Zooplankton community shifts along a natural humic substances gradient

Background/Question/Methods

Aquatic humic substances represent about 60-95% of the dissolved organic carbon present in coastal environments of northern Rio de Janeiro state, Brazil. They are very important as regulators of processes involving aquatic communities. Thus, humic substances act in aquatic ecosystems altering trophic structure, stressing organisms, altering the pH and limiting nutrients bioavailability. The response of the zooplankton community to such substances remains ubiquitous and poorly understood, especially in coastal lagoons regarded as ‘highly humic’ environments. Therefore, our objective is: determine whether the zooplankton community can be pointed out as a potential indicator of natural humic gradients. Considering that, due to humic substances’ direct and indirect effects, the zooplankton composition would be altered and consequently its biomass and abundance would be indirectly correlated to the increase of humic substances concentration.  A natural humic gradient was obtained in 7 freshwater shallow lakes with distinct dissolved organic carbon concentrations (20-200mg.L^-1). The zooplankton community of each environment was sampled during February 2012 through 50μm mesh filtration of 100L of water. Species richness, abundance and diversity were analyzed and integrated with environmental variables through a Canonical Correspondence Analysis. Additionally, the origin of the organic carbon was also analyzed through the comparison of the fluorescence spectra. 

Results/Conclusions

Results show a decrease in zooplankton density along the studied gradient as hypothesized: highly humic environments had lower zooplankton abundance. However, planktonic testate amoeba had higher abundance in the most humic environments. Microphages organisms were most present in lakes with higher concentration of humic substances, probably due to a greater importance of the microbial-loop in these environments. Some species – such as Scapholeberis armata (Cladocera) and Lecane boettgeri (Rotifera) were only accounted for in highly humic ecosystems, illustrating their potential as bioindicators. Species diversity and evenness of the zooplankton community were not clearly correlated to the humic substances content. On the other hand, zooplankton biomass, as total abundance, was higher in less humic and non-humic lakes. Furthermore, the comparison of the fluorescence spectra reveled that the organic carbon of all studied environments had the same origin, confirming a uniquely quantitative variation of humic substances concentration.  Finally, we point out that along an increasing natural humic substances gradient, an effect on the structure and composition of primary consumers was observed, high lightening a shift in the trophic structure of each environment.