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.
