Tropical Floodplain Research Articles

Riverine connectivity modulates elemental fluxes through a 200- year period of intensive anthropic change in the Magdalena River floodplains, Colombia

Tropical floodplain lakes are increasingly impacted by human activities, yet their pathways of spatial and temporal degradation, particularly under varying hydrological connectivity regimes and climate change, remain poorly understood. This study examines surface-sediment samples and 210Pb-dated sediment cores from six floodplain lakes, representing a gradient in hydrological connectivity in the lower Magdalena River Basin, Colombia. We analysed temporal and spatial variations in several sediment biogeochemical indicators: the concentration and flux of nutrients, heavy metals, and organic matter (OM), and redox conditions, flooding and erosion. Multiple factor analysis (MFA) of surface-sediments identified redox conditions, OM, flooding, heavy metals and lake connectivity as the main contributors to spatial variability within- and between-lakes sediments, accounting for 48% of the total variation. Additionally, no clear distinction was found between littoral and open-water sediment characteristics. Isolated lakes sediments exhibited reductive conditions rich in OM and nutrients, whereas connected lakes sediments showed greater heavy metal enrichment and higher concentrations of coarse river-fed material. Generalised additive models identified significant changes in the biogeochemical indicators since the late 1800s, that accelerated post-1980s. Shifts in OM, erosion, flooding, redox conditions, land-cover change, heavy metals and climate were identified by MFA as the main drivers of change, explaining 60%-71% of the variation in the connected lakes and 53%-72% in the isolated lakes. Post-1980s, connected lakes transitioned from conditions of higher accumulation of OM and little erosion to higher accumulation of heavy metals and river-fed material. Conversely, isolated lakes, shifted from detrital-heavy metal-rich sediments to OM-, and nutrient-rich, reductive sediments. Sedimentation rates also surged post-1980s, particularly in highly connected lakes, from 0.14 ± 0.07 g cm² yr⁻¹ to 0.5 ± 0.5 g cm² yr⁻¹, with elevated fluxes of metals, OM and nutrients. These changes in sediment biogeochemistry align with deforestation, river regulation and prolonged dry periods, highlighting the complexities behind establishing reliable reference conditions for pollution assessments in large, human-impacted tropical river systems.

PERFORMANCE OF SOYBEAN CULTIVARS FOR SEED PRODUCTION IN TROPICAL FLOODPLAINS UNDER DIFFERENT GROWING CONDITIONS

Soybeans are a major crop in Brazilian agribusiness, with cultivar selection, location, and sowing date significantly impacting seed yield and quality. This study aimed to evaluate the yield and quality of soybean seeds under different growing environments, combining sowing time and location in tropical floodplain conditions in Tocantins in 2022. Four field experiments were conducted in two locations, with sowing dates of May 17 and May 31 in Lagoa da Confusão, TO. The cultivars Bônus, Extrema, Domínio, SYN1687, and SYN2282 were assessed. Each experiment followed a randomized block design with five treatments and four replications. After full maturity, the crops were harvested and evaluated for yield parameters and seed physiological quality. The cultivars Bônus, Domínio, and SYN1687 produced the highest yields, unaffected by sowing dates. However, the growing environment influenced seed vigor and viability, with the May 31 sowing favoring seed quality.

Hydrodynamic modeling of floodplains with inundated trees and floating plants

Hydrodynamic influences of inundated forests and floating plants, common on floodplains, include modifying exchange flows and thermal structure. This study added algorithms to represent these plants in a three-dimensional hydrodynamic model, applied the model to a tropical floodplain, and validated the results with high resolution field measurements. The simulations reproduced temporal and spatial variation of thermal structures in vegetated and open water areas and demonstrated that vertical and horizontal temperature gradients were enhanced in the presence of vegetation. The study extends research findings obtained from laboratory experiments to in-situ situations and to vegetation not represented in the laboratory. The results are relevant to lateral and vertical exchanges of dissolved gases and solutes, and indicate that loss of inundated forests could lead to enhanced lateral exchange.

