Photochemical alterations of dissolved organic matter : Impact on heterotrophic bacteria and carbon cycling in lakes

Sammanfattning: Globally, the amount of carbon in dissolved organic matter (DOM) is similar to the total amount in the atmosphere. DOM also plays a key role in structuring natural surface waters and affects the physical, chemical, and biological character of these systems. In this thesis, the impact of solar UV radiation on the dynamics and tumover of these substances in lakes was studied, in particular concerning the influence of photochemical alterations of DOM on the transfer of carbon to heterotrophic bacteria for transfer through food webs. Substantial photoproduction of a number of low molecular weight organic acids was demonstrated, and I also showed that these photoproducts may be major substrates for heterotrophic bacterioplankton in humic surface waters. The photoproduction of these acids and the complete oxidation of DOM to CO2 occurred in waters from all studied lakes, and it was concluded that these processes could be a major sink for aquatic DOM. The impact of UV irradiation on the ability of DOM to support heterotrophic bacterial growth was strongly positive in all freshwaters with a high content of terrestrially derived organic matter, while the opposite was true for waters with a high relative contribution of algal material. Hence the quality of the DOM is probably an important factor governing the net effect of irradiation on DOM bioavailability. In conclusion, photochemical processes play a major role in the dynamics of DOM in surface waters and also influence bacterial growth by altering the size of the total pool of organic compounds available for bacterial utilization.