Impacts of the 2023 Amazon drought in contrasting aquatic ecosystems ( INCT ADAPTA – Scientific Expedition 2023)

This special issue of the Journal of Fish Biology presents 12 original research articles derived from a scientific expedition conducted in November and December 2023 under the auspices of the National Institute of Science and Technology – Center for Studies on the Adaptations of Aquatic Biota of the Amazon (INCT ADAPTA). The expedition investigated the ecophysiological, ecotoxicological and ecological impacts of the unprecedented drought that affected the Amazon basin in 2023, one of the most extreme drought events ever recorded in the region. Field studies were carried out in two ecologically contrasting Amazonian floodplain environments: the Lago do Prato, in the Anavilhanas National Park (Figure 1) located in the lower Rio Negro (blackwater), and the entrance of Lago Janauacá, in the lower Rio Solimões (whitewater). These environments differ significantly in physical and chemical characteristics such as pH, ion concentration, turbidity and dissolved organic carbon (DOC), offering a unique natural laboratory for comparative analysis under extreme hydroclimatic stress. Opening this special issue, the first article provides an integrative overview of environmental conditions during the peak of the 2023 drought, focusing on water quality, zooplankton community structure and their implications for fish health (Johannsson et al., 2026 [in press]). In the Rio Negro, increased total suspended solids reduced underwater visibility, impairing fish foraging while simultaneously degrading the quality of DOC, reducing its protective capacity against metal toxicity and acidification. In contrast, the Rio Solimões exhibited unexpectedly elevated DOC concentrations, but with low molecular quality, and the microbial trophic pathways appeared to be functionally imbalanced. This contribution provides the ecological backdrop for the studies that follow. Subsequent articles address how fish physiologically respond to elevated temperatures, metal exposure and natural water chemistry. The tambaqui (Colossoma macropomum), a key Amazonian species, was used to assess changes in transepithelial potential (TEP), a rarely studied yet critical parameter in ion regulation. TEP was shown to shift significantly under variations in pH, oxygen and temperature, highlighting its sensitivity as a biomarker of environmental stress (Wood et al., 2025). Other experiments explored the combined effects of heat and copper exposure. Juvenile tambaqui experienced intensified ionic imbalances and reduced thermal tolerance in the presence of copper, and these effects were more pronounced in blackwater than in whitewater, confirming the compounded risk posed by climate stress and metal pollution (Crémazy et al., 2025). Further research showed that DOC-rich blackwater can protect fish from ionoregulatory disturbances induced by copper and acidic pH, but this protective effect was not observed with whitewater (Morris et al., 2025) despite the fact that whitewater DOC, freshly collected in an earlier non-drought year, was very protective (Sadauskas-Henrique et al., 2025). The latter study provided evidence that the protective effects weaken when DOC is degraded during storage; the same phenomenon was likely explained by the degradation of whitewater DOC during extreme drought. Hypoxia tolerance emerged as a central theme, with several studies revealing the remarkable adaptability of Amazonian catfishes. Under severely hypoxic conditions, a range of catfish species from both black- and whitewater environments exhibited reduced oxygen uptake yet maintained consistent ammonia and relatively high rates of urea excretion, suggesting strong metabolic plasticity (Pelster et al., 2025a, 2025b). Facultative air-breathing species like Pterygoplichthys altered their respiration patterns and sodium flux rates during hypoxia and recovery, demonstrating finely tuned physiological regulation. Complementing these findings, experiments comparing congeneric species from contrasting river types revealed that blackwater species exhibited lower thermal and hypoxic tolerance than their whitewater counterparts, suggesting distinct physiological strategies shaped by their native environments (Braz-Mota et al., 2025). A novel cellular-level investigation on freshwater stingrays (Potamotrygon) identified two distinct types of ionocytes responsible for acid and base secretion (Rossi et al., 2025). These cells responded selectively to acid–base challenges, underscoring evolutionary specializations for osmoregulation in dilute, low-pH waters such as the Rio Negro. From an ecotoxicological perspective, alarming rates of microplastic ingestion were recorded across fish species in both systems: over 85% of sampled individuals contained plastic particles in their digestive tracts (Tuvikene et al., 2025). Particle types and sizes varied by river and feeding guild, with predators in whitewater and planktivores in blackwater showing the highest loads. This evidence highlights that even remote Amazonian waters are not immune to pervasive anthropogenic contamination. A comparative analysis of fish body condition between 2019 and 2023 revealed a significant decline across both environments, especially in the blackwater floodplain. Reduced productivity, habitat contraction and competition for scarce resources were among the likely drivers of this observed deterioration during extreme drought (Zingel et al., 2025). The final contributions delve into microbial and planktonic dynamics, showing how drought conditions altered trophic flow efficiency at the base of the aquatic food web. In the Rio Negro, microbial energy pathways remained partially functional, whereas in the Solimões, bacterial overgrowth decoupled energy transfer to zooplankton, reducing food availability for higher trophic levels and visual predators (Agasild et al., 2025). This collection of studies represents a multidisciplinary, field-based scientific effort to document in real time the effects of extreme climatic events on tropical aquatic ecosystems. By integrating physiological, ecological and chemical perspectives, the special issue provides a unique and valuable resource for understanding the vulnerability and resilience of Amazonian fish communities in the face of environmental change. We extend our special thanks to the Brazilian National Research Council (CNPq) and the Amazonas Research Foundation (FAPEAM) for their essential support of the INCT ADAPTA project. We also gratefully acknowledge Katherine Sloman (University of the West of Scotland, UK) and Carolina A. Freire (Federal University of Paraná, Brazil) for their careful and dedicated editorial work. These findings underscore the urgency of continued environmental monitoring and adaptive conservation strategies to safeguard the Amazon's unparalleled freshwater biodiversity under a rapidly changing climate. Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.