels in the macro-tidal estuaries during PM indicated anthropogenic contributions from domestic and industrial effluents, which significantly impacted the mesozooplankton community structure. Nutrient enrichments favored higher phytoplankton standing stock leading to low DO levels. In the micro- and meso-tidal estuaries, meso- and euryhaline copepods dominated whereas in the macro-tidal estuaries, the copepod community was dominated by euryhaline and coastal species. Furthermore, the high-saline eutrophic environment of macro-tidal estuaries formed congenial for the increased jellyfish preponderance during PM. The predation pressure exerted by the jellyfish population on the crustacean zooplankton and ichthyoplankton exerted an adverse impact on the potential fishery stock in the macro-tidal estuaries. Thus, the study reveals that the nutrient enrichment favoring a shift in the mesozooplankton community structure from nutritionally superior crustacean plankton to less desirable jellyfishes, which in turn, may lead to a threat on the estuarine pelagic energy transfer and ecosystem deliverables.

Nekton community dynamics within active and inactive deltas in a major river estuary: potential implications for altered hydrology regimes

High fisheries production within estuaries is associated with coastal upwelling, tidal mixing, and land-based runoff facing increasing impacts from climate and human activities. Active river deltas receive large riverine inflows compared to inactive river deltas, providing contrasting estuaries to compare impacts of river inflow on estuarine nekton. We quantified nekton assemblages and stable isotopes (δ13C, δ15N) of commercially important blue crab Callinectes sapidus Rathbun, 1896 within an active and inactive delta in coastal Louisiana to explore the impacts of differing riverine inflow. Crustaceans dominated estuarine assemblages, differing only by season and not delta type, with summer and fall supporting highest densities. Fish density and assemblages differed by the interaction of season and delta due to differences during the 2019 record high spring river inflow. During this period, the active delta supported reduced fish densities and richness compared to the inactive delta. Nekton densities across deltas and seasons reflect a combination of species life history characteristics and habitat conditions. The high spring river discharge in 2019 impacted habitat availability (reduced presence of submerged aquatic vegetation), water conditions (decreased temperature and salinity), and potentially displaced nekton to unsampled habitat areas (i.e. interior marsh surface) within the active delta. While differences in nekton density and assemblages were only evident during the high spring river discharge, δ15N values of blue crabs were approximately 1.5 times higher in the active delta, potentially indicating more terrestrial influence. Understanding how altered inflow impacts environmental variables supporting estuarine nekton production remains critical for supporting management within these hydrologically managed regions.

Sea level rise inducing tidal modulation along the coasts of Bengal delta

The Bengal delta, the largest delta on the Earth, is subject to a marked coastal flooding hazard and associated with widespread vulnerability. The situation will expectedly deteriorate in the ongoing context of sea level rise. This sea level rise will not only have a direct effect on the coastal flooding, but will also have indirect effects, through the alteration of the coastal hydrodynamics. In the present study, we investigate the impact of sea level rise on tide, which is the largest source of variability of sea level along the macro-tidal coast of Bengal delta. Through a comprehensive modelling framework comprising the coastal delta, major estuaries, as well as the intricate hydraulic network of the delta, we assess the future changes of tidal properties to be expected for various sea level rise scenarios, representative of the end of the 21st century and beyond. It is found that the effect is large, and regionally dependent. Over both the south-western and south-eastern parts of the delta, the amplitude of the tide is expected to increase when the sea level is higher, which is bound to aggravate the tidal flooding hazard. In contrast, the central part of the delta will potentially experience massive flooding of river banks and adjoining lands in the scenarios exceeding 0.5 m of sea level rise. Consequently, this flooding induces a decay of the tidal amplitude in the central part. Our study shows that the tidal modulation is a significant ingredient that needs to be accounted for in the evolution of the future hydrodynamics of the Bengal delta. The friction-dominated and regionally contrasting damping and amplification mechanism also underscores the potential application of managed realignment strategy for a sustainable delta management in the future.

Inter-estuarine Variation in Otolith Chemistry in a Large Coastal Predator: a Viable Tool for Identifying Coastal Nurseries?

