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

Abstract

Humic substances, a component of terrestrial dissolved organic matter (tDOM), contribute to dissolved organic matter (DOM) and chromophoric DOM (CDOM) in coastal waters, and have significant impacts on biogeochemistry. There are concerns in recent years over browning effects in surface waters due to increasing tDOM inputs, and their negative impacts on aquatic ecosystems, but relatively little work has been published on estuaries and coastal waters. Photodegradation could be a significant sink for tDOM in coastal environments, but the rates and efficiencies are poorly constrained. We conducted large-scale DOM photodegradation experiments in mesocosms amended with humic substances and nutrients in the Gulf of Finland to investigate the potential of photochemistry to remove added tDOM and the interactions of DOM photochemistry with eutrophication. The added tDOM was photodegraded rapidly, as CDOM absorption decreased and spectral slopes increased with increasing photons absorbed in laboratory experiments. The in situ DOM optical properties became similar among the control, humic-and humic+nutrients-amended mesocosm samples toward the end of the amendment experiment, indicating degradation of the excess CDOM/DOM through processes including photodegradation. Nutrient additions did not significantly influence the effects of added humic substances on CDOM optical property changes, but induced changes in DOM removal.

The response of aquatic ecosystems to the interactive effects of stratospheric ozone depletion, UV radiation, and climate change

“…However, to be ecologically relevant, these enclosures need to be transparent to UV radiation. Unfortunately, some common designs for mesocosms use UV-opaque materials (e.g., [ 73 ]) or covers (e.g., [ 74 ]), which leaves open the question of how representative the results of some multi-stressor experiments are with respect to natural conditions that include exposure to UV radiation—such as the finding that ocean acidification encourages the growth of toxic microalgae [ 75 ].…”

Dataset from a mesocosm experiment on brownification in the Baltic Sea

“…Empty cells indicate no data. The primary data is discussed in [ 15 ], and additional results on photochemical degradation of dissolved organic matter is presented in [ 16 ].…”