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 Amazon River. 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 consumers by investigating the density, biomass and distribution of mesozooplankton and ichthyoplankton communities across the two rivers during the rainy season. Our results show that mean mesozooplankton density (2,730 inds. m−3) and biomass (4.8 mg m−3) were higher in the black-water river compared to the white-water river (959 inds. m−3; 2.4 mg m−3); however an exceptionally high mesozooplankton density was not observed in the confluence boundary. Nonetheless we found the highest density of ichthyoplankton in the confluence boundary (9.7 inds. m−3), being up to 9-fold higher than in adjacent rivers. The confluence between white and black waters is sandwiched by both environments with low (white water) and high (black water) zooplankton concentrations and by both environments with low (white water) and high (black water) predation pressures for fish larvae, and may function as a boundary layer that offers benefits of both high prey concentrations and low predation risk. This forms a plausible explanation for the high density of ichthyoplankton in the confluence zone of black and white water rivers.
Highlights
The region where two different hydrological regimes meet is characterized by strong physical and biological processes (Walkusz et al, 2010; Bolotov, Tsvetkov & Krylov, 2012)
This study describes the density and biomass of mesozooplankton and ichthyoplankton across the Negro River and the Amazon River in the center of the Amazon basin to elucidate the distributional differences between the two rivers and their confluence zone, which were not previously well-described quantitatively
As the multidimensional scaling (MDS) and analysis of similarities (ANOSIM) analyses clearly indicated, the present study revealed that the compositions of mesozooplankton assemblages differ between the white water of the Amazon River and black water of the Negro River
Summary
The region where two different hydrological regimes meet is characterized by strong physical and biological processes (Walkusz et al, 2010; Bolotov, Tsvetkov & Krylov, 2012). The water properties of the white and black waters are different in terms of many parameters such as flow speed, conductivity, turbidity, pH, water temperature, nutrient concentrations, and dissolved and particulate organic matter concentrations (Laraque et al, 1999; Moreira-Turcq et al, 2003; Leite, Silva & Freitas, 2006; Filizola et al, 2009; Laraque, Guyot & Filizola, 2009; Franzinelli, 2011; Röpke et al, 2016). The black and white water rivers are not completely mixed until over 100 km beyond the confluence (Laraque, Guyot & Filizola, 2009)
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