Enhanced greenhouse gas emissions and changes in plankton communities following an experimental increase in organic carbon loading to a humic lake

Organic carbon concentrations in the surface waters of the boreal region have increased during the past two decades. We investigated the impact of elevated dissolved organic carbon (DOC) loading to a humic lake by a whole-lake experiment in which DOC in the form of cane sugar was added monthly during the ice-free period over two consecutive years. The sugar addition represented an increased concentration of 2 mg l(-1) of DOC in the epilimnion and led to an increase in CO2 emission and also an apparent increase in CH4 emission to the atmosphere from the lake surface. The composition of the bacterial, phytoplankton and zooplankton communities altered during the study period and the bacterial abundance in the metalimnion and hypolimnion of the lake decreased. No changes were detected in epilimnetic primary production or respiration, but there was an increase in bacterial production in the epilimnion. The nutrient and particulate organic carbon concentrations also suggested possible changes in the activity of heterotrophic bacteria in the metalimnion. Carbon stable isotope analyses indicated transfer of some added sugar carbon through the food web to zooplankton consumers. Overall the results suggest that future increases in organic carbon loading to boreal lakes will increase greenhouse gas emissions, although the magnitude of any change is likely to depend on the availability of nutrients like phosphorus and nitrogen which influence organic matter processing and the development of plankton communities.