The Marine Nitrogen Cycle

Nitrogen is required for all plants and animals to exist because it forms part of the proteins that are needed for our growth and repair. Almost all of the nitrogen within marine life originates from the nitrogen that is bio-available (which means, in a form that the biology can use) as nutrients. These nutrients may come from a variety of sources. Using chemical techniques, we can identify these different sources within the nutrients dissolved in seawater.

If we want to understand how the Arctic food web may be changing, we first have to understand how the nutrients, which are used to form the organic matter at the base of the food web, are sourced and cycled in the ocean.

Most of the nitrogen on the planet is in the form of N2, which is formed by a triple bond between the two nitrogen atoms. This bond is very difficult to break, therefore, a very large proportion of the nitrogen on the planet remains unused by plants.

Nitrogen cycle

There are a few ways which bioavailable nitrogen is added to the ocean, and all may change with human interference to the natural cycle and climate change:

  1. Terrestrial sources: The land can add nutrients to the sea from a number of sources. With humans farming to make enough food for our growing population, fertilisers are added to the soils, which can reach the rivers and run into the ocean. Organic nitrogen is also transported in rivers (both naturally and from human inputs), which can be an important source of nitrogen to marine life. Nitrogen from the land can be identified by distinct chemical signatures.
  2. Atmospheric sources: Industrial processes add reactive nitrogen to the atmosphere. This nitrogen can be transported to the sea as dust (‘dry deposition’) and with rain (‘wet deposition’.) Natural sources of nitrogen can also be transported by both wet and dry deposition.
  3. Biological sources: There are a few types of algae, called nitrogen fixers, that can break down the strong N2 bond in seawater. Nitrogen fixers can convert N2 into the nitrogen which forms organic matter (on land this can happen too – peas and beans can fix N2 from the atmosphere!) Once nitrogen has been ‘fixed’, the organic nitrogen produced is within the food web and can be passed up into higher trophic levels. This nitrogen can be traced in the foodweb by its different chemical make up to nutrients that have come from other sources.
  4. Glacial and sea ice inputs: In the Arctic, glaciers and sea ice can also provide an important source of nitrogen to the ocean, particularly in summer when the ice melts. This is likely to change with increasing loss of sea ice and glaciers.
  5. Recycling within the ocean: A large proportion of the nutrients that are used by algae in the ocean are recycled from organic matter. Once a plant or animal dies, the organic matter within it can be respired and the organic matter turns back into carbon and nutrients (such as nitrogen). A large proportion of these recycled nutrients are stored in the deep ocean and can be supplied to the sunlit surface ocean (where photosynthesis can occur) by sea currents.

Once nutrients are taken up by algae, the nitrogen (and carbon and other elements) can be eaten by animals and be passed up the food web. Some of the organic matter within the ocean (from plants all the way to seals) might not be eaten by other animals, but may sink to the seafloor. This organic matter in sediments can also be studied using chemical techniques to help us to understand the Arctic food web.

In this project, we aim to work out the relative importance of all of these nitrogen sources, and the processes within the ocean that can affect the nutrients. We are doing this by taking part in a number of Arctic research cruises over a two-year period, to try to understand how each of the sources and processes within the ocean may respond to a changing Arctic Ocean.

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