Physicochemical characterization of the aquatic-terrestrial transition zone of a tropical wetland (Ayapel-Colombia)

Contextualization: The aquatic-terrestrial transition zone (ZTAT) in the floodplain (alluvial plain) Ramsar Ayapel (Colombia) remains flooded for more than half of the year, and its coastal strip generates a strong influence on the ecosystem. An approach to the spatial and temporal characterization of this zone is crucial for further functional analysis, especially when tropical floodplains are little explored except for some Amazonian ecosystems. Knowledge gap: Floodplain dynamics are mainly regulated by flood pulses, directly influencing he physical and chemical conditions of water and sediment. Therefore, the biota is adapted to the flooding and waterlogging conditions of the ecosystem. However, research has addressed the terrestrial and aquatic phases as separate systems, making it necessary to resort to methodologies that demonstrate connectivity in the system and facilitate the interpretation of key factors in the ecological functioning of tropical wetlands. Purpose: The research analyzed the structure and functioning of the ZTAT through physical and chemical variables at different times of the flood pulse. Methodology: In two wetlands of the muddy complex, sensitive to the flood pulse, three samplings were carried out: rising water (July/2021), high water (September/2021), and declining water (March/2022). The ZTAT was monitored by delimiting it into four subzones: water, sediments, floodable soil, and soil. Results and conclusions: Significant changes in time associated with water level and nitrogen concentrations were revealed; while in space, assisted by the contrast of the ZTAT at the two sites, differences were reflected through in situ variables and soil characteristics. It was detected that depth, transparency, and a higher concentration of nitrogen forms, and some ions such as aluminum and magnesium generated differences on a time level. On the contrary, conductivity, calcium concentration, and variables associated with soil characteristics expressed it at the spatial level due to exploring the ZTAT. Therefore, the physicochemical dynamics of moments and sites in a wetland require a sampling design that, using the ZTAT, facilitates the scale of physicochemical analysis.

Fluvial avulsions influence soil fertility in the Pantanal wetlands (Brazil)

River avulsions drive important changes in the Pantanal wetlands, owing to their role in the hydro-sedimentology of the region. Although relevant to numerous ecosystem services, few studies have analyzed the influence of river avulsions on soil fertility in the Pantanal. Here, we use the largest ongoing avulsion in the Taquari River (Caronal region) to evaluate the effects on soil fertility, considering two factors: avulsion stage (1) and aquatic-terrestrial succession (2). Since both factors are influenced by macrophyte abundance, an incident map was created through tasseled cap indices from Sentinel 2 images to guide sampling efforts in flooded soils. The mapped area was split into two zones of alluvial processes, the first from the apex of the Caronal lobe corresponding to the Taquari River megafan (TRM), and the second as the distal Paraguay River floodplain (PRF). Soil macro- and micronutrient levels were evaluated from 42 surface samples (0–0.2 m) distributed across the two alluvial process zones. The macrophyte map's overall accuracy (OA) was analyzed by a confusion matrix using the Sentinel 2 imagery. Finally, we used Random Forest regressions to determine the influence of response variables on soil attributes, including tassel indices, distance from the Caronal crevasse, macrophyte density, and an existing soil fertility map. The macrophyte map obtained an OA of 93 %. Some parameters such as pH (r = −0.62; R2 = 0.57), effective cation exchange capacity (r = −0.49; R2 = 0.79), Mn (r = −0.71; R2 = 0.6), Zn (r = −0.69; R2 = 0.54), and base saturation (r = −0.7; R2 = 0.93) were influenced by the distance or level of maturation of the avulsion stage in the TRM. Our scattering of soil collections was insufficient to test the terrestrialization hypothesis (2). The study results show that river channel avulsions influence the accumulation of mineral and organic nutrients in tropical floodplain soils, which has implications for fertility and biodiversity.

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.

Ecosystem-based fishery enhancement through pen culture of Indian major carp Labeo catla in a tropical floodplain wetland of North Eastern Region, India, during COVID pandemic.