Coastal estuaries provide essential juvenile habitat for many commercially and recreationally important fish, which may move between estuarine and coastal environments throughout their life. Identifying the most important estuarine nurseries that contribute to the broader stock can support targeted management of juvenile and spawning populations. The objective of this study was to (1) compare chemical fingerprints within sagittal otoliths of juvenile Mulloway (Argyrosomus japonicus) sampled from putative south-eastern Australian nurseries, (2) assess their potential as natural tags to distinguish nursery grounds for the broader coastal Mulloway stock and (3) assess the viability of otolith chemistry as a fisheries management tool when limited to opportunistic, fisheries-dependant, otolith sample collection from by-catch. Otoliths from juvenile Mulloway (0 to 3 years, 4 to 44.8 cm total length) were obtained from 8 major estuaries and 2 inshore ocean locations along coastal south-eastern New South Wales, Australia, from April 2015 to July 2018. Concentrations of Sr, Ba, Mg, Mn and Li in the otolith region corresponding to the juvenile nursery stage were determined using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The element to Ca ratios of fish from coastal estuaries differed significantly among collection areas, based upon multivariate elemental fingerprints, with some exceptions. When the otoliths of fish were analysed in a multinomial logistic regression (MLR) classifier, there was an overall mean allocation success of 59% to the estuary of capture. This study highlights the use of otolith ‘fingerprints’ as natural tags in Mulloway, and contributes to progressive research in environmental reconstruction applications of otolith chemistry.

Bacterioplankton abundance and community structure during post-monsoon in mangrove dominated estuaries of the Indian Sundarbans; An insight to biogeochemical processes

Bacterioplankton abundances in three major estuaries of the Indian Sundarbans (Saptamukhi, Thakuran and Matla) were determined by flow cytometry and fluorescence microscopy in a post-monsoonal study followed by microbial community composition (16S rRNA clone library), during the period 2012–2016. All three estuaries were oxygenated and meso - to polyhaline. They had similar levels of dissolved inorganic carbon and nutrients and were net heterotrophic, as evidenced by net negative community productivity. Mean post-monsoon bacterioplankton abundances (x 109 cells L−1) in these estuaries were: Saptamukhi (1.30 ± 0.78 × 109 cells L−1); Matla (1.38 ± 1.34 × 109 cells L−1); Thakuran, (0.92 ± 0.46 × 109 cells L1). Bacterioplankton abundance was associated with water temperature coupled with dissolved inorganic nutrients and carbonate parameters. Highest abundances were at the most upstream sites in the Saptamukhi and Matla estuaries, while a mid-estuarine peak was evident in Thakuran. In the Saptamukhi and Thakuran the bacterioplankton was dominated by Proteobacteria (mostly Deltaproteobacteria), with a contribution from Bacteroidetes in the Saptamukhi and from Cyanobacteria and Actinobacteria in the Thakuran. Despite the oxygenated nature of these estuaries, clones resembling the sulfate reducer Desulfovibrio sp. were detected, and this could be a consequence of anoxic pore water exchange across the sediment-water interface. Sulfate reduction may thus be an important pathway for carbon metabolism in these ecosystems.

The impact of Sea level rise in the guadiana estuary

Understanding the impact of sea level rise on coastal areas is crucial as a large percentage of the population live on the coast. This study uses computational tools to examine how two major consequences of sea level rise: salt intrusion and an increase in water volume affect the hydrodynamics and flooding areas of a major estuary in the Iberian Peninsula. A 2D numerical model created with the software MOHID was used to simulate the Guadiana Estuary in different scenarios of sea level rise combined with different freshwater flow rates considering varying tidal amplitudes. An increase in salinity was found in response to an increase in mean sea level in both high and low freshwater flow rates at all areas around the estuary. An increase in flooding areas around the estuary was also positively correlated with an increase in mean sea level.

