Aquatic environments require nutrients such as nitrogen and phosphorus but an excess of these elements can be harmful. In recent years, high levels of nitrogen and phosphorus (from man-made sources like artificial fertilizers) in water bodies around the world have produced dangerous algal blooms. These lead to "dead zones" in the water where oxygen levels are too low for most organisms to survive.
Now, Susan Ziegler of Memorial University in Newfoundland, Canada, and colleagues at the University of Arkansas, US, have looked at how dissolved organic carbon is released and recycled in two different aquatic environments – one that is nutrient rich and one that is nutrient poor. In particular, the researchers studied how carbon is released from biofilms and exchanged between algae and bacteria in two headwater streams in Arkansas: Moore Creek (nutrient rich) and Huey Hollow (nutrient poor).
The team obtained its results by building enclosures in the streams. These chambers were then incubated on site with rocks and stream water labelled with carbon-13, a radioactive isotope of carbon. This allowed the scientists to follow how phototrophs (algae and cyanobacteria) on the rocks take up and release both inorganic and organic carbon. Net changes in carbon, nitrogen and phosphorus between the biofilms on the rocks and stream water were also tracked.
Ziegler and colleagues observed lower carbon exchange between organisms in the nutrient-rich stream compared to that in the nutrient-poor one. The nutrient-rich stream waters also lost more newly fixed carbon to dissolved organic carbon. These results suggest that some mechanisms are at play in the nutrient-rich stream that reduce carbon exchange within the biofilms and increase losses of newly fixed carbon, Ziegler told environmentalresearchweb.
The results also help us better understand carbon cycling in streams by unravelling some of the factors that regulate carbon flow between algae and bacteria, she added. Dissolved organic carbon represents the largest actively cycling reservoir of organic carbon in the aquatic environment and bacteria are the main way by which this source of energy finds its way into the food web.
"Our work certainly points to the need for more coupled research that addresses carbon cycling by microorganisms and fauna and how this role will be affected by future environmental change," said Ziegler.
The team now plans to assess the possible role of other organisms, such as invertebrates, in cycling dissolved organic carbon in streams. It will also study more northern, boreal, ecosystems where climate change is already having an impact on sources of dissolved organic carbon.
The work was published in Inter Research Aquatic Microbial Ecology.