Influência da sazonalidade pluviométrica e variáveis ambientais na diversidade ictiofaunística de igarapés de terra firme: abordagem via DNA ambiental.

Abstract

The distribution of biodiversity worldwide is governed by environmental factors such as latitude and temperature, influencing its occurrence and distribution across various biogeographic regions. The Neotropical region holds the greatest biodiversity among all biogeographic regions, encompassing an area from southern California, North America, through Central America, to South America. This region possesses an abundance of biomes and, therefore, is proportionally rich in terms of biological diversity. Among the diverse biomes, the Amazon biome stands out as the largest area, harboring one of the highest biodiversities in the Neotropical region, with a vast and heterogeneous set of aquatic and terrestrial ecosystems. Among them, aquatic environments stand out as the most diverse, ranging from large rivers with headwaters in the Andes, to black-water streams (igarapés de terra firme), white-water streams (igarapés de várzea), lakes (permanent and temporary), and their estuaries. In these aquatic environments, we find a unique diversity of fish species, surpassing that found in other continents and ecoregions. Due to its vastness, the efforts already undertaken to inventory the Amazonian ichthyofauna remain understudied, and there is still a need to understand how the occurrence and distribution of aquatic species are affected by factors (biotic or abiotic). In this context, it is essential to understand the composition of aquatic communities in relation to environmental changes (anthropogenic and non-anthropogenic) and the effects that climate change can cause, such as the loss of environmental health and consequently the loss of local biodiversity. In this study, we directed efforts to understand the relationship between fish diversity and the preservation conditions of the Adolpho Ducke Forest Reserve (RFAD), allowing us to estimate how fish diversity is distributed in a considered healthy environment. The Adolpho Ducke Forest Reserve is located in the peri-urban region of Manaus-AM and has black-water streams belonging to the Acará hydrographic basin. The composition of fish assemblages was assessed using the environmental DNA method, which detects the presence of organisms by identifying DNA fragments extracted from environmental samples, such as stream water, based on the principle that every organism releases genetic material into the environment. Thus, to obtain a representation of the ichthyofauna in the Acará basin, we collected water samples from 12 streams, seven of which were first-order streams, two second- order, and three third-order, respectively. In addition to the ichthyofaunal survey, the streams were characterized by identifying the area (m²), dissolved oxygen (mg·dL-1), pH, mean depth (m), temperature (°C), current velocity (m/s), and flow rate (m³/s). Our results suggest that rainfall variation influences diversity indices when comparing the rainy and dry seasons, reducing diversity in second-order streams during the dry season, and is probably related to the reduction of connectivity between streams. Furthermore, during the rainy season, although no difference was identified in diversity indices, the composition of fish assemblages in the three stream orders differs in their richness and abundance. Highlighting the heterogeneous nature of black-water stream basins in their healthy condition.