Biomassa de peixes é subsidiada por recursos alóctones em um reservatório subtropical recém-formado: aumento da aloctonia para áreas ripárias preservadas

Abstract

Ecological quality and the functioning of rivers are strongly influenced by their riparian region, which can directly affect the distribution of food resources and consequently the feeding relationships of biological communities living in aquatic environments. In this sense, the aim of this research was to identify the basal resources that originated the fish biomass in the Passo São João reservoir (Ijuí River-RS, Brazil) comparing littoral areas with riparian forests and of open canopy. Sampling was carried out in two seasons of the year (spring/2017 and summer/2018). A total of 177 muscle samples belonging to nine species of fish and 51 bulk samples of basal resources (benthic algae, fine particulate organic matter, terrestrial vegetation and debris) were collected in four areas for preserved riparian forest and four areas of open canopy. The carbon and nitrogen isotopic signatures of the fish generally reflected the importance of terrestrial vegetation as a basal energetic resource, however, differences were observed when isotopic signatures of fish coming from areas of riparian forest and open canopy were compared. The origin of the total biomass of fish showed clearly that there is an increasing allochthonous support to biomass towards the riparian forest, subsidizing around 70% of the total fish biomass in the riparian forest areas vs. around 57% of biomass in the open canopy areas. Most herbivorous-detritivorous fish increased their autochthonous supported biomass towards the open canopy areas. However,the omnivores-carnivorous showed a consistent pattern of allochthonous supported biomass for all species in both areas, but with a slightly higher contribution of allochthonous resources in riparian areas 83.4 (74.3-92.7) than in open canopy 70.5 (56.7-83.0), considering average values and a 95% credibilty intervals. On the other hand, the results obtained from the volumes of stomach contents of the species studied did not present differences between the food items consumed in both areas. The present study shows that the riparian forest represents an important energetic subsidy for several fish species and its removal may cause a limitation in the supply of food items, affecting the trophic structure of the fish community