Amazon Floodplain Lake Research Articles

Niche partitioning and seasonality may mediate coexistence of piranha species in Amazonian floodplain lakes

Niche partitioning and seasonality may mediate coexistence of piranha species in Amazonian floodplain lakes

Hydrodynamic Modeling of Stratification and Mixing in a Shallow, Tropical Floodplain Lake

AbstractFloodplain lakes are widespread and ecologically important throughout tropical river systems, however data are rare that describe how temporal variations in hydrological, meteorological and optical conditions moderate stratification and mixing in these shallow lakes. Using time series measurements of meteorology and water‐column temperatures from 17 several day campaigns spanning two hydrological years in a representative Amazon floodplain lake, we calculated surface energy fluxes and thermal stratification, and applied and evaluated a 3‐dimensional hydrodynamic model. The model successfully simulated diel cycles in thermal structure characterized by buoyancy frequency, depth of the actively mixing layer, and other terms associated with the surface energy budget. Diurnal heating with strong stratification and nocturnal mixing were common; despite considerable heat loss at night, the strong stratification during the day meant that mixing only infrequently extended to the bottom at night. Simulations indicated that the diurnal thermocline up and downwelled creating lake‐wide differences in near‐surface temperatures and mixing depths. Infrequent full mixing creates conditions conducive to anoxia in these shallow lakes given their warm temperatures.

Effects of hydrological and spatial seasonality on taxonomic and functional diversity of fish assemblages in an Amazonian floodplain lake

AbstractThe floodplain lakes of the Amazon constitute one of the most important aquatic systems in terms of fish biodiversity. These ecosystems are influenced by the flood pulse, which in turn, affects the behaviour of fish communities within this lake ecosystem. In this study, we tested the hypothesis that the facets of taxonomic and functional diversity may be influenced by the conditions of ‘patches’ of internal habitats, considering the margins and central environments of a floodplain lake in the Amazon. Lake Cujubim, located on the right bank of the Madeira River, was evaluated via fisheries carried out in the high‐water and low‐water phases, during the day and night, and considering two location strata. The results suggest that the absence of significant differences in alpha taxonomic diversity between the two phases may be a possible seasonal mischaracterisation in Lake Cujubim attributed mainly to dam constructions along the Madeira River. Apparently, no major differences occur in the assembly of fish in terms of their taxonomic and functional characteristics, regardless of luminosity. In the strata of the locations, demonstrates noticeable habitat patches, with the margins being bioregions with high incidences of individuals of the Engraulidae family, confirmed by both taxonomic and functional diversity (alpha and beta). It can be seen that, even in connected lakes, there are differentiated metacommunities between the bioregions, thus forming patches of habitat, and that the shores of the lake, especially those located in proximity or distant from the river, are more suitable areas for food and refuge.

Can species guilds act as hubs for energy transfer in macrophyte meadows of Amazonian floodplain lakes?

Aquatic macrophytes are the main autochthonous component of primary production in the Amazon Basin. Floating meadows of these plants support habitats with highly diverse animal communities. Fishes inhabiting these habitats have been assumed to use a broad range of food items and compose a particular food web. We employed carbon (δ13C) and nitrogen (δ15N) stable isotope analysis to draw the trophic structure of these habitats and to trace the energy flow by its trophic levels. Fishes and other animals from 18 independent macrophyte meadows of a floodplain lake of the Solimões River (Amazonia, Brazil) were analyzed. The food web of macrophyte meadows consists of four trophic levels above autotrophic sources. In general, primary consumers exhibited a broader range of food sources than the upper trophic levels. Some fish species depended on a large number of food sources and at the same time are consumed by several predators. The energy transfer from one trophic level to the next was then mainly accomplished by these species concentrating a high-energy flux and acting as hubs in the food web. The broad range of δ13C values observed indicates that the organisms living in the macrophyte meadows utilize a great diversity of autotrophic sources.

Littoral areas of Amazonian floodplain lakes: a biological reserve to biodiversity loss

Carbon from aquatic plants and periphyton in littoral zones subsidize food webs. Field work was carried out in two different lake types in the Colombian Amazon: a várzea (Yahuarcaca) and an igapó (Pacatúa). Plants in the littoral zone of várzea Lake had higher photosynthetic biomass compared to the plants of the littoral zone of igapó, but differences in root biomass were not significant. The two littoral zones contained a relatively high species richness and diversity (Shannon-Wiener Index) of organisms associated with the roots, although no significant differences existed between the two lakes. Littoral areas may be important in relatively isolated and physically fragmented large terrestrial landscapes such as the Amazon basin.

