Amazonian drought of 2023: Environmental conditions relevant to fishes.

This paper provides a platform for the following studies within this Special Issue. 'Ecophysiology of fishes in the two great tributaries of the Amazon in the Anthropocene'. It documents the water quality conditions and accompanying zooplankton community structure and biomass relative to fish health in the Rio Negro and Rio Solimões during the extreme drought of 2023. Our goals were to document conditions, compare them with historical data and assess their implications for the resident fishes. During the 2023 drought, fishes in the Rio Negro came under more pressure from environmental changes than fishes in the Rio Solimões. Habitats became smaller and less diverse than normally observed by the end of the annual drought in both rivers. In some areas, fishes experienced elevated temperatures and/or reduced oxygen especially in the floodplain lakes and shallower areas. In the Rio Negro, increases in total suspended solids (TSS) decreased the range of vision for resident fishes, which would have both protected them from predators and shielded their prey. The coincidence of higher temperatures and higher oxygen in the afternoon in the Rio Negro suggests that primary production still occurred in the shallower, lit surface waters. This would explain the presence of zooplankton near the water's surface. Predation was likely intense as the macro-zooplankton were small, although, the effect of high temperatures on metabolic rates could also influence size. Cladoceran biomass showed a negative relationship with the mass of the 1.2-50 μm fraction of TSS. Higher TSS concentrations likely meant resuspension of sediments. Spectroscopic analysis showed that dissolved organic carbon (DOC) was more degraded than normal, and thus would have provided less protection against the effects of low pH on ionoregulation. Low pH and low conductivity put extreme stress on maintaining ion balance in the black waters. In the Rio Solimões, fish suffered less environmental change. Water temperature and oxygen concentrations were more moderate, although evidence suggests that temperature and conductivity have increased compared to historic data. The concentration of TSS was normal. The greatest change was a 200% to 300% increase in DOC, albeit the DOC was very degraded. This may have spurred the high biomass of bacteria which was out of balance with the other components of the microbial loop. The passage of energy from bacteria to higher zooplankton trophic levels worked efficiently in the Rio Negro but not the Rio Solimões. This study allows a better understanding of the stresses on the fish community in these adverse conditions and the implications for their future.