Distribution, seasonality, and fluxes of dissolved organic matter in the Pearl River (Zhujiang) estuary, China

Abstract. Dissolved organic carbon (DOC) concentration in the Pearl River estuary (PRE) of China was measured in May, August, and October 2015 and January 2016. Chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the latter three seasons were characterized by absorption and fluorescence spectroscopy. CDOM and FDOM exhibited negligible seasonal variations, while DOC displayed a significant seasonality, with the average concentration being highest in May (156 µmol L−1), lowest in November (87 µmol L−1), and comparable between January (118 µmol L−1) and August (112 µmol L−1). Although DOC, CDOM, and FDOM in surface water were generally higher than in bottom water, the difference between the two layers was statistically insignificant. DOC showed little cross-estuary variations in all seasons, while CDOM and FDOM in January were higher on the west side of the estuary than on the east side. All three variables showed rapid drawdowns in the head region of the estuary (salinity <5); their dynamics in the main estuary were primarily controlled by conservative mixing, leading to linearly declining or relatively constant (for DOC in May and November only) contents with increasing salinity. The decrease in FDOM with salinity was 5 %–35 % faster than that of CDOM, which in turn was 2–3 times quicker than that of DOC. Salinity and CDOM absorption coefficients could serve as indicators of DOC in August and January. Freshwater endmembers in all seasons mainly contained fresh, protein-rich DOM of microbial origin, a large part of it likely being pollution-derived. Protein-like materials were preferentially consumed in the head region but the dominance of the protein signature was maintained throughout the estuary. Exports of DOC and CDOM (in terms of the absorption coefficient at 330 nm) into the South China Sea were estimated as 195×109 g and 266×109 m2 for the PRE and 362×109 g and 493×109 m2 for the entire Pearl River Delta. The PRE presents the lowest concentrations and export fluxes of DOC and CDOM among the world's major estuaries. DOM delivered from the PRE is, however, protein-rich and thus may enhance heterotrophs in the adjacent coastal waters. Overall, the PRE manifests lower abundance and smaller spatiotemporal variability of DOM than expected for a sizable estuary with a marked seasonality of river runoff due supposedly to the poorly forested watershed of the Pearl River, the rapid degradation of the pollution-derived DOM in the upper reach, and the short residence time of freshwater.

Field Mark-Recapture of Calcein-Stained Larval Oysters (Crassostrea virginica) in a Freshwater-Dominated Estuary

Knowledge of larval transport is important for restoration and management efforts; yet, there are no established methods to determine larval transport in situ. Calcein staining of oyster larvae may help fill this void, and a two-part study was conducted to determine its effectiveness at tracking larval oyster transport in the field. First, it was tested whether oysters could be successfully stained, survive, and grow at estuarine salinities (15, 20, 26), and at sufficiently large numbers (millions of oysters) to support field mark-recapture studies. Second, the field-based application was tested by releasing 22 million stained larvae twice (high and low salinity) into a major estuary, and two methods (fluorescent microscopy and FlowCam) were used to detect recaptured larvae. Results were compared with expected larval movement patterns simulated by an existing larval transport model. Calcein concentrations (100 mg L−1) did not affect larval growth or survival, but handling conditions (water salinity manipulations and tank size) did affect growth and survival during the post-staining period. Microscopy had double the detection capacity, but FlowCam was more practical and time efficient for the large-volume, high particulate load field samples. Larvae (n = 2) were recaptured during the second, higher salinity release, and model comparison showed a 1–2-day time-lag between field recapture and model predictions, suggesting need for model refinement. Calcein has potential to be a useful marker to track larval movement at large scales needed for field-based studies, providing critical information to aid in selection of restoration sites and management of commercially important shellfish species.

Quantitative analysis of erosion and accretion (1975–2017) using DSAS — A study on Indian Sundarbans

Sundarbans, world’s largest mangrove forest is now struggling to maintain its existence in the face of severe erosion. Any management process to protect this vulnerable ecosystem and to rescue millions of people living here, require place specific erosion measurement. This paper attempted to assess point specific erosion/accretion rate between 1975–2017 using LANDSAT Images. Permanent waterline was demarcated from the rectified images using the algorithm Land Surface Water Index (LSWI) > Normalize Differential Vegetation Index (NDVI). 178 inhabited Mouzas (smallest revenue unit) and 48 islands located along the major estuaries from Hooghly to Raimangal are taken for predicting shore/bank movement using the Digital Shoreline Analysis System (DSAS). 7723 transects along the main river and 855 along the coast are generated at an interval of 100m for calculating End Point Rate (EPR), Net Shoreline Movement (NSM) and Land net Loss/Land Gain (LL/LG). From the EPR we found 62% and 85% transects are eroding along the estuarine channels and coast respectively. Multiplying mean EPR and transect interval (100m) between two transect, inter transects net land loss or gain were calculated. Among the channels intensive erosion were observed along the eastern bank of Muriganga in all successive years. Accretion increases landward from the coast in almost all channels. A sharp difference of erosion/accretion observed between reclaimed mouzas and islands covered by dense mangrove swamp. 54% mouza having embankments are retreating at an average rate of 6.14 metre/year, contrary to this only 35% mangrove islands are moving landward at a much slower rate (4.78 m/yr.) in comparison to habitat areas. Process of new land formation is also very steady in mangrove areas (4.78 hec./yr.) than habited areas (6.84 hec./yr.) which indicates channel stability and erosion resistance provided by Mangrove. Mangrove plantation can be a useful option for erosion management.