The present study was conducted with the objective of developing ecologically and economically feasible pen culture protocols for Labeo catla as an alternate income source for wetland fishers in the context of the COVID-19 pandemic. Yearlings of L. catla (12.33 ± 1.99 cm mean total length and 26.05 ± 6.57 g mean weight) were reared in HDPE pens (500 m2 area each) at three different stocking densities of 3 (SD3), 6 (SD6) and 9 (SD9) no. m-2 in triplicates. Fishes were fed with floating pelleted feed containing 28% crude protein and 5% crude lipid two times daily at 1.5-3% of body weight. During the culture period, fish grew from 26.05 ± 6.57 to 434.61 ± 30.63 g, 306.13 ± 10.68 g and 221.13 ± 14.92 g, respectively, at stocking densities of 3, 6 and 9 no. m-2 respectively. Weight gain percentage and specific growth rate declined with increase in stocking density. Gross fish yield increased with increase in stocking density and was highest at SD9 (657.92 ± 53.55 kg pen-1), while net fish yield increased initially from SD3 to SD6 (594.31 ± 29.72 kg pen-1) and then declined with further increase in stocking density. Important water quality parameters influencing fish growth were measured, and significant difference (p > 0.05) was not observed between treatments (inside pens) and reference site (outside pen at 10-m distance). Weight gain was positively correlated (p < 0.05) to water temperature (r = 0.989) and total phosphorus (r = 0.81). Benefit cost ratio and net return was highest at SD3 (1.61; US $518.88, respectively). Stocking density of 3 no. m-2 can be considered economically feasible for table fish production of L. catla in pens. Post pen culture, monthly income of fishers increased by 10.76-179.11%, with a mean increase of 90.57%, compared to the period of first COVID-19 wave in India. The present findings can provide an impetus for effective utilization of pen enclosures for income generation and livelihood enhancement of small-scale wetland fishers during pandemic.

The diversity and use of dwarf swamp forest vegetation in a tropical floodplain lake in West Kalimantan, Indonesia

To alleviate the consequence of severe biodiversity threats, fifteen national priority lakes to be rehabilitated have been declared in Indonesia. Lake Sentarum National Park (LNSP), one of the lakes, is a distinctive floodplain lake that exhibits significant vegetation and flora diversity. One particular ecosystem type in the area is dwarf swamp forest vegetation, which displays enormous amounts of floral vegetation in that area. This study intends to uncover vegetation data in the dwarf swamp forest habitat, which currently has relatively limited data series about its diversities. The vegetation specimens were collected using a 50 x 50 m line transect. Fourteen species from ten families were collected from six observation stations. The most prevalent vegetation is C. cf. ensifolius, I. mentangis, and B. acutangula. The species diversity index (H') is 1.78 (low category), and the small lake area has a greater species variety than the river area in the LNSP area. Furthermore, we found that dwarf swamp forest vegetation is mainly used as food for Apis dorsata honey bees to produce forest honey. The findings of this study will be helpful as a fundamental reference for future monitoring, research, and conservation efforts in the park.

Linking Biogeochemical and Hydrodynamic Processes to Model Methane Fluxes in Shallow, Tropical Floodplain Lakes

AbstractFloodplains lakes are abundant in the Amazon basin and are important methane sources to the atmosphere. Existing biogeochemical models require modifications and inclusion of hydrodynamic processes operative in shallow, warm waters to be applied to these aquatic ecosystems. We modified a 1‐dimensional process‐based, lake biogeochemical model and combined a 3‐dimensional hydrodynamic model to suit Amazon floodplains. We evaluated the combined model's performance simulating methane concentrations and fluxes and several related processes in the open lake and an embayment of a well‐studied Amazon lake. Parameters for calibration were selected through sensitivity tests using a machine learning‐based algorithm, classification, and regression trees. Comparison between simulated and measured fluxes indicate generally good agreement in seasonal patterns and magnitudes. Comparisons of near‐surface concentrations varied with no clear patterns. Simulations of methane concentrations at near‐surface and near‐bottom, and diffusive emissions are most sensitive to carbon mineralization rate, Q10 factors for methanogenesis and oxidation, and methane oxidation potential. Modeled rates of planktonic photosynthesis were generally lower than measurements, though simulated planktonic respiration was often similar to measurements. Simulated rates of methane oxidation were considerably lower, with a few exceptions, than measurements of methane oxidation in oxic water of the lake. Improvements of results of the linked hydrodynamic‐biogeochemical model will result from inclusion of advective transport, use of parameter values appropriate for tropical waters, especially for methane oxidation and photosynthesis, and addition of changes in hydrostatic pressure to model of ebullition.