Evaluation of fisheries management strategies using a biotic integrity index in floodplain lakes in the lower Solimões River, Amazonas, Brazil

ABSTRACT Fisheries is an important activity in the Amazon basin and potentially has a high impact on ichthyofauna. With the aim of achieving sustainability in fisheries of target species, protected areas were established within a fisheries management framework known as the Amazon Lakes Management System (ALMS). With the aim to evaluate the effect of ALMS on fish assemblages as a whole, we compared floodplain lakes with different levels of fish management in the lower Solimões River, in the Brazilian Amazon, developing an index of biotic integrity, named ALMS-IBI. We sampled fish in three lake categories: protected (PR), managed (MN), and commercially fished (CM). The ALMS-IBI was developed by selecting and testing fish assemblage metrics based on samplings carried out in 2017 and 2018, using gillnets with different mesh sizes. We captured 4565 fish of 113 species and seven trophic categories. The final index is composed of seven metrics related to species richness, trophic structure, and fish abundance, and characterized the fish assemblage in the PR and MN lakes as acceptable, and in the CM lakes as poor. Our results indicate that the ALMS-IBI can be an efficient method to monitor the whole fish assemblage in Amazonian floodplain lakes, and can be used as a complementary tool in the ALMS to assess environmental sustainability.

Flood-pulse and trophic position modulate mercury concentrations in fishes from an Amazon floodplain lake

Mercury (Hg) bioaccumulation can be influenced by biological characteristics (e.g., trophic position) and environmental changes (e.g., seasonal flood-pulse), as it can create site conditions conducive to methylation and alter the availability of food to biota. This study aimed to relate total mercury (THg) and methylmercury (MeHg) concentrations in fishes of an Amazonian lake (Catalão Lake) to their dietary changes during different hydrological seasons (rising-, high-, falling-, and low-water). Levels of THg, MeHg, and stable isotopes of nitrogen and carbon (δ15N and δ13C) were determined in the muscles of six fish species: Rhytiodus microlepis, Hemiodus sp., Triportheus albus, Anodus elongatus, Pellona flavipinnis and Acestrorhynchus falcirostris. Triportheusalbus, an omnivorous fish, showed a significant relationship between Hg concentrations and δ13C signatures (negative regression; THg: p = 0.0001 and MeHg: p = 0.0002), as well as between Hg and δ15N signatures (positive regression; THg: p = 0.003 and MeHg: p = 0.021), indicating that changes in feeding habits influenced the Hg concentrations in this species. The herbivorous R. microlepis presented the highest MeHg concentrations during the high-water season, reflecting Hg methylation in its diet (macrophyte roots) during flooded season. The piscivorous A. falcirostris had the lowest THg concentrations during this season because of lower prey availability. Slopes of the regressions between the Hg concentrations and δ15N values followed the sequence of rising- < high- < falling-water seasons (THg: 0.06 < 0.11< 0.15; MeHg: 0.07 < 0.10 < 0.17), indicating that the difference between the THg and MeHg concentrations observed for lower- and higher-trophic-level fish increased during the falling-water season, resulting in trophic biomagnification. The present study shows that feeding habit and the flood-pulse influence mercury concentrations in the muscle of fishes from Catalão Lake.

Influence of Limnological Processes and Water Level Variation on the Biodiversity of Brazilian Amazon Lakes: An Ecological Approach

This research makes a brief review of the limnology of lakes in wetlands in the Western Amazon, associating physical-chemical and morphological aspects to the biodiversity of black-waters lakes and whitewaters lakes. Lakes in these areas can show different patterns of vertical stratification, both in dissolved oxygen concentration and temperature. Furthermore, there are different mixing patterns. These events depend mainly on the way lakes connect to rivers, the time and extent of flooding, as well as the water levels variation. The availability of nutrients, particularly of phosphate, controls the biodiversity of the lakes and it is directly related with the thermal stratification and oxygen level in water column. In general, the most of the Amazon floodplain lakes can be classified as polymictic. Nevertheless, there are lakes of black-waters with trend to meromixy with permanent stratification. Under these conditions, stratification of other limnological variables also occurs, e.g., pH and electrical conductivity. Consequently, black-waters lakes systems with high species richness result in contrast to whitewaters lakes systems with lower species richness and high abundance of specimens.