Genetic connectivity of Fenneropenaeus indicus (H. Milne Edwards 1837) among three prawn fishing grounds of Tanzanian coastal waters

The penaeid prawn fishery of Tanzanian coastal waters occurs within three main fishing grounds referred to as zone 1, 2 and 3. The three zones are connected to major estuaries of Ruvu, Rufiji and Matandu rivers, respectively. The genetic connectivity of Fenneropenaeus indicus population in those areas had not been previously studied. The population study was performed to test the hypothesis that only a single population exists in the area, utilizing the mitochondrial DNA (mtDNA) sequence data. There were 89 sequences of F. indicus used in the analysis. Results indicated that there were 36 haplotypes and the mean haplotype diversity (h) was relatively moderate when there was a low level of nucleotide diversity (π). The FST values ranged from −0.00798 to 0.00918 indicating the absence of differentiations between localities. The Tajima’s D and Fu’s Fs statistic showed the presence of recent expansion of the population. An analysis of molecular variation (AMOVA) indicated a higher variation within a population than among populations. The study established that the populations existing in the three prawn fishing grounds were a panmictic population. Thus, the reconsideration of the geographical distribution of the fishing grounds (Zones 1–3) and extension of closed season management strategy to the artisanal fishers are recommended. Further study is recommended using nuclear markers to infer population structure in the coastal waters of Tanzania.

Massive production of heavy metals in the Ganga (Hooghly) River estuary, India: Global importance of solute-particle interaction and enhanced metal fluxes to the oceans

The Ganga River System is a major contributor to the global sediment and water discharge to the oceans. The estuary of Ganga (Hooghly) River in India is under increasing influence of anthropogenic contributions via discharge of the industrial and urban effluents. Here we document, based on the investigation of water and suspended sediment samples collected during six periods over two years, that there is extensive production of heavy metals (Co, Ni and Cu) in the estuary such that the annual dissolved fluxes of metals from the Hooghly River are enhanced by up to 230–1770%. Furthermore, the estuarine dissolved metal fluxes, when normalized with water fluxes, are the highest among estuaries of the major rivers in the world. Our simultaneous data on the dissolved, suspended particulate and exchangeable phases allow us to identify the ion-exchange process (coupled adsorption and desorption) as the dominant contributor to the generation of heavy metals in the middle and lower estuary where the estimated anthropogenic contribution is negligible. The estimated contributions from the groundwater are also insufficient to explain the measured metal concentrations in the estuary. A strong positive correlation that is observed between the dissolved heavy metal fluxes and the suspended particulate matter (SPM) fluxes, after normalizing them with the water fluxes, for estuaries of the major global rivers imply that the solute-particle interaction is a globally significant process in the estuarine production of metals. Based on this correlation that is observed for major estuaries around the world, we demonstrate that the South Asian Rivers which supply only ∼9% of the global river water discharge but carry elevated SPM load, contribute a far more significant proportion (∼40 ± 2% Ni and 15 ± 1% Cu) to the global supply of the dissolved metals from the rivers.

