Estradas, alagados antrópicos, peixes e a fragmentação de redes fluviais em uma paisagem agrícola do Nordeste do Pará

Abstract

Badly planned roads crossing streams can generate small impoundments in channel segments upstream these crossings, turning lotic environments into lentic ones. The aim of this work was to study the diversity and functional structure of fish communities in anthropic impoundments in northeastern Pará state. These impoundments are formed due to roads construction on the riverine network, with large or small impacts on the hydrological connectivity, subject that is largely ignored for Neotropical lotic systems. Eighteen reaches were sampled in seven streams: six crossed by roads, and one not crossed by roads, used as a reference. Five streams are located in the cities of Igarapé-Açu and Marapanim in an agricultural landscape, and two in the city of São Francisco do Pará in a forest matrix. Five of six streams crossed by roads showed the formation of large impounded areas. In these locations three reaches were selected to sample fish: one upstream, one inside, and another one downstream the impoundment. In the not impounded stream only two reaches, upstream and downstream the road crossing the channel, were selected. The selected environments were sampled once between June and November, 2010, using visual census techniques (ad libitum and linear transect) in 200 m extension reaches. Seventy three species distributed in six orders, 26 families and 63 genera were registered. Characiformes (38.4%) and Siluriformes (31.5%) were the most representative orders in species number. Characidae, with 15 species (20.5%), e Cichlidae, with 10 (13.7%), were the most representative families in species number. These species were organized in 18 functional trophic groups (FTG) formed according to the most frequency observed feeding tactics to and its observed spatial distribution during the diving sessions. The impoundments enforced modifications in the streams biotic (assemblage taxonomic and functional structure) and physical structure of the streams. These environments showed lower species richness, higher abundance and lower FTG diversity in relation to the upstream and downstream reaches. The impoundments showed larger width, depth and high temperature besides less average speed and dissolved oxygen when compared to the upstream and downstream reaches. The use of functional trophic groups approach allowed to understand how the riverine network fragmentation process interfere in the studied fish assemblage structure.