Comprehensive analysis of chemical and biological problems associated with browning agents used in aquatic studies

Abstract

Inland waters receive and process large amounts of colored organic matter from the terrestrial surroundings. These inputs dramatically affect the chemical, physical, and biological properties of water bodies, as well as their roles as global carbon sinks and sources. To understand the complex changes associated with allochthonous inputs, experiments are needed. However, manipulative studies, especially at ecosystem scales, require large amounts of dissolved organic matter with optical and chemical properties resembling indigenous organic matter. Here we compared the chemical and biological impacts of two leonardite products (HuminFeed (HF) and SuperHume (SH)) and a freshly derived reverse osmosis concentrate of organic matter (RO) in a set of comprehensive mesocosm- and laboratory-scale experiments and analyses. The chemical properties of RO concentrate and the leonardite products were very different with leonardite products being low and RO being high in carboxylic functional groups. Light had a strong impact on the properties of leonardite products, including loss of color and increased particle formation. Furthermore, HF had drastic impacts on bacteria as light stimulated bacterial production and modified community composition, while dark conditions appeared to inhibit bacterial processes. While none of the browning agents inhibited the growth of the tested phytoplankton, Gonyostomum semen, leonardite products had detrimental effects on zooplankton abundance and Daphnia reproduction. We conclude that the effects of browning agents extracted from leonardite are in sharp contrast to those originating from terrestrially-derived DOM. Hence, they should be used with great caution in experimental studies on the consequences of terrestrial carbon for aquatic systems.

Effects of Added Humic Substances and Nutrients on Photochemical Degradation of Dissolved Organic Matter in a Mesocosm Amendment Experiment in the Gulf of Finland, Baltic Sea

“…Other factors and processes will likely affect the in situ photodegradation of DOM (75–77), but high laboratory photodegradation rates suggest that there is a high potential for photodegradation of the added humic substances, at least of the colored portion. While the samples in our Baltic Sea study are different, previous work found very little degradation of HuminFeed when kept in the dark, but observed CDOM and DOC decreases accompanied by increases in particulate matter, when exposed to visible light (68).…”

“…7 and Table 1). These trends were again consistent with the addition of humic substances to the mesocosm, and may suggest an increase in aromaticity when humic substance was added (65) and a decrease in aromaticity during subsequent in situ degradation of the added humic substance (79,83), as HuminFeed is abundant in aromatic compounds (68). The SUVA 254 values became similar again between the control and humic‐amended mesocosms toward the end of the amendment experiment (Fig.…”

“…1) despite the higher absorption, most likely due to the higher attenuation coefficients in the amended samples, so light decreased with depth much faster in the amended mesocosms. The higher attenuation in turn was likely due to the higher CDOM absorption from humic substance addition, but possibly also from particle formation induced by the added HuminFeed (68), although the estimation of in situ light dose in this study assumed a constant contribution from CDOM absorption to attenuation. The decrease in light availability at depth caused by higher light attenuation is also consistent with the effect of browning on light availability in the water column (69).…”

“…If the dominant CDOM loss process in situ were photodegradation, we should have seen differences in rates similar to those in laboratory experiments when comparing the loss rates among different treatments. These results suggest that the in situ CDOM fading might not be entirely due to photodegradation, but have contributions from other processes such as biological degradation (68,70,71) or particle formation (33,34,68), especially considering the reduced light availability in the mesocosms because of the cutoff of short‐wavelength UV light by the acrylic cover.…”

“…Spectral slopes were lower in amended samples than in control samples (Figs. 4 and 5 and Figure ), likely due to the increased absorption from the addition of HuminFeed, but may also suggest an addition of higher molecular weight CDOM to the amended samples (64), as HuminFeed is characterized as having a high abundance of higher molecular weight aromatic compounds (68). Spectral slopes increased with photons absorbed and time in amended samples in situ and in laboratory irradiation experiments (with some exceptions) (Figs.…”