Hybrid Semi-Analytical Algorithm for Estimating Chlorophyll-A Concentration in Lower Amazon Floodplain Waters

The Amazon Basin is the largest on the planet, and its aquatic ecosystems affect and are affected by the Earth’s processes. Specifically, Amazon aquatic ecosystems have been subjected to severe anthropogenic impacts due to deforestation, mining, dam construction, and widespread agribusiness expansion. Therefore, the monitoring of these impacts has become crucial for conservation plans and environmental legislation enforcement. However, its continental dimensions, the high variability of Amazonian water mass constituents, and cloud cover frequency impose a challenge for developing accurate satellite algorithms for water quality retrieval such as chlorophyll-a concentration (Chl-a), which is a proxy for the trophic state. This study presents the first application of the hybrid semi-analytical algorithm (HSAA) for Chl-a retrieval using a Sentinel-3 OLCI sensor over five Amazonian floodplain lakes. Inherent and apparent optical properties (IOPs and AOPs), as well as limnological data, were collected at 94 sampling stations during four field campaigns along hydrological years spanning from 2015 to 2017 and used to parameterize the hybrid SAA to retrieve Chl-a in highly turbid Amazonian waters. We implemented a re-parametrizing approach, called the generalized stacked constraints model to the Amazonian waters (GSCMLAFW), and used it to decompose the total absorption αt(λ) into the absorption coefficients of detritus, CDOM, and phytoplankton (αphy(λ)). The estimated GSCMLAFWαphy(λ) achieved errors lower than 24% at the visible bands and 70% at NIR. The performance of HSAA-based Chl-a retrieval was validated with in situ measurements of Chl-a concentration, and then it was compared to literature Chl-a algorithms. The results showed a smaller mean absolute percentage error (MAPE) for HSAA Chl-a retrieval (36.93%) than empirical Rrs models (73.39%) using a 3-band algorithm, which confirms the better performance of the semi-analytical approach. Last, the calibrated HSAA model was used to estimate the Chl-a concentration in OLCI images acquired during 2017 and 2019 field campaigns, and the results demonstrated reasonable errors (MAPE = 57%) and indicated the potential of OLCI bands for Chl-a estimation. Therefore, the outcomes of this study support the advance of semi-analytical models in highly turbid waters and highlight the importance of re-parameterization with GSCM and the applicability of HSAA in Sentinel-3 OLCI data.

Understanding the effects of environmental heterogeneity on the morphofunctional structure of the phytoplankton community during the hydrological year in an Amazon floodplain lake, Brazil

Understanding the effects of environmental heterogeneity on the morphofunctional structure of the phytoplankton community during the hydrological year in an Amazon floodplain lake, Brazil

Flood-Pulse and Trophic Position Modulate Mercury Concentration in Fishes from an Amazon Floodplain Lake

Flood-Pulse and Trophic Position Modulate Mercury Concentration in Fishes from an Amazon Floodplain Lake

Phytoplankton Genera Structure Revealed from the Multispectral Vertical Diffuse Attenuation Coefficient

The composition of phytoplankton and the concentration of pigments in their cells make their absorption and specific absorption coefficients key parameters for bio-optical modeling. This study investigated whether the multispectral vertical diffuse attenuation coefficient of downward irradiance (Kd) gradients could be a good framework for accessing phytoplankton genera. In situ measurements of remote sensing reflectance (Rrs), obtained in an Amazon Floodplain Lake (Lago Grande do Curuai), were used to invert Kd, focusing on Sentinel-3/Ocean and Land Color Instrument (OLCI) sensor bands. After that, an analysis based on the organization of three-way tables (STATICO) was applied to evaluate the relationships between phytoplankton genera and Kd at different OLCI bands. Our results indicate that phytoplankton genera are organized according to their ability to use light intensity and different spectral ranges of visible light (400 to 700 nm). As the light availability changes seasonally, the structure of phytoplankton changes as well. Some genera, such as Microcystis, are adapted to low light intensity at 550–650 nm, therefore high values of Kd in this range would indicate the dominance of Microcysts. Other genera, such as Aulacoseira, are highly adapted to harvesting blue-green light with higher intensity and probably grow in lakes with lower concentrations of colored dissolved organic matter that highly absorbs blue light (405–498). These findings are an important step to describing phytoplankton communities using orbital data in tropical freshwater floodplains. Furthermore, this approach can be used with biodiversity indexes to access phytoplankton diversity in these environments.

