Relation between flood pulse and functional composition of the macroinvertebrate benthic fauna in the lower Rio Negro, Amazonas, Brazil

Abstract

In a twelve-month study carried out at the archipelago of Anavilhanas, lower Rio Negro, State of Amazonas, Brazil, the macroinvertebrate fauna was sampled from the margins of a lake (Redondo lake) permanently connected to the river. The total biomass of the macroinvertebrates sampled during this period was 9,985.5 mg (dry weight) and 19,314 specimens were collected. Decapods (52.9 %) were the most important group regarding biomass while dipterans (21.1 %), ostracods (20.0%), cladocerans (14.0 %), and oligochaetes (13.4%) were the most abundant macroinvertebrates. All taxa showed frequency variations related to the high-low water cycle of Rio Negro and the substrate at the lake margins varied in quantity as well as in composition and structure. The greatest amounts of litter occurred in the flood period, with a higher proportion of new and whole leaves. The degree of litter fragmentation increased towards the low water period. The functional structure of the macroinvertebrate community showed adaptation to the available substrate in each season. A gradient of dominance between the functional categories, related to the distance from the central area of the water body, was observed. Shredders were sampled mainly inside the "igapó" (high water period), in a similar proportion to collectors. On the other hand, collectors were clearly predominant in the dry period. Scrapers were limited to the rising and falling water periods. The same trend is probably found in all the corresponding sections of the Rio Negro