Amazon Estuary Research Articles

Spatial and temporal variation of water reflectance in an Amazonian estuary: a case study around Tatuoca Island

Abstract. Studies of water color are related to its optically active components, including suspended sediments. The correlation of this parameter with reflectance data is well documented in the literature, to understand its concentrations and transport in the water body. The spatio-temporal variations of water color in an estuary are affected by various physical and hydrological components, which makes it complex to understand. In this study, we sought to understand the spatio-temporal variation of water color, for sampling purposes, in an estuary on the Amazon coast. To achieve our objectives, we applied geostatistical and statistical tools to test the spatio-temporal variability of water color, based on multisensor reflectance data, Landsat-8/9 and Sentinel-2 in the red band. The spatial variation showed distances of more than 48 m ±4.26 at high tide and 44 m ±7.62 at low tide. For the temporal variation, we found that the monthly variations are significant according to seasonality and have the same variation at different stages of the tide. Therefore, the color of the water in the Tatuoca Island region shows geostatistically and statistically significant spatio-temporal variation. The distances and temporal frequency of sampling should be adjusted according to the tides and seasonality to ensure that data is sampled in accordance with the variations in the environment.

Quantitative impacts of hydroelectric dams on the trans‐Amazonian migrations of goliath catfish

AbstractHydropower expansion is increasingly responsible for connectivity and biodiversity loss in freshwater ecosystems. The Amazon basin, which supports the highest level of freshwater biodiversity globally, faces such unplanned expansion. Here, we demonstrate and quantify the impacts of two major hydroelectric dams on the Madeira River on the trans‐Amazonian movements of one apex predatory catfish (Brachyplatystoma rousseauxii) performing the longest migration known in freshwaters. Using otolith microchemistry, we show that above the dams, the former basin‐wide homing migration between the breeding grounds in the upper Madeira and the nursery in the Amazon estuary has now been replaced by residency. We found evidence suggesting downstream migration of juvenile fish past dams into the Amazon estuary and that some of them home back to the Madeira. However, we did not find evidence that the fish homing back from the estuary can access their breeding grounds in the upper Madeira; they remain blocked below the dams. Our results provide undisputable evidence that the conservation of the species primarily requires the reestablishment of connectivity with the construction of efficient fishways.

Seasonal and macrotidal influence on the marphodynamics of estuarine beaches (Marajó island - Eastern amazon - Brazil)

The Marajó island situated at the mouth of the Amazon estuary, is recognized as the largest estuarine island in the world. It is surrounded by the Amazon Estuary to the west, Pará estuary to the east, the Tocantins River to the south, and the Atlantic Ocean to the north. The Pará Estuary Border of the Island is divided by the Paracauari channel, creating a division between two distinct coastlines: the coast with active cliffs and narrow beaches like Grande beach (35 m) and Joanes beach (58.4 m) located in Salvaterra. In the other side, extensive beaches such as Barra Velha beach (285 m) and Pesqueiro beach (240/300 m) with low topography and paleo mangrove terraces, located in Soure. The unique hydrodynamic conditions include tides reaching up to 5.7 m, currents of 1.7 m/s, and waves up to 1.5 m high originated from the northeast, along with average wind speeds of 7 m/s. The sedimentary equilibrium is more pronounced in the low sector. The low sector (Pesqueiro beach = 525.38 m3/m) exhibits a higher sedimentary balance compared to the high sector (Joanes beach = 250.07 m3/m and Grande beach = 88.58 m3/m). The beaches displayed an intermediate morphodynamic character, except for Grande beach (reflective/rainy season) and Joanes beach (reflective/dry). According to the RTR model, these beaches had an intermediate nature characterized by runnels and bars, except for Joanes beach (reflective/dry).

