Draw a microbial-based nitrogen cycle. In the open ocean there is plenty of sunlight and carbon dioxide for primary production, yet photosynthesis is limited in the open ocean compared to other aquatic environments

Question

What limits photosynthesis in the open ocean?
What will be an ideal response?

Answer 1

Answer: The textbook illustrates the links between the C and N cycles in Figure 20.4. Photosynthesis in the open ocean is not limited by carbon or carbon cycling, but is limited by other nutrients; thus, the C cycle is linked to the cycling of other nutrients. In particular the N cycle is the most obvious cycle that can control primary production because a large amount of N is present in biomass; yet sources of N can be low in some environments such as the ocean. In order to supply enough N for primary production, N2 gas must be fixed by a biologically expensive process (nitrogen fixation) by specialized microbes. This is a key step in the N cycle that may limit primary production and thus limits the first step of the carbon cycle. Other steps of the N cycle can remove N from the ecosystem, which would also limit primary production. For instance, denitrification converts nitrate back into nitrogen gas and removes nitrogen from the ecosystem.