Hydrological Dynamics of the Pantanal, a Large Tropical Floodplain in Brazil, Revealed by Analysis of Sentinel-2 Satellite Imagery

Extensive tropical floodplain wetlands, such as the Brazilian Pantanal, are complex ecosystems composed of mosaics of permanently and seasonally flooded habitats and are increasingly threatened by land use and climate change. Spatial and interannual variability in the seasonal flood pulse is a fundamental ecological driver in these ecosystems. This study analyzes optical imagery from the Sentinel-2 satellite to determine the extent and seasonal patterns of inundation over five years in the northern Pantanal, a Ramsar site renowned for its wildlife. The study site is bordered by the Cuiabá and São Lourenço rivers, each with distinct flow regimes. Inundation patterns were revealed with a combination of water indices, supervised classification of land cover, and a digital elevation model. Total extent of flooding was underestimated by the optical imagery, but open water bodies were readily delineated with the land cover classification.

Sustainable irrigation management in tropical lowland rice in Brazil

Drought events and water use conflicts drive the need for more efficient water management in rice-growing areas of the Brazilian Cerrado. Recent studies have shown the advantages of adopting water-saving irrigation in the region but a comprehensive assessment is needed. This study aims to model the performance of rice cultivation and water productivity in the tropical floodplains of the Cerrado biome of northern Brazil in response to irrigation management under contrasting seasonal rainfall levels. Twenty-seven scenarios of rice cultivation, resulting from the combination of three sites, three irrigation treatments, and three rainfall regimes, were simulated with the calibrated and validated hydrological model SWAP/WOFOST. The rainfall levels high (1501 mm), intermediate (952 mm), and low (510 mm) were relative to 120 days and obtained from weather stations located in the region. Two irrigation methods (flooding and water-saving irrigation) were compared against rainfed cultivation. Actual transpiration of the flooding and water-saving irrigation was 9 % and 4 % higher in the intermediate and high rainfall scenarios while it was 30 % and 20 % higher in the low rainfall scenario compared to the rainfed treatment. The largest deep percolation loss was 12700 mm per season for flood irrigation in the low rainfall scenario, whereas the lowest one was 349 mm for the rainfed treatment in the low rainfall scenario. Changing from flooding to water-saving irrigation increases water productivity by an average of 9 % and decreases relative grain yield by 5–12 %. Water productivity based on bottom flux increased on average by about five times (high rainfall scenario) to ten times (low rainfall scenario) when comparing flooding with water-saving irrigation. Results suggest that saving irrigation based on crop transpiration can reduce deep percolation losses and increase water productivity in the rice-growing areas of the Brazilian Cerrado.

Assessment of cardinal temperatures of Egeria najas Planchon and its potential growth in a tropical floodplain lagoon

Assessment of cardinal temperatures of Egeria najas Planchon and its potential growth in a tropical floodplain lagoon

Extreme Drought Event Affects Demographic Rates and Functional Groups in Tropical Floodplain Forest Patches

Extreme Drought Event Affects Demographic Rates and Functional Groups in Tropical Floodplain Forest Patches

Standardizing Pen Culture of Small Indigenous Fish Labeo bata in the Tropical Floodplain Wetland of the North Eastern Region, India: A Step towards Sustainable Fisheries Management