Variations in macrobenthic fauna of mangrove and unvegetated habitats in an Amazon estuary

Variations in macrobenthic fauna of mangrove and unvegetated habitats in an Amazon estuary

Ups and downs: An insight on the stranding pattern of Guiana dolphins, Sotalia guianensis, in the Amazon Estuary, Northern Brazil

Abstract Long‐term systematic records of cetacean strandings are valuable indicators of temporal responses of specific cetacean populations and communities to natural or anthropogenic pressures. They also help guide management objectives and conservation actions. Generalized additive models (GAMs) were used to analyse 10 years of systematic and opportunistic surveys of Guiana dolphin strandings in the Amazon Estuary region, Northern Brazil. The spatial distribution of strandings was assessed through kernel density maps, and age‐at‐death structure of this population was described. The model results revealed marked seasonality in the strandings and, by extension, the mortality of Guiana dolphins in the Amazon Estuary. This seasonality varied between the inner estuarine regions and Atlantic coastal area, reflecting the environment's characteristics. Long‐term decline of strandings in the Amazon Estuary inner region and unusual high mortality of young sexually mature individuals were detected. These results, besides the high stranding rate detected for the species, are mostly attributed to the species' interaction with intense fishing activity in the region. Conservation efforts should consider the seasonality of Guiana dolphin strandings in the Amazon Estuary. The systematic monitoring of strandings, especially in high‐frequency areas, is recommended for assessing long‐term species mortality. Additionally, implementing management actions related to the species' interaction with fishing activities, such as regulatory enhancements, gillnet modifications and seasonal no‐fishing zones, is advised. Educational campaigns targeting fishermen and riverine communities are essential to reduce bycatch and the folkloric use of the species. This study underscores the effectiveness of stranding data as a source of biological and ecological information, and as a tool for understanding human actions affecting coastal populations of aquatic mammals. It emphasizes the need for urgent studies to characterize this population, clarify the influence of bycatch as an anthropogenic pressure and implement effective conservation actions to preserve the Guiana dolphin population.

Opportunistic sightings of manta rays on Brazil's Amazon Coast.

Little is known about the ecology and distribution of mobulid rays along Brazil's extensive coastline. Here we report opportunistic sightings of manta rays (Mobula cf. birostris) in the Brazilian Amazon estuaries and the Great Amazon Reef System. These sightings consist of manta ray individuals stranded in tide pools, caught in artisanal fisheries, and footage obtained with a submersible. Future investigations on the spatial, temporal, and environmental drivers of manta rays' distribution on the northern Brazilian coast and the threats posed by fishing gear are warranted.

Population dynamics of Pentaclethra macroloba, a hyperdominant tree in the Amazon River estuary

Pentaclethra macroloba is a hyperdominant tree of the Amazon estuary and highly exploited in the region, because the oil of its seeds has medicinal properties. Little is known about the influence of the flood polymodal cycle on the population dynamics of trees in the Amazon estuary. We evaluated the influence of tree diversity, climate, soil properties and flood level on the population dynamics of P. macroloba. During five years of monitoring (from 2011 to 2015), population density and mortality, recruitment and growth rates data were obtained of three populations of P. macroloba (Mazagão Velho, Maracá and Ajuruxi areas). Four transects were used in each area and 20 permanent plots were installed in each transect to study the population dynamics. Linear models were used to evaluate the relationship between biotic/abiotic factors and population density of P. macroloba. The population density of the three populations analyzed showed a significant decrease between the period from 2011 to 2015 (F = 381.3; p < 0.001). The Maracá area had a higher mortality rate (4.50% year−1) compared to Mazagão Velho (3.24% year−1) and Ajuruxi (1.96% year−1). The recruitment rate in Ajuruxi and Mazagão Velho were 1.05% year−1 and 0.32% year−1, respectively, and in Maracá there were no recruits. Annual growth rate of the Maraca area was higher (0.93 cm−1 year−1) compared to Ajuruxi (0.75 cm−1 year−1) and Mazagão Velho (0.65 cm−1 year−1) populations. The flood level had a negative and significant correlation with the population density of P. macroloba in the Maracá (F = 39.7; p < 0.01), Ajuruxi (F = 9.9; p < 0.05) and Mazagão Velho (F = 19.6; p < 0.05) areas. Only the Ajuruxi area had a positive and significant correlation with arboreal richness (β = 0.14; p < 0.05). The flood gradient is an environmental driver that regulates the population growth of P. macroloba. The hyperdominant characteristic of this species is the result of its adaptability and interactions between soil fertility and tree diversity.

