The Nitrogen cycle is a beautifully complex part of our natural world that has direct impacts on the productivity of the oceans, and by extension, the amount of carbon in the Atmosphere. It has been suggested that if the N cycle were to break down the amount of global CO2 would increase by nearly 50% (Gruber 2008).
Because of the importance of the N cycle we are going to spend some time looking at it in detail in class. It is important to note that although N is the most common element in the atmosphere (N2 making up ~78% of air) it is largely biologically unavailable. Plants are literally bathing in fertilizer but it is inaccessible to them until the process of Nitrogen fixation. This process, carried out by a polyphyletic assemblage of microbial species, splits gaseous N2 into biologically available N molecules.
This N is usually available in Ammonium NH4+ or Nitrate NO3-. From these forms N can be used by a variety of phytoplankton, which in turn, are used by zooplankton and bacteria. Nitrogen tends to bounce around among these forms and is highly sought after. But remember, because the phytoplankton are photosynthetic this can only take place in the photic zone. Once the biologically available N sinks below the photic zone (in the form of Dissolved or Particulate Organic Material) it becomes sequestered from the phytoplankton. This is why deeper waters tend to be nutrient rich and why upwelling areas tend to be productive.
To complete the N cycle gaseous N2 must be returned to the atmosphere. This takes place through two microbial processes Denitrification and Anammox. In addition to completing the N cycle these process are important in reducing nutrient loads in eutrophic coastal waters (Smith et al. 2015)
To gain a better understanding of N flux we carried out a (very) active learning module in class today: We start with labeling ping pong balls as Nitrogen
First I taped two balls together to represent N2 gas. Then we N-4 students stand in a circle, with three students in the middle and one with a bag of N2 gas ping pong balls. The student with the bag splits the ping pong balls apart and throws them at the students in the circle. The students in the circle represent phytoplankton, zooplankton and bacteria and catch the balls, quickly passing them to other students in the circle. The students in the middle catch the fallen balls and tape them back together, handing them to the student with the N2 bag.
Questions for students:
1) What did the person playing the N fixing bacteria do?
2) How did we model the flow of N among phytoplankton, zooplankton and bacteria?
3) What did the balls falling to the floor represent?
4) The various forms of N differ in their residency time in the water column. NO3– gets turned over in the water column every 400 years, while NH4+ gets turned over every two weeks. Which form of these forms of N do you think is most biologically preferred? Why?
One of the predicted impacts of climate change is a increased stratification of the ocean (Wang et al. 2015). With this decreased mixing of waters…
5) What do you predict will happen to phytoplankton populations and what impacts will that have to water chemistry/nutrient availability?
6) If the upper layer of warm water grows thicker, what do you think will happen to the productivity of coastal upwelling?
One controversial approach to ameliorating climate change is ocean fertilization where large quantities of reaction limiting elements (N, P, Fe) are dumped into the open ocean to stimulate phytoplankton blooms.
7) Critique the science behind this and talk about whether you think this is a viable climate change mitigation scheme. Why or why not and use two peer-reviewed sources to help support your arguments.
8) Lastly, given that in winter large storms break down stratification and create a more well mixed ocean, while in summer warm temperatures help increase stratification, graph out how populations of phytoplankton, nutrients and the thermocline vary over one year in the high latitude northwest Atlantic (say in Halifax, NS).
9) For each season describe what is the most limiting resource.
Gruber, Nicolas. “The marine nitrogen cycle: overview and challenges.” Nitrogen in the marine environment (2008): 1-50.
Smith, Richard L., et al. “Role of Anaerobic ammonium oxidation (anammox) in nitrogen removal from a freshwater aquifer.” Environmental Science & Technology 49.20 (2015): 12169-12177.
Wang, Daiwei, et al. “Intensification and spatial homogenization of coastal upwelling under climate change.” Nature 518.7539 (2015): 390-394.