Labeo bata fry were reared in high-density polyethylene (HDPE) pens at different stocking densities to determine growth, survival, feed utilization efficiency and economics in Borkona beel, Barpeta, Assam. Pens (500 m2 each) were stocked with fry at four stocking densities, 3 (SD3), 5 (SD5), 7 (SD7) and 9 (SD9) no. m−2 in triplicates. Feeding was performed twice a day at a rate of 3–5% body weight with floating pelleted feed containing 28% crude protein. Fish grew from 2.38 ± 0.30 g to 82.78 ± 3.18, 75.94 ± 0.89, 71.94 ± 0.89 and 61.81 ± 3.35 g at stocking densities of 3, 5, 7 and 9 no. m−2, respectively. Weight gain per cent ranged from 2491.85 ± 140.56 to 3371.33 ± 133.16 and specific growth rate from 1.41 ± 0.02 to 1.55 ± 0.02, both of which decreased with increasing stocking density. The net and gross yields increased with increasing stocking density and were highest at SD9. The benefit-cost ratio was maximum at SD7 (1.42), followed by SD9 (1.41). Post-pen culture, the monthly income of fishers increased by 6.10% (SD3) to 40.50% (SD9). Significant differences (p &gt; 0.05) in water quality parameters were not observed between treatments (inside pens) and reference site (outside pen at 10 m distance). Weight gain exhibited a significant positive correlation with temperature (r = 0.92; p = 0.029) and alkalinity (r = 0.95; p = 0.014). The present study can provide impetus towards species diversification in pen enclosures, income enhancement of small-scale wetland fishers and sustainable ecosystem-based floodplain wetland fisheries management.

Fire on a tropical floodplain: a fine-scale fire history of coastal floodplains in the Northern Territory, Australia

Background The coastal floodplains of northern Australia are fire-prone, but the impact of fire on floodplain biota is not well understood. Aims In this study, we sought to characterise the fire history of six adjacent floodplains in coastal Northern Territory, Australia. Methods We built a fine-scale 31-year fire history (1988–2018) to compare fire regimes on floodplains across land tenures and floodplain fire regimes with the surrounding savanna fire regime, determine the extent to which current fire regimes are meeting existing ecological fire thresholds, and investigate the relationship between rainfall totals and the extent of burning. Key results Floodplains in conservation reserves burnt more frequently than those on pastoral lands, and savannas burnt more often than floodplains. Current floodplain fire regimes comfortably meet existing ecological fire thresholds. The proportion of floodplain burning is inversely proportional to the amount of rain in the previous wet season. Conclusions Floodplain fire regimes vary markedly between land-use types, and floodplain fire regimes differ to those of savannas. The current management thresholds for floodplain fire regimes would benefit from further evidence of conservation outcomes. Implications For more effective floodplain fire management, research is needed to generate floodplain-specific thresholds that best conserve their considerable conservation value.

Diversity of eukaryotic and prokaryotic microbiota revealed by metabarcoding in Neotropical floodplain lakes.

The diversity of eukaryotic and prokaryotic communities has been assessed by morphological and genetic approaches, which are used to characterize the microbiota in different environments. Here, planktonic prokaryotic and eukaryotic communities of the Araguaia River, located in the Central region of Brazil, were analyzed based on metabarcoding analysis of rRNA genes to evaluate the diversity of these groups in tropical floodplain lakes. Also, we tested their spatial concordance throughout the Araguaia river. Water samples were collected from 8 floodplain lakes in Araguaia River. The 16S and 18S rRNA genes were amplified and sequenced using Illumina MiSeq. For eukaryotes, 34,242 merged reads were obtained and 225 distinct OTUs were delineated, of which 106 OTUs were taxonomically classified. For prokaryotes, 26,426 sequences were obtained and 351 OTUs were detected. Of them, 231 were classified in at least one taxonomic category. The most representative eukaryotes belonged to Ciliophora, Chlorophyta and Charophyta. The prokaryotic phylum with the most OTUs classified were Proteobacteria, Actinobacteria and Bacteroidetes. The lakes did not show spatial concordance when comparing the similarity between their microbiota. The knowledge of freshwater biodiversity using DNA sequencing for important rivers, such as Araguaia River, can improve microbiota inventories of tropical biodiversity hotspots.