Simultaneous preconcentration of 9Be and cosmogenic 10Be for determination of the 10Be/9Be ratio in (coastal) seawater

AbstractBeryllium isotopes have emerged as a quantitative tracer of continental weathering, but accurate and precise determination of the cosmogenic 10Be and stable 9Be in seawater is challenging, because seawater contains high concentrations of matrix elements but extremely low concentrations of 9Be and 10Be. In this study, we develop a new, time‐efficient procedure for the simultaneous preconcentration of 9Be and 10Be from (coastal) seawater based on the iron co‐precipitation method. The concentrations of 9Be, 10Be, and the resulting 10Be/9Be ratio for Changjiang Estuary water derived from the new procedure agree well with those obtained from the conventional procedure requiring separate preconcentration for 9Be and 10Be determinations. By avoiding the separate preconcentration, our newly developed procedure contributes toward more time‐efficient handling of samples, less sample cross‐contamination, and a more reliable 10Be/9Be ratio. Prior to this, we validated the iron co‐precipitation method using artificial seawater and natural water samples from the Amazon Estuary regarding: (1) the “matrix effect” for Be analysis, (2) its extraction efficiency for pg g−1 levels Be in the presence and absence of organic matter, and (3) the data comparability with another preconcentration method. We calculated that for the determination of 9Be and 10Be in most open ocean seawater with typical 10Be concentrations of > 500 atoms g−1, good precisions (< 5%) can be achieved using less than 3 liters of seawater compared to more than 20 liters routinely used previously. Even for coastal seawater with extremely low 10Be concentration (e.g., 100 atoms g−1), we estimate a maximum amount of 10 liters to be adequate.

Molecular characterization of juvenile fish from the Amazon estuary using DNA barcoding approach.

The efficiency of the DNA barcoding relies on sequencing fragment of the Cytochrome C Subunit I (COI) gene, which has been claimed as a tool to biodiversity identification from distinct groups. Accordingly, the goal of this study was to identify juvenile fish species along an estuary of Caeté River in the Brazilian Blue Amazon based on. For this purpose, we applied the DNA barcoding and discuss this approach as a tool for discrimination of species in early ontogenetic stages. A 500-bp fragment was obtained from 74 individuals, belonging to 23 species, 20 genera, 13 families and seven orders. About 70% of the 46 haplotypes revealed congruence between morphological and molecular species identification, while 8% of them failed in identification of taxa and 22% demonstrated morphological misidentification. These results proved that COI fragments were effective to diagnose fish species at early life stages, allowing identifying all samples to a species-specific status, except for some taxa whose COI sequences remain unavailable in public databases. Therefore, we recommend the incorporation of DNA barcoding to provide additional support to traditional identification, especially in morphologically controversial groups. In addition, periodic updates and comparative analyses in public COI datasets are encouraged.

The influence of river-derived particles on estuarine and marine elemental cycles: Evidence from lithium isotopes

To examine the alteration of river-derived sediments through a large estuary and the implications for elemental cycling and global climate, this study analyses lithium (Li) isotopes and elemental concentrations (e.g., Li, Na, Mg, K, Ca, Fe and Al) of both the dissolved load and different phases of the sediment load (i.e., exchangeable, carbonate, oxide, clay and residue) in the Amazon estuary. The results show that river-derived sediments remove Li from the dissolved load, largely due to cation retention in secondary clays. By modelling the Li mass-balance and isotope fractionation, we estimate that the river-derived sediments gain 3–4 μg/g Li from the dissolved load in the Amazon estuary, with a Li isotope fractionation factor (αclay-solution) of approximately 0.975. Considering the whole Amazon estuary, the river-derived sediments remove around 3.6–4.8 × 109 g/yr of Li from the dissolved load. Specifically, around 1.0–1.7 × 108 g/yr of Li is removed from river water (∼1.8–3.0% of the dissolved Li discharge flux of the Amazon River) and around 3.5–4.7 × 109 g/yr of Li is removed from seawater, which represents a significant sink from the ocean. This estuarine Li sink is likely to be related to continental erosion rates; thus, continental weathering and erosion regimes could influence not only riverine Li input, but could also directly affect the Li sink, leading to a dual control on the Li budget and isotope composition in the ocean.