Mixing processes in modern estuarine sediments from the Gulf of Khambhat, western India

Sandy and muddy detritus from two distinctive end-member provenance terrains, basaltic and granitic, are transported to the estuaries of the Gulf of Khambhat by fluvial discharge and opposing tidal currents making this embayment ideal for investigating the effects of mineralogical and geochemical mixing on sediment composition in estuarine depositional systems. Rivers flowing predominantly from the north over Precambrian metasediments and granites supply a felsic plutoniclastic sand to the gulf, whilst a mafic volcaniclastic sand, sourced by Deccan basalts, is transported by rivers draining the Western Ghats and from the local coastal erosion in the southern part of the gulf.Sediments from the Tapti estuary are rich in basaltic rock fragments (∼50%) and are characterised by a distinctly basaltic source composition (La/Sc∼0.25–0.86, Th/Sc∼0.12–0.40, Cr/Th∼13–121, Eu/Eu*∼0.93–1.68 and LREE/HREE∼2.96–6.57). By contrast, the sediments of the Sabarmati inner estuary are rich in quartz grains (80%) as evident from the major and trace element compositions (La/Sc∼1.38–6.63, Th/Sc∼0.58–2.98, Cr/Th∼0.58–19.39, Eu/Eu*∼0.46–0.86 and LREE/HREE∼2.90–11.40). The detrital compositions of the remaining major estuaries, the Narmada, Dhadhar and Mahi, scatter across a broad range in a QFR diagram and contain varying amounts of quartz, feldspar and rock fragments. In the Narmada Estuary the mixed signature characterised by REE values including Eu/Eu* (∼0.42–2.28) and LREE/HREE (∼4.37–14.80) is caused by the presence of diverse rock types that crop out in the source area. In the Mahi estuary, which largely flows across felsic source terrains the compositional signature is controlled by mixing of felsic detritus from riverine input with mafic sediments from seaward side in the estuary.The two end-member detrital compositions, plutoniclastic and volcaniclastic, are mixed in the macro-tidal estuaries. Mixing of sediment takes place where fluvial currents and tidal flows interact and the proportion of mixing varies systematically from outer to central to inner estuary. A high degree of mixing of felsic sediments with Deccan basalt derived sediments takes place in the outer estuary resulting in felsic:mafic ratios of ∼40:60. Sediments in the central estuaries (∼65:35) and in the fluvial-dominated inner estuaries (∼90:10) are characterised by moderate to minor degrees of mixing. The mixing in estuaries is systematic and is reflected in both the mineralogy and geochemistry of the sediments. Provenances of ancient estuarine successions should therefore be interpreted with great care incorporating awareness of the mixing processes which influence sediment compositions to different degrees in estuaries, especially those characterised by strong tidal currents.

Differences in rocky reef habitats related to human disturbances across a latitudinal gradient

This study tested for differences in the composition of intertidal and shallow subtidal rocky reef habitats subjected to a range of human pressures across ∼1000 km of coastline in New South Wales, Australia over 5 years. Percentage covers of habitats were sampled using aerial photography and a large grain size (20 m2 intertidal; 800 m2 subtidal) in a nested hierarchical design. Results were consistent with anthropogenic impacts on habitat structure only around estuaries with the most heavily urbanised or agriculturally-intense catchments. The most convincing relationships documented here related to environmental variables such as SST, latitude, reef width and proximity to large estuaries irrespective of human disturbance levels. Moreover, there were suggestions that any influences of estuarine waters (be they anthropogenic or natural) on reef assemblages could potentially extend 10s of kilometres from major estuaries. In general, our results supported those of studies that utilised smaller grain sizes (greatest variability often at smallest spatial scales), but we found that variability over scales of 100s of km can be similar to or greater than variability over scales of 10s of metres.

Capacity of humic substances to complex with iron at different salinities in the Yangtze River estuary and East China Sea

The iron binding capacities (IBC) of fulvic acid (FA) and humic acid (HA) were determined in the salinity range from 5 to 40. The results indicated that IBC decreased while salinity increased. In addition, dissolved iron (dFe), FA and HA were also determined along the Yangtze River estuary’s increasing salinity gradient from 0.14 to 33. The loss rates of dFe, FA and HA in the Yangtze River estuary were up to 96%, 74%, and 67%, respectively. The decreases in dFe, FA and HA, as well as the change in IBC of humic substances (HS) along the salinity gradient in the Yangtze River estuary were all well described by a first-order exponential attenuation model: y(dFe/FA/HA, S) = a0 × exp(kS) + y0. These results indicate that flocculation of FA and HA along the salinity gradient resulted in removal of dFe. Furthermore, the exponential attenuation model described in this paper can be applied in the major estuaries of the world where most of the removal of dFe and HS occurs where freshwater and seawater mix.