Effect on Carbon Cycle Organic carbon

Classical Swedish studies some 70 years ago, already noticed that oxygen is undersaturated in the epilimnion of humic-rich lakes. Approximately 40 years later, it has been suggested that an increased degradation of dissolved

Dissolved Organic Carbon Degradation

Figure 6 Photolysis of chromophoric organic substances as source of inorganic nutrients in aquatic ecosystems (particularly phosphorus and nitrogen) and organic substrates (small organic acids, amino acids, amines), which sustain net-heterotrophy in noneutrophicated systems. Heterotrophic production is indicated by black, and photoautotrophic by gray arrows. hv is solar energy.

Figure 6 Photolysis of chromophoric organic substances as source of inorganic nutrients in aquatic ecosystems (particularly phosphorus and nitrogen) and organic substrates (small organic acids, amino acids, amines), which sustain net-heterotrophy in noneutrophicated systems. Heterotrophic production is indicated by black, and photoautotrophic by gray arrows. hv is solar energy.

Dissolved Organic Carbon

Figure 7 In theepilimnion of very acidic lakes, a major source of Fe(ii) is photoreduction, which requires dissolved organic carbon and simultaneously oxidizes it. This photooxidation is the main reason for very low concentrations of dissolved organic carbon in the very acidic Lausatian postmining lakes (Germany). hv is solar energy. The ecological consequences are outlined in the text.

Figure 7 In theepilimnion of very acidic lakes, a major source of Fe(ii) is photoreduction, which requires dissolved organic carbon and simultaneously oxidizes it. This photooxidation is the main reason for very low concentrations of dissolved organic carbon in the very acidic Lausatian postmining lakes (Germany). hv is solar energy. The ecological consequences are outlined in the text.

organic substances is the potential cause for this under-saturation. This increased degradation may be facilitated, when chromophoric organic substances are split by UV radiation and the photodegradation products become more easily available to microorganisms.

The increased bioavailability is associated with significant increases of bacterial numbers and biovolume of the bacteria. In humic lakes, the direct coupling of photochemical production to biological use of organic C accounts for the comparatively constant activity of bacteria that is independent of phytoplankton primary production. The following fatty acids and further organic compounds are released from sunlit chromophoric organic substances: acet-aldehyde, acetate, acetone, citrate, formaldehyde, formiate, glyoxal, glyoxalate, levulinate, malonate, methylglyoxal, oxalate, propanal, and pyruvate. Many, but not all (such as oxalate), of these compounds are valuable substrates for microbial growth.

In nutrient-poor waters, the contribution of bacteria to total planktonic respiration ranges up to as much as 90%. This applies even to some regions of the oceans. These systems are net heterotrophic, where planktonic communities respire more organic C than they produce by photosynthesis. The organic carbon derives from the surrounding terrestrial environment and continents. Photolysis of chromophoric organic carbon is the process which makes the autochthonous carbon available to microorganisms, which form one major basis of the aquatic food webs. For boreal freshwater systems, there is an estimate that only 2% of terrestrial production suffices to sustain net-heterotrophy in the lakes.

A graphical summary of photolytic pathways for the provision of inorganic and organic nutrients is given in Figure 6.

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