Hydrological seasonality of the river affecting fish community structure of oxbow lakes: A limnological approach on the Amapá Lake, southwestern Amazon

The hydrological regime represents the driver of ecological function and biodiversity in tropical river floodplains. This study aimed to document the species composition and evaluate the effect of the flood pulse on the fish community of Amapá Lake, an oxbow lake temporarily connected to the Acre River. Between October 2008 and September 2009, fish were sampled monthly with gill nets, and seven physical-chemical variables were recorded at three sampling stations along the lake. The fish community structure and physical-chemical variables were compared among the hydrological phases of pre-flooding, minor and major flood and post-flooding, and the degree of association between the limnological characteristics and species composition was explored with a canonical correspondence analysis. A total of 2131 specimens belonging to 53 species were captured. The numerical abundance and biomass of fish and water temperature were higher in the pre and post-flooding phases. Samples of the pre-flooding phase comprised more than 40% of the overall numerical abundance (115 ind. 1000 m2 24 h−1 ± 16 se) and biomass (9949 g 1000 m2 24 h−1 ± 1.816 se) due to the greater dominance of few species, mainly small (the Siluriformes Hypoptopoma gulare) to medium sized (the Characiformes Triportheus curtus) during low waters. With the onset of the minor flood, conductivity decreased and depth increased in the lake, whereas total phosphorous and pH reduced significantly at major flood. The composition and abundance of fish community changed more among hydrological phases than species richness and 26% of this annual variability in the biotic component was shaped exclusively by the physical-chemical variables. As fish community and physical-chemistry of the Amapá Lake undergo to the hydrological cycle of the Acre River, these results reinforce the general pattern of predictable seasonal alterations in the functioning of tropical floodplains. Based on the general decrease in dominance during the long flood pulse and the occurrence of piscivorous migrators among the five most abundant species, we conclude that the seasonal variation in the hydrometric level of the Acre River plays an important role in maintaining the high fish diversity observed in the Amapá Lake all year round.