New insights on the relation between zooplankton and humic substances in tropical freshwater ecosystems.

Clarice C Nova,Reinaldo L Bozelli,Adriana M Rocha,Christina W.C Branco

doi:10.1590/0001-3765202120190409

Abstract

Aquatic humic substances (HS) represent about 60-95% of the dissolved organic carbon (DOC) present in coastal environments of northern Rio de Janeiro state, Brazil. Although they are important regulators of processes involving aquatic communities, the response of the zooplankton community to their presence remains poorly understood, especially in natural tropical environments. Therefore, our objective was to elucidate zooplankton community responses along a natural gradient of HS. Such natural humic gradient was obtained in coastal freshwater environments with distinct DOC concentrations (20-200 mg L-1). Results show a decrease in zooplankton density and biomass along the HS gradient. However, microphages organisms (e.g. non-predatory rotifers and smaller testate amoeba, such as Difflugia) were most present in environments with higher concentration of HS, probably due to a stronger importance of the microbial-loop in these environments. Some species - such as Scapholeberis armata (Cladocera) and Lecane boettgeri (Rotifera) were only accounted for environments with high HS concentration, illustrating their potential as bioindicators for HS presence. Nevertheless, we were able to observe the effects of HS on the structure and composition of primary consumers and how these substances might indirectly affect species dynamics. We point out to new findings in highly humic tropical environments, which are still poorly studied and understood.

Highlights

Humic substances (HS) are heterogeneous compounds with high molecular weight, biologically stable and which have a coloring trait, formed in the environment as a byproduct of organic matter decomposition (Druvietis et al 1998)
This table shows that that was only a subtle variation in physical and chemical parameters measured in the field in a seven-day interval, illustrating the constancy of water column conditions for zooplankton
The humic gradient, which is mentioned here, was obtained due to the variation in color and dissolved organic carbon content, since HS could represent over 95 % of the DOC in most water bodies (Steinberg 2003, Suhett et al 2011)

Summary

Introduction

Humic substances (HS) are heterogeneous compounds with high molecular weight, biologically stable and which have a coloring trait, formed in the environment as a byproduct of organic matter decomposition (Druvietis et al 1998) These substances are complex organic molecules which represent most (up to 95%) of the dissolved organic carbon in aquatic ecosystems (Wetzel 2001, Steinberg et al 2006) reaching this percentual contribution in tropical coastal ecosystems (Farjalla et al 2009). HS have been historically considered biologically inert substances in aquatic ecosystems and their ecological functions are generally related to nutrients bioavailability, pH control and light penetration (Steinberg et al 2006, Steinberg 2003) Due to these effects on the water column, these substances may act limiting the autotrophic primary production and its related food web (Jones 1992). The main and most direct role usually attributed to HS is associated to its use as primary source

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