Changes in Rainfall Patterns Affect Stream Fish in Central Amazon

With the increase in rainfall in recent years, the streams (igarapés) of Central Amazon have undergone some changes, influencing the types of fish found in these environments.
Text and Illustration: Gabriel Verçoza

Climate change has transitioned from a future concern to one of the primary threats to biodiversity in the present. One significant consequence of the observed climate crisis is the shift in rainfall patterns, marked by increased intense droughts and storms. Among the Amazonian ecosystems most affected by these changes are the igarapés, as their dynamics directly depend on the amount of rainfall. One of our recent studies indicates how these alterations in the Amazon's rainfall pattern can impact fish populations in these environments.

Forest Fish
When we think of forests, fish rarely come to mind. However, the igarapés that cut through the upland forests (those not in flooded areas) harbor a rich and diverse set of fish species, some of them remarkably beautiful. Dozens of fish species inhabit various aquatic environments in igarapés, from the open waters of channels to the small spaces between the dead forest leaves that accumulate on the bottom. In general, the most distinctive ecological feature of these organisms is their strong dependence on the forest, both for obtaining food, such as insects and plants falling into the water, and for shelter, provided by leaves, branches, and trunks from the forest. This makes fish assemblages especially sensitive to changes occurring within the igarapés and the surrounding land.

The Research
To understand how the fish community is responding to changes in rainfall, we conducted four collections in the Adolpho Ducke Forest Reserve between 2001 and 2018, examining how the species and types of fish living there change over time. Over these 18 years, we observed an increase in the total annual rainfall and the number of stormy days. Simultaneously, as these stronger rains increased, the bottom of the igarapés lost its leaf cover, revealing more exposed sand.

Section of an igarapé in the Ducke reserve with a sandy bottom and reduced leaf cover.Section of an igarapé in the Ducke reserve with a sandy bottom and reduced leaf cover.

The fish community adapted to climate and bottom conditions changes over the years. These changes were mainly evident in the shift in the number of individuals of some common species (such as piabas and acarás). The types of fish also changed over time, with species residing in the water column (laterally flattened fish with smaller fins and mouths at the front of the head) giving way to species adapted to lateral pools and shallow areas (cylindrical bodies, larger fins, and upward-facing mouths). These pools are complex environments that offer new habitats and food sources, situated on the sides of the igarapés.

Com o aumento das chuvas, a quantidade de folhas no canal do igarapé diminui.

With increased rainfall, the amount of leaves in the igarapé channel decreases. Consequently, we observe a reduction in the number of fish occupying these environments and an increase in the population using lateral pools.

Images of fish

What about the Future?
Considering the speed of climate changes observed in this study and the notion that Amazonian igarapés are historically stable environments (changing slowly and little), these results are surprising and indicate that igarapé fish communities can respond rapidly to climate changes. Intense rainfall and stormy days significantly increase sediment discharge, altering the igarapés' bed by moving sediments from the bottom and eroding the banks. This temporarily increases the connection between the main channel and temporary lateral pools. This may favor species with characteristics conducive to using shallow pools, such as smaller and less laterally compressed bodies and larger fins. The overall effect of such changes on ecosystems is not yet fully understood.

Our study did not detect any cases of extreme drought in the area. Despite being severe issues in other Amazon regions, drought events have not yet proven problematic in the igarapés where we conducted the research, possibly due to the protection of watercourses by densely forested areas and the intense water replenishment from springs. This emphasizes the importance of conserving areas around springs and monitoring these environments to understand the short-, medium-, and long-term effects that climate changes can have on water bodies and the organisms that depend on them.

For additional information, read the article: "Temporal changes in rainfall affect the taxonomic and functional composition of stream fish assemblages in central Amazonia," published in the Freshwater Biology journal in 2020 by Gabriel Borba from PELD IAFA and other collaborators.