Digestive tube of an herbivorous fish (Rhytiodus microlepis) from the Amazonian floodplain lakes: A morphological and histochemical study.

This study describes the morphology and histochemistry of the digestive tube of the herbivorous fish Rhytiodus microlepis, which is commonly found in the Amazonian floodplain lakes, Brazil. Thirty-eight fish were used in this study. The digestive tube was measured and dissected for anatomical description, and stained with (i) haematoxylin and eosin, for histological analyses; (ii) periodic acid-Schiff, for detected neutral mucins; and (iii) Alcian Blue (AB) pH 1.0 and 2.5 for acid and sulphated acid mucins. The oesophagus, with its secretory cells and taste buds, is adapted for lubrication of vegetable matter, resistance to friction and reception of stimuli. The stomach is U-shaped and rich in gastric glands, particularly in cardiac and fundic regions, which are adapted to receive large amounts of food, and promotes the digestion. The intestine comprises more than 70% of the digestive tube. All portions of the intestine, except the rectum, have the same pattern of mucosal folds, and the goblet cells reacted positively to all histochemical methods (PAS, AB 1.0 and 2.5), while rodlet cells reacted only to periodic acid-Schiff (PAS) staining. Neutral mucins and sulphated acid mucins predominated in the anterior portion of the intestine and a high concentration of carboxylated acid mucosubstances were present in the other portions. The rectum showed a strong reaction to all histochemical methods. The muscular layer of the intestine consists of three layers, which showed features presumably related to the rigid nature of the food. A simple squamous epithelium constitutes the serous layer over the entire length of the tube.

Inundation, Hydrodynamics and Vegetation Influence Carbon Dioxide Concentrations in Amazon Floodplain Lakes

Extensive floodplains and numerous lakes in the Amazon basin are well suited to examine the role of floodable lands within the context of the sources and processing of carbon within inland waters. We measured diel, seasonal and inter-annual variations of CO2 concentrations and related environmental variables in open water and flooded vegetation and estimated their habitat area using remote sensing in a representative Amazon floodplain lake, Lake Janauacá. Variability in CO2 concentrations in open water resulted from changes in the extent of inundation and exchange with vegetated habitats. Depth-averaged values of CO2 in the open water of the lake, 157 ± 91 µM (mean ± SD), were less than those in an embayment near aquatic vegetation, 285 ± 116 µM, and were variable over 24-h periods at both sites. Within floating herbaceous plant mats, the mean concentration was 275 ± 77 µM, while in flooded forests it was 217 ± 78 µM. The best statistical model that included CO2 in aquatic plant mats, water clarity, rate of change in water level and chlorophyll-a concentrations explained around 90% of the variability in CO2 concentration. Three-dimensional hydrodynamic modeling demonstrated that diel differences in water temperature between plant mats and open water as well as basin-scale motions caused lateral exchanges of CO2 between vegetated habitats and open water. Our findings extend understanding of CO2 in tropical lakes and floodplains with measurements and models that emphasize the importance of flooded forests and aquatic herbaceous plants fringing floodplain lakes as sources of CO2 to open waters.

Do fish isotopic niches change in an Amazon floodplain lake over the hydrological regime?

