Our Work

Background

Nitrogen is an essential nutrient in marine ecosystems. Human activities have significantly increased the input of nitrogen into both coastal and open ocean regions. Anthropogenic nitrogen deposition is potentially changing marine biogeochemical cycles (the ways that biological, physical, geological, and chemical processes all interact with each other in the natural environment) and thus, altering ocean ecosystems.

Although the early work of WG 38 did consider some aspects and impacts of atmospheric nitrogen species on the ocean, nitrogen was soon recognized as a significant and complex scientific issue that required more in-depth study.

Workshop and report

A highly successful and productive workshop on “The Atmospheric Deposition of Nitrogen and its Impact on Marine Biogeochemistry” was held at the University of East Anglia (UEA) in Norwich, United Kingdom, from 11 to 14 February 2013. Twenty-three scientists participated in the workshop.

The results of the nitrogen workshop were synthesized in GESAMP Reports and Studies 97, “The Magnitude and Impacts of Anthropogenic Atmospheric Nitrogen Inputs to the Ocean”. Results were also published in seven papers listed at the end of this webpage (numbers 5 through 12).

Please find WG 38’s updated Terms of Reference on page 6 of this report.

Main findings

Atmospheric nitrogen deposition has increased due to human activity.

WG 38 estimates that the amount of nitrogen entering the ocean from the atmosphere every year is almost 4 times that in 1850, and even in 1850, conditions were not pristine. Additionally, atmospheric inputs are now the main way that anthropogenic nitrogen reaches the open ocean, having overtaken river inputs. However, river nitrogen inputs have also increased substantially due to human activity.

At the time of this workshop, WG 38 estimated that 39 million tonnes of nitrogen enters the ocean from the atmosphere ever year (39 TgN y^-1). Riverine inputs of nitrogen are 34 million tonnes per year. Additionally, 164 million tonnes enter the ocean every year due to the natural biological activity of certain bacteria, via a process known as biological nitrogen fixation.

Most of the atmospheric nitrogen input reaches the open ocean beyond the boundary between the continents and the ocean floor (known as the shelf break). Whereas a substantial amount of the riverine input is trapped on the shelf.

Consequences of increased nitrogen in oceans.

The impacts of increased nitrogen in oceans may be widespread. It increases ocean productivity but may contribute to undesirable algal blooms. WG 38 identified that increased nitrogen availability supports increased growth of phytoplankton species that produce oxygen. In regions where deep water oxygen levels are naturally low, such as the north-western Indian Ocean, the sinking of this increased biomassmay then create conditions where greenhouse gas nitrous oxide (N₂O) emissions are likely to increase. Thus, increased nitrogen deposition may contribute to climate change pressures as well as impacting on ocean biogeochemical function.

Where nitrogen in the atmosphere comes from.

Atmospheric deposition of nitrogen to the oceans can come from several distinct chemical components, each in approximately the same amounts:

· Oxidized nitrogen, primarily nitrate aerosol and nitric acid. The main anthropogenic source of oxidized nitrogen is fossil fuel combustion, both on land and increasingly from ships.

· Reduced nitrogen, primarily ammonium aerosol and ammonia. The main anthropogenic course of reduced nitrogen is intensive agriculture.

· There is also an important but poorly understood natural recycling of ammonia and organic nitrogen between the atmosphere and the oceans.

Regions most impacted by atmospheric nitrogen deposition.

The largest emission sources are in North America, Europe, India and South-East Asia because these are the areas with the highest fossil fuel combustion and most intensive agriculture. The largest inputs of nitrogen to the oceans occur downwind of these large emission sources over the North Atlantic, Northern Indian and north-west Pacific Oceans.

Future modelling suggests that although total nitrogen inputs to the oceans will change little between now and 2050, emissions will likely increase over southern Asia and decline over North America and Europe. Therefore, the impacts of atmospheric deposition will likely increase in the future, particularly in the north-west Pacific and the Northern Indian Ocean.

Remaining Scientific Gaps

WG 38 recommends additional work on the following scientific topics:

• The sources of atmospheric organic nitrogen.
• The magnitude and significance of recycling of ammonia and organic nitrogen from the oceans.
• The ways to estimate dry deposition from atmospheric concentrations.
• An improved data base, particularly on wet deposition over the oceans

The extent and thresholds for suppression of nitrogen fixation by ambient surface water dissolved nitrogen.

• The retention of nitrogen within shelf systems, and particularly how the rates of bacterial processes that convert fixed nitrogen back to dinitrogen gas (denitrification and the related anammox bacteria, both of which convert N_r into inert nitrogen gas) vary with temperature.
• The impacts of possible changes in aerosol acidity over coming decades on the atmospheric delivery of nutrients to the oceans.
• Increasing atmospheric carbon dioxide concentrations are already driving ocean acidification and this is likely to increase in coming decades. The impacts of the changing ocean pH on ocean emissions of atmospherically important gases including nitrogen species, but also other gases, needs to be evaluated.

We also recommend further work in the following regions, which are particularly sensitive to likely future changes in atmospheric deposition:

• The Northwest Pacific, where deposition fluxes are expected to grow and where there may already be impacts from the current inputs.
• The northern Indian Ocean, an important source region for N₂O, which receives a large atmospheric input that is argued to already be increasing in plankton productivity.
• Areas of the Mediterranean and North Atlantic where primary production is phosphorus- or iron-limited and hence where additional nitrogen deposition may lead to different nutrient biogeochemical responses to those in other ocean areas where nitrogen is the primary limiting nutrient.

Please find a full list of peer-reviewed publications of GESAMP Working Group 38 here.