Dissolved silicon in a lake-floodplain system: Dynamics and its role in primary production

Dissolved silicon (DSi) is essential for aquatic primary production and its limitation relative to nitrogen (N) and phosphorus (P) facilitates cyanobacterial dominance. However, the effects of DSi on phytoplankton growth and community structure have yet to be fully determined in tropical lakes, particularly in relation to N and P. Therefore, this study investigated the role of DSi in Tonlé Sap Lake, Cambodia, a tropical floodplain system well known for its flood-pulse characteristics and high productivity. To that end, seasonal water sampling and in situ water quality measurements were performed around the floating villages of Chhnok Tru region. The concentration of DSi was significantly higher in the dry season than in the wet season at 16.3–22.1 versus 7.2–14.0 mg/L, respectively; however, both sets of measurements were comparable with lakes in other parts of the world. Meanwhile, the average molar ratio of TN:TP:DSi was 69:1:33 in the dry season and 39:1:24 in the wet season, which compared with the Redfield ratio of 16:1:16, suggested limitation of TP and DSi in both seasons. In addition, phytoplankton biomass in terms of chlorophyll-a was found to be a collective function of DSi, TN:TP, dissolved oxygen, and water temperature in both seasons. Taken together, these results suggest that DSi is affected by the annual hydrological cycle in the Tonlé Sap Lake flood-pulse ecosystem, serving as a secondary limiting nutrient of primary production during both the dry and wet seasons.

Water Quality Indicators in Three Surface Hydraulic Connection Conditions in Tropical Floodplain Lakes

Water quality indicators have been tied to natural or man-made surface hydraulic connection (SHC) conditions. Among these, temporally connected lakes (TCL) are hydraulic intermediates between isolated (IL) and permanently connected lakes (PCL). Therefore, the aim of this study is to answer if water quality indicators can estimate the possible overlap between the two opposed conditions of SHC (IL and PCL) with the intermediate one (TCL) in lakes with similar modifications in the water level regulation at the basin level. Among nine water variables sampled in six lakes with the three SHC conditions mentioned, chlorophyll a (Chl-a), Secchi disk (SD), and total phosphorus (TP) were identified as quality water indicators through principal component analysis. Furthermore, said indicators were used to measure their overlap and trophic state index deviations. The Chl-a, SD, and TP values in TCL showed a 0.72 overlap of PCL and IL. TP surplus measured in all the lakes was meaningful in urbanized ILs and lessened in a rural lake (PCL6) with submerged rooted macrophytes. The estimated overlap of trophic indicators between TCL, IL, and PCL in this study must be verified at a global representative scale for predictive and preventive use in the conservation of tropical coastal plain lakes.

Sediment organic carbon stock and carbon sequestration estimate of a tropical Floodplain Wetland in Assam, India

Sediment organic carbon stock and carbon sequestration estimate of a tropical Floodplain Wetland in Assam, India

Change in Structural Components Due to Seasonal Flooding Governs Provisioning Ecosystem Service Delivery and Livelihood Diversification: A Case Study from a Tropical Floodplain Wetland in Barak Valley Region of India

Annual flood events lead to periodic changes in structural components within seasonal floodplain wetlands. Such change in structural components may determine the type and quantity of provisioning ecosystem services (ES) that offer resident communities opportunities for diverse livelihood activities. We elucidate this hypothesis using the case study of a seasonal floodplain wetland—Chatla—located in Assam, northeast India, by integrating spatially explicit data of structural components of the wetland with social-ecological information following standard methods. The structural components of Chatla during dry and wet seasons were mapped using LANDSAT 8 satellite data. The provisioning ES was quantified and the corresponding economic value was estimated through household- and market surveys. About 51% of Chatla remained inundated during the wet season, which served as a capture fishery source for the riparian communities. Only 6% of the wetland area retained water during the dry season, supporting culture fishery, while a substantial area was brought under paddy cultivation. Both fishery and paddy cultivation served as the major source of income for riparian households. In addition, various non-timber forest products (NTFPs) harvested from the wetland were used for subsistence during both seasons. The total annual value of provisioning ES in harvesting paddy, fish, NTFPs, and soil extraction was estimated as USD 387,487. Our study confirms that the change in structural components of the wetland driven by the annual flooding ensures the delivery of provisioning ES that facilitates livelihood sustainability. We suggest that national and international policies focus on the conservation and wise use of such seasonal wetlands to ensure a sustainable future for the wetland-dependent population in the tropics.