The abundance and biomass of mesozooplankton and ichthyoplankton in the confluence boundary of the Negro and the Amazon Rivers

Abstract

The boundary zone between two different hydrological regimes is often a biologically enriched environment with distinct planktonic communities. In the center of the Amazon River basin, muddy white water of the Amazon River meets with black water of the Negro River, creating a conspicuous visible boundary spanning over 10 km along the AmazonRiver. Here, we tested the hypothesis that the confluence boundary between the white and black water rivers concentrates prey and is used as a feeding habitat for juvenile fish by investigating the abundance, biomass and distribution of mesozooplankton and ichthyoplankton communities across the two rivers. Our results show that mesozooplankton abundance and biomass were higher in the black-water river compared to the white-water river; however an exceptionally high mesozooplankton abundance was not observed in the confluence boundary. Nonetheless we found the highest abundance of ichthyoplankton in the confluence boundary, being up to 9-fold higher than in adjacent rivers. The confluence boundary between black and white water rivers may function as a boundary layer that offers benefits of both high zooplankton prey concentrations (blackwater) and low predation risk (white-water). This forms a plausible explanation for the high abundance of ichthyoplankton in the confluence zone of black and white water rivers. 22 Abstract 23 The boundary zone between two different hydrological regimes is often a biologically enriched 24 environment with distinct planktonic communities. In the center of the Amazon River basin, 25 muddy white water of the Amazon River meets with black water of the Negro River, creating a 26 conspicuous visible boundary spanning over 10 km along the Amazon River. Here, we tested the 27 hypothesis that the confluence boundary between the white and black water rivers concentrates 28 prey and is used as a feeding habitat for juvenile fish by investigating the abundance, biomass 29 and distribution of mesozooplankton and ichthyoplankton communities across the two rivers.30 Our results show that mesozooplankton abundance and biomass were higher in the black-water 31 river compared to the white-water river; however an exceptionally high mesozooplankton 32 abundance was not observed in the confluence boundary. Nonetheless we found the highest 33 abundance of ichthyoplankton in the confluence boundary, being up to 9-fold higher than in 34 adjacent rivers. The confluence boundary between black and white water rivers may function as 35 a boundary layer that offers benefits of both high zooplankton prey concentrations (black-water) 36 and low predation risk (white-water). This forms a plausible explanation for the high abundance 37 of ichthyoplankton in the confluence zone of black and white water rivers. & Kingsford, 1996). Thus, the boundary zone is important for local 55 ecosystem functioning. 56The Amazon River is well-known for its largest and most dense river network in the 57 world and has the highest level of discharge, contributing ca. 20% of the global continenta...