Courtney Dean (Research Assistant, SEOS, UVic)
Simplified diagram of the global nitrogen cycle, detailing nitrogen pathways based on oxygen conditions. While bacterially-mediated nitrogen transformations are concentrated in the sediments, these processes also occur in the water column.
Diagram created by Courtney Dean (the Dr. Juniper Lab, UVic).
Acknowledgements:
I would like to thank Dr. Kim Juniper (Department of Biology, SEOS, UVic), Dr. Richard Dewey (Associate Director, Research, VENUS), Dr. Roberta Hamme (SEOS, UVic), and Dr. Jody Klymak (SEOS, UVic).
Nitrogen, an essential element for all life forms, is bio-available in the marine environment as ammonia (NH3), nitrite (NO2-) and nitrate (NO3-). These nitrogen salts primarily enter the marine system through runoff from land and are then taken up and transformed by phytoplankton, that form the base of the food chain. The decomposition of plankton debris in seafloor sediments releases dissolved nitrogen salts back into the water column when oxygen is abundant, but during oxygen depletion, the nitrogen is lost from the ecosystem as nitrogen gas (Figure). Since oxygen depletion events are becoming more common, and the productivity of marine ecosystems is usually limited by nitrogen salts, it is important to understand how oxygen levels can drive nitrogen availability, particularly in the more productive coastal ecosystems that are influenced by nitrogen inputs from anthropogenic sources.
To study the influence of oxygen levels on the movement of nitrate in and out of bottom sediments, we used a nitrate sensor deployed just above the seafloor at 99 m on the VENUS observatory in Saanich Inlet, which goes through a cycle of oxygen depletion each year. Nitrate, oxygen, temperature, salinity, density and pressure data were obtained from the VENUS data archive for October 2008 to January 2009. The results suggested that the physical movement of the water masses in and out of Saanich Inlet created a complex environment, where variations in nitrate concentration were not just caused by biological processes, but were also strongly influenced by the physical mixing of water masses. As a result, the effects of physical mixing and biological processes on concentrations of dissolved nitrate could not be separated in the data set.
Our results contributed to a new proposal to deploy the nitrate sensor on a water column vertical profiler that will be part of the VENUS Phase II expansion, expected to be in operation in the summer of 2012. By measuring nitrate concentrations throughout the water column, together with other indicators of water mass intrusions into the inlet, we should be better able to separate local production and consumption of nitrate from external inputs.