AbstractFloodplain lakes are ecosystems characterised by annual flood and dry cycles. Fish ecology is influenced by the flood pulse due to the large influx of allochthonous food resources and diversification of habitats during the flood cycle, while during the dry cycle, fishes tend to be confined in reduced habitat. The aim of this study was to evaluate the seasonal variation in trophic niche width and overlap of four species—Mylossoma duriventre, Prochilodus nigricans, Cichla.pleiozona and Serrasalmus rhombeus—in an Amazonian floodplain lake. Stable isotope analyses were used to estimate trophic niche width and overlap during the flood and dry seasons. We hypothesised broader niche width for all species during the flood cycle and a higher degree of overlap between the two piscivorous fishes during the dry cycle. Isotopic niche width was 72% broader for P. nigricans, 61% for S. rhombeus and 54% for C. pleiozona during the dry cycle, which did not support our hypothesis. Core niche width overlaps were not observed between piscivorous species in either flood or dry cycle. The results indicate that seasonal variation in isotopic niche width is specific to feeding habit. Understanding how fish trophic ecology responds to changes in the hydrological regime during the seasons is crucial for sustainable fishery management in a region where many people rely heavily on fish for nutritional and economic purposes.

Limnological perspectives on conservation of floodplain lakes in the Amazon basin

Abstract Limnological aspects of Amazon floodplain lakes are examined in the context of aquatic conservation. A prerequisite to detecting and evaluating changes that could threaten the ecological health and organisms in floodplain lakes is understanding variation under present conditions. Based on one of the few studies with regular measurements over 2 years, chlorophyll, total phosphorus, dissolved oxygen, transparency, and total suspended solids in Lake Janauacá indicate that the lake is naturally quite variable with a mesotrophic to eutrophic status. Direct threats to ecological health of floodplain lakes include mining operations that can increase turbidity and trace metals and reduce nutritional quality of sediments. Mercury contamination and methylation leads to bioaccumulation in aquatic organisms. Deforestation in uplands increases nitrogen and phosphorus inputs to floodplain lakes and can alter trophic status. Deforestation in floodable forests alters the habitat and food of the fish that inhabit these forests. Cumulative limnological responses as catchments are altered by urban, agricultural, and industrial developments, and as inundation is altered by changes in climate and construction of dams, have major implications for the ecology of floodplain lakes. To improve understanding and management of threats to the conservation of aquatic Amazon biota and ecosystems requires considerably expanded and coordinated research and community‐based management that includes the spectrum of floodplain lakes throughout the basin.

Use of optical absorption indices to assess seasonal variability of dissolved organic matter in Amazon floodplain lakes

Abstract. Given the importance of dissolved organic matter (DOM) in the carbon cycling of aquatic ecosystems, information on its seasonal variability is crucial. In this study we assess the use of optical absorption indices available in the literature based on in situ data to both characterize the seasonal variability of DOM in a highly complex environment and for application in large-scale studies using remote sensing data. The study area comprises four lakes located in the Mamirauá Sustainable Development Reserve (MSDR). Samples for the determination of colored dissolved organic matter (CDOM) and measurements of remote sensing reflectance (Rrs) were acquired in situ. The Rrs was used to simulate the response of the visible bands of the Sentinel-2 MultiSpectral Instrument (MSI), which was used in the proposed models. Differences between lakes were tested using the CDOM indices. The results highlight the role of the flood pulse in the DOM dynamics at the floodplain lakes. The validation results show that the use of the absorption coefficient of CDOM (aCDOM) as a proxy of the spectral slope between 275 and 295 nm (S275–295) during rising water is worthwhile, demonstrating its potential application to Sentinel-2 MSI imagery data for studying DOM dynamics on the large scale.

Mobile piscivores and the nature of top-down forcing in Upper Amazonian floodplain lakes

Shallow lakes can change states in response to manipulations of top predators. In most reported experiments, the top piscivore has been a fish. However, low-latitude lakes typically support non-piscine piscivores, including mammals, birds, and reptiles. The roles of these groups have been little investigated, and whether they are more important as piscivores than fish remains unknown. We report both a longitudinal (2001–2018) and a cross-sectional (2012) study of the bird, caiman, and giant otter populations of floodplain lakes in Peru’s Manu National Park. We compare the three groups, after removing the effects of taxonomic status and body mass, using allometric equations to estimate the field metabolic rate (FMR) for each group in each lake. Giant otters emerge as the dominant piscivore in lakes that support a resident family, with an energy requirement more than twice that of piscivorous birds and more than seven times that of caiman. However, giant otters were resident in only eight of 27 surveyed lakes. Indirect evidence suggests that these eight lakes are more productive than lakes not occupied by otters. The fact that lakes occupied by otters are dominated by phytoplankton suggests an underlying five-tiered trophic structure in keeping with the prominence of alternating levels in the structure of food webs.

