Riverine sources of bioreactive macroelements and their impact on bacterioplankton metabolism in a recipient boreal estuary

The loading of macroelements such as, dissolved organic carbon (DOC), total nitrogen (N) and total phosphorus (P) from terrestrial soils to aquatic systems is increasing in the northern hemisphere. This phenomenon has several ecological and biogeochemical consequences for inland and coastal water systems, which are linked to bacterial cycling of DOC, N and P. During growth, bacteria assimilate macroelements into biomass (a process known as bacterial production), removing DOC, N and P from the water column and channelling these elements to consumers. Bacteria can also utilize organic carbon as a source of energy for respiration. For this purpose they take up oxygen dissolved in the water and release CO2 to the atmosphere. Increases in bacterial production have consequences for aquatic food web structures, whereas increases in bacterial respiration have implications for greenhouse gas emissions and for oxygen concentrations of estuarine waters. Currently, it is still unclear the degree to which land derived macroelements can be utilized by bacteria. Moreover, it is also not understood how bacterial metabolism (production and respiration) will respond to increased terrestrial macroelement fluxes.
In this thesis, I aimed to determine the fraction of the terrestrially derived macroelement loading that can be utilized by bacteria. Moreover, I investigated whether riverine inflows of DOC, N and P, alone or combined, limited the metabolism of bacteria in estuarine waters. I found that on average only 2% of the DOC exported from land was utilized by bacteria for production of biomass during a seven-day period. This share amounted to approximately 50% in the case P, which was thus the macroelement mostly available to bacteria among the three macroelements studied. Yet, the relatively low DOC quality increased downstream for rivers with long river water residence time and high catchment proportions of agricultural and urban land. These riverine catchment features are thus important to predict the export of oxygen consuming organic carbon (C) from rivers to estuaries.
The DOC transported in the Öre river, was less important for support of bacterial respiration than nutrients. These findings agree well with predictions of DOC quality for the Öre river, based on the its catchment features. Riverine nutrients stimulated primary production at the Öre estuary, which in turn supplied organic carbon to bacteria. In general, bacterial production in the Öre estuary was limited by P, while bacterial respiration was limited by organic C. In light of predicted increases of riverine macroelement deliveries and expected reductions of estuarine primary production, bacterial production will likely increase, as well as the bacterial respiration of riverine delivered organic C. Both estuarine bacterial production and respiration are expected to increase the most in response to combined increases of riverine deliveries of DOC, N and P.
In summary, land derived macroelements can be substantially available to freshwater and estuarine bacteria and impact their metabolism. Given the large role of bacteria in food web structures and aquatic CO2 emissions, continued changes in the input of terrestrial macroelements may have large implications for boreal aquatic ecosystems.