The density and biomass of mesozooplankton and ichthyoplankton in the Negro and the Amazon Rivers during the rainy season: the ecological importance of the confluence boundary

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.

Mesozooplankton of the Damietta Branch, River Nile, Egypt.

“…The CCA ordination analysis revealed that the considerable variations in different mesozooplankton groups among sampling stations were significantly correlated with the variations of temperature, conductivity, and nitrate concentrations. This confirms with the observations in other areas, where the water temperature was a keystone element in the seasonal dynamics of mesozooplankton (Jerling and Weerts, 2018;Nakajima et al, 2017), and may regulate most ecological mechanisms in temperate areas (Sellami et al, 2010). As mentioned by El-Tohamy (2018a), the study area receives relatively high anthropogenic disturbance, directly or indirectly resulting in high values of nutrients and conductivity.…”

Ontogenetic structure and distribution patterns of ichthyoplankton in the confluence zone of two river systems in the Eastern Amazon

“…This dynamic allows the transport of organisms to flooded wetlands, with high productivity and complex habitats (e.g. aquatic macrophytes), which favors biological recruitment (Nakajima et al., 2017; Oliveira et al., 2020; Stuart & Sharpe, 2020; Zacardi et al., 2020).…”

“…These ecotones function as a key habitat structure and work as filters in the dispersion of fish in river systems, influencing changes in species composition at small spatial scales (Czeglédi et al., 2016; Thornbrugh & Gido, 2010). In addition, they constitute spatial elements of the organization and structuring of fish populations in the different phases of their life cycle (Kiffney et al., 2006; Nakajima et al., 2017). They also act as important feeding and spawning areas for several species of migratory fish of economic importance, widely exploited by commercial and subsistence fishing in the Amazon (Barthem et al., 2016; Zacardi, 2020).…”

“…Despite its relevance to adult individuals of the ichthyofauna, little is known about the functioning and structure of the community of fish eggs and larvae in the confluence zones between rivers with different trophic state indices of the Amazon region (Nakajima et al., 2017; Röpke et al., 2016; Zacardi & Ponte, 2016). This information becomes crucial when it comes to detecting areas of spawning, nursery, growth and development; to understand the ecological aspects that modulate and structure the ichthyoplankton community; and to supporting measures for the management and conservation of species.…”

“…organization and structuring of fish populations in the different phases of their life cycle (Kiffney et al, 2006;Nakajima et al, 2017). They also act as important feeding and spawning areas for several species of migratory fish of economic importance, widely exploited by commercial and subsistence fishing in the Amazon (Barthem et al, 2016;Zacardi, 2020).…”