Fisheries management influences phytoplankton biomass of Amazonian floodplain lakes

Abstract Tropical floodplains secure the protein supply of millions of people, but only sound management can ensure the long‐term continuity of such ecosystem services. Overfishing is a widespread threat to multitrophic systems, but how it affects ecosystem functioning is poorly understood, particularly in tropical freshwater food webs. Models based on temperate lakes frequently assume that primary producers are mostly bottom‐up controlled by nutrient and light limitations, with negligible effects of top‐down forces. Yet this assumption remains untested in complex tropical freshwater systems experiencing marked spatiotemporal variation. We use consolidated community‐based fisheries management practices and spatial zoning to test the relative importance of bottom‐up versus top‐down drivers of phytoplankton biomass, controlling for the influence of local to landscape heterogeneity. Our study focuses on 58 large Amazonian floodplain lakes under different management regimes that resulted in a gradient of apex‐predator abundance. These lakes, distributed along ~600 km of a major tributary of the Amazon River, varied widely in size, structure, landscape context, and hydrological seasonality. Using generalised linear models, we show that community‐based fisheries management, which controls the density of apex predators, is the strongest predictor of phytoplankton biomass during the dry season, when lakes become discrete landscape units. Water transparency also emerges as an important bottom‐up factor, but phosphorus, nitrogen and several lake and landscape metrics had minor or no effects on phytoplankton biomass. During the wet‐season food pulse, when lakes become connected to adjacent water bodies and homogenise the landscape, only lake depth explained phytoplankton biomass. Synthesis and applications. Tropical freshwaters fisheries typically assume that fish biomass is controlled by bottom‐up mechanisms, so that overexploitation of large predators would not affect overall ecosystem productivity. Our results, however, show that top‐down forces are important drivers of primary productivity in tropical lakes, above and beyond the effects of bottom‐up factors. This helps us to understand the enormous success of community‐based ‘fishing agreements’ in the Amazon. Multiple stakeholders should embrace socio‐ecological management practices that shape both bottom‐up and top‐down forces to ensure biodiversity protection, sustainable fisheries yields and food security for local communities and regional economies.

Mapping of diffuse attenuation coefficient in optically complex waters of amazon floodplain lakes

The modeling of underwater light field is essential for the understanding of biogeochemical processes, such as photosynthesis, carbon fluxes, and sediment transports in inland waters. Water-column light attenuation can be quantified by the diffuse attenuation coefficient of the downwelling irradiance (Kd) using semi-analytical algorithms (SAA). However, the accuracy of these algorithms is currently limited in highly turbid environments, such as Amazon Floodplains, due to the SAA parametrization steps. In this study, we assessed an SAA approach for Kd retrieval using a sizeable (n = 239) and diverse dataset (e.g., Kd (490) ranging from almost 0 to up to 30 m−1 with mean values of 5.75 ± 3.94 m−1) in Amazon freshwater ecosystem. The main framework of this study consists of i) re-parametrization of a quasi-analytical algorithm using regional in-situ inherent optical properties (IOPs) and ii) application and validation of SAA for Kd retrieval using in-situ and Sentinel-2/MSI (n = 49) derived from Remote Sensing Reflectance (Rrs). Overall, the performance of the calibrated SAA was satisfactory for both in-situ and satellite Rrs. The validation results with in-situ data achieved a Mean Absolute Percentage Error (MAPE) lower than 22%, Correlation Coefficient (R) > 0.80, Root Mean Square Error (RMSE) lower than 1.7 m−1, and bias between 0.73 and 1.34 for simulated visible bands of Sentinel-2/MSI (490, 560 and 660 nm) (VIS). The results using MSI imagery were similar to those of in-situ, with R > 0.9, MAPE < 20%, RMSE < 1.25 m−1, and bias between 0.98 and 1.10 for VIS bands, which illustrate the viability of this methodology for Kd mapping in Amazon Floodplain Lakes. Therefore, this study demonstrates a successful application of satellite remote sensing data for the spatialization of the Kd in the optically complex waters of Amazon Basin, which is essential for the ecological management of the Amazon Floodplain Lakes.