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Biogeochemical Cycles

Nitrogen Fixation

Perhaps nothing excites the Howarth-Marino lab group more than nitrogen fixation and its controls at the ecosystem scale. The vast majority of nitrogen on Earth occurs as molecular N2, which is available only to specialized bacteria that can convert it (fix it) into ammonium. Thus, this process is essential to all life on the planet, yet it is scarcely a ubiquitous process. Rates are high in some types of ecosystems (some lakes, tropical forests and savannas, subtropical ocean gyres) yet very low in others (the planktonic portion of estuaries, boreal forests, temperate and arctic oceans). The controls on nitrogen fixation that lead to these patterns remain poorly known in many cases. Howarth wrote his first published paper on rates of nitrogen fixation in salt marshes (where rates are among the highest anywhere), and Marino wrote her Ph.D. thesis on the question of why there is not more nitrogen fixation in strongly nitrogen-limited estuaries (concluding that an interaction of a top-down grazