How do extreme fluctuations in water level affect fish condition in Amazonian Floodplain Lakes?

The annual flood pulse is a defining feature of Amazonian floodplain lakes, creating a highly variable environment that influences resource availability, such as food and habitat. These cyclical changes necessitate a high degree of adaptability among fish species, many of which have evolved specialized strategies to cope with the fluctuating conditions. In 2023, the Amazon basin experienced a record-breaking drought event, leading to mass mortality of Amazonian fish and other wildlife. This study examines the effect of this extreme event on fish condition in white-water (Rio Solimões basin) and black-water (Rio Negro basin) floodplain lakes. These contrasting environments provide a unique opportunity to study how different water qualities and extreme water-level fluctuations impact fish condition. Research was conducted during the normal low-water period in November 2019 and the drastically decreased water levels in November 2023. The main objective was to understand how extreme water-level fluctuations affect fish health and nutritional status. A total of 585 fishes were analyzed, with 294 from white-water and 291 from black-water, representing different feeding types to provide a comprehensive picture of changes in fish condition. Water-level changes had a statistically significant impact on fish condition in both areas. Comparing low-water and extreme low-water levels, fish condition was consistently higher during the normal low-water period. The linear mixed-effects model revealed that the intensity of the low-water season had a significant effect on fish length-adjusted mass, suggesting that the decrease in water level is associated with an overall decrease in fish length-adjusted mass. When comparing the mean water-level effect (Glass's Δ) between low-water and extreme low-water levels, we found a bigger effect in the black-water system than in the white-water system. This difference may be attributed to the lower nutrient content and higher levels of humic acids and refractory dissolved organic matter in black-water, which can further limit primary productivity and food availability for fishes.