Secondary production of Acartia tonsa Dana, 1849 and A. lilljeborgii Giesbrecht, 1889 in an Amazonian estuary (Brazil)

The dynamics of the copepods A. tonsa and A. lilljeborgii were described for the first time in the Taperaçu Estuary. The acartiids were collected using plankton nets (200 μm) in June 2012, March 2013 (rainy season) and September 2012 and 2013 (dry season). The oscillations in rainfall and the fluctuations in hydrological variables influenced the abundance, biomass, and production of both A. tonsa (17 ± 23 to 8501 ± 13,248 ind.m−3; 16,385.29 mg.C.m−3; 0.09 ± 0.21 to 355.17 ± 590.84 mg.C.m−3.d−1) and A. lilljeborgii (14 ± 11 to 1470 ± 1591 ind.m−3; 22,398.40 mg.C.m−3; 177.99 ± 263.13 mg.C.m−3.d−1) with clear monthly, seasonal, and spatial patterns. The high levels of production observed may be related to the presence of waters rich in particulate organic material derived from the adjacent mangrove forests. This material is consumed by a number of copepod species, in particular A. tonsa and A. lilljeborgii, favoring the development and reproduction of both species which are characterized by high rates of productivity in the study estuary. The present results indicate that the biomass and productivity in equatorial mangrove estuaries may be relatively high in comparison with the levels observed in other coastal systems around the world and that earlier stages of both species have a great relevance for biomass and production in Amazonian estuaries.

Impacts of Inherited Morphology and Offshore Suspended-Sediment Load in an Amazon Estuary

Impacts of Inherited Morphology and Offshore Suspended-Sediment Load in an Amazon Estuary

Overlooked riverine contributions of dissolved neodymium and hafnium to the Amazon estuary and oceans

The Amazon River accounts for 20% of global freshwater runoff and supplies vital trace metals to the Atlantic Ocean. Suspended particles within its plume are thought to partially dissolve, constituting a large potential source of metals, which is, however, not well constrained. Here we used combined neodymium (Nd) and hafnium (Hf) isotopes to disprove the release of Nd and Hf from particles as the cause of the observed dissolved concentration increases and isotopic variability across the plume. Instead, the changes reflect admixture of nearby Pará River freshwater with exceptionally high dissolved Nd and Hf concentrations contributing 45–100% of the riverine fraction to the southern and outer estuary. This result led us to develop an empirical relationship between riverine Nd concentration and pH to revise the global dissolved riverine Nd flux, which accordingly is at least three times higher than commonly used estimates. Future work should focus on contributions of low-pH rivers to global metal fluxes.

The bycatch of piramutaba, Brachyplatystoma vaillantii industrial fishing in a salinity and depth gradient in the Amazon estuary, Brazil

ABSTRACT The piramutaba, Brachyplatystoma vaillantii is a freshwater catfish that is the most abundant fishery resource in the Amazon estuary. Piramutaba trawling is done on industrial fishing scale and is characterized by the presence of many freshwater and marine bycatch species, with and without commercial value. Here we describe the bycatch of the industrial fishery of piramutaba in the Amazon estuary and evaluate the relationship of two important environmental factors, depth and salinity, with the accidental capture of freshwater and marine fishes in the Amazon estuary in the rainy and dry seasons. We identified 21 cartilaginous fish species (19.1% freshwater and 80.9% marine) and 125 bony fish species (25.6% freshwater and 74.4% marine). The bycatch included 64 species without commercial value (43% of all bycatch species), which are always discarded. Freshwater and estuarine fishes exhibited significantly higher abundances in shallower environments, while marine fishes were similarly abundant along the entire depth gradient. On the contrary, the abundance of freshwater fishes significantly decreased, and that of estuarine and marine fishes significantly increased with increasing salinity. Regarding the conservation status of the bycatch species, one is classified as vulnerable (VU), and seven as critically endangered (CR). The information on the bycatch of piramutaba fishery in the Amazon estuary is important to subsidize regional fisheries policies and the management of protected areas.

Insights about levels and sources of organic pollution in an urbanized Amazon estuary (Belém, PA, Northern Brazil)

Amazon aquatic systems have been affected by organic pollution from urbanized regions. This study was conducted to determine the levels, sources, and distribution patterns of 16 polycyclic aromatic hydrocarbons (PAHs) and 6 steroid markers in surficial sediments from an important urbanized Amazon estuarine system (Belém, PA, Northern Brazil). Total PAH concentration (∑PAH) ranged from 878.2 to 9905.7ngg-1, 3295.2ngg-1 on average, suggesting a highly contaminated environment. PAH molecular ratios and statistical analysis indicated that PAH originated from a mixture of local sources emissions, mainly related to the combustion of fossil fuels and biomass. Coprostanol levels (maximum concentration = 292.52ngg-1) could be compared to the mid-range reported in the literature. Studied stations, except for one, presented sterol ratio data indicating organic matter related to untreated sewage. Sterols indicative of sewage contamination showed a correlation with pyrogenic PAH amounts which are transported by the same channels where sewage is discharged.

Effect of climate and flooding on the phenology of Attalea phalerata (Arecaceae) in a white‐water floodplain forest of the Amazon River

AbstractLittle is known about the effect of the polymodal tide of the Amazon estuary on the phenology of palms. We evaluated the effect of climate/flood and biotic factors on the phenology of 80 Attalea phalerata palms over 1‐year period. Bract emission and young leaves percentage were negatively correlated to accumulated rainfall. Inflorescence and leaf production occurred mainly in the dry season. Immature fruit was positively correlated with rainfall, showing that fruit development occurs during the rainy season. The production of flowers, unripe fruit, and young leaves were positively correlated to flooding. The production of infructescence, per palm, varied from 0 to 4 and 18% of the variance was explained by variables of stem diameter, number of new leaves, altitude, and flood amplitude. The number of infructescence increased for large palms and decreased for individuals subjected to high amplitudes of flooding. These findings reveal that phenological patterns of A. phalerata are driven by seasonal variation in rainfall and flooding. Furthermore, infructescence production can be predicted by plant size and this production is sensitive to variations in flood amplitude. Alteration in rainfall and flooding due to climate change may have a negative impact on fruit production if extreme events increase in frequency.

Challenges to smallholder forestry policy reform on a postindustrial logging frontier: lessons from the Amazon estuary

In 2013, policy makers from the Brazilian State of Amapá launched regulatory reforms intended to streamline options to formalize smallholder forest management. This paper reviews that policy reform process and analyses observations from local stakeholders to identify lessons for the promotion of smallholder forestry. In 2014, 2017 and 2021, interviews were conducted with family timber producers and sawmill operators in floodplain communities and regional timber buyers to evaluate the impact of the policy reform on their operations. Interviews with policy makers sought to understand the rationale behind the reform and how the process played out. Although the r eform efforts were well intentioned, results illustrate how bureaucratic inertia and complexity obstructed efforts to simplify policy and how policy did not address smallholder needs. While the reforms did not have the intended effect, the case offers lessons for future policy reform efforts.

Population structure and dynamics of Petrolisthes armatus (Gibbes, 1850) (Anomura: Porcellanidae) in an Amazon estuary

The aim of this study was to assess the environmental factors that modulate the spatiotemporal distribution of Petrolisthes armatus and analyze their population structure, dynamics and secondary production in an Amazon estuary. Monthly samplings were carried out over one year in the intertidal rocky outcrop considering the following environmental features; season (dry and rainy), areas (middle and lower estuary), banks (east and west), substrate with different rock sizes, and the upper and lower intertidal zones. Crab density was positively correlated with salinity and negatively correlated with temperature. Higher densities occurred during dry season, on the eastern bank, in the lower estuary, in the lower intertidal zone, and in the larger rock substrate. The high abundance of P. armatus indicates that this estuary is a suitable habitat for the development of this anomuran, and salinity and substrate were the most important factors driving population structure. High production reflects high density and biomass and the fast growth of this crab indicated that this filter feeder generates substantial production to the benthos. This study corroborated the hypothesis that the high metabolism of P. armatus combined with the high temperature in the Amazon region results in short life span (∼one year), high mortality rate (∼6 year −1), and high P/B ratio (∼4 year −1).

Implications of a Large River Discharge on the Dynamics of a Tide-Dominated Amazonian Estuary

Estuaries along the Amazonian coast are subjected to both a macrotidal regime and seasonally high fluvial discharge, both of which generate complex circulation. Furthermore, the Amazon River Plume (ARP) influences coastal circulation and suspended sediment concentrations (SSCs). The Gurupi estuary, located south of the mouth of the Amazon River, is relatively unstudied. This study evaluates how the Gurupi estuary dynamics respond to seasonal discharge and the varying influence of the ARP using cross-sectional and longitudinal surveys of morphology, hydrodynamics, and sediment transport. The Gurupi was classified as a tide-dominated estuary based on morphology and mean hydrodynamic conditions. However, the estuary was only partially mixed during both the wet and dry seasons. The tides propagated asymmetrically and hypersynchronously, with flood dominance during the dry season and ebb dominance during the rainy season. Seasonal variations of the ARP did not significantly affect the hydrodynamic structure of the lower Gurupi estuary. Estuarine turbidity maxima (ETM) were observed in both seasons, although the increase in fluvial discharge during the wet season attenuated and shifted the ETM seaward. Little sediment was delivered to the estuary by the river, and the SSCs were higher at the mouth in both seasons. Sediment was strongly imported during the dry season by tidal asymmetry. The morphology, hydrodynamics, and sediment dynamics all highlight the importance of considering both fluvial discharge and coastal influences on estuaries along the Amazon coast.

Seasonal to interannual variability of the tide in the Amazon estuary

The Amazon River exports the largest volume of fresh water to the ocean worldwide. Although previous studies have revealed the spatiotemporal tidal variability of the estuary, its hydrodynamics is still poorly understood. Here we evaluate the seasonal and interannual variability of the tide from Óbidos (800 km upstream) to the Atlantic Ocean and show how it is affected by the hydrological regime of the Amazon River. A high-resolution 2D hydrodynamic model was applied in this region at the scale of the whole estuary. The tide model is validated using data from 14 water level stations and shows an average complex error of 16 cm in the low flow season and 23 cm in the high flow season. The semi-diurnal tide is highly variable at seasonal timescales, and the seasonality of the discharge affects the tidal amplitude, the geographic extent of tidal influence, the tidal wave celerity, and the tidal flow reversal. Notably, the tidal influence on water level remains detectable up to Óbidos during the low flow season while during the high flow season it extends from the ocean to only 300 km downstream of Óbidos. On the other hand, the upstream limit of the domain where the tide induces a periodic flow reversal is different from the limit of tidal influence on the water level. The upstream limit of this flow reversal is shifted by 170 km (from 500 km to 670 km downstream of Óbidos) along the year due to the seasonality of the discharge. At interannual scale, anomalous hydrological discharges affect the tidal amplitude by up to 30% in the central reach of the estuary. Our findings open unprecedented opportunities to understand biogeochemical and geomorphological processes, help navigation, and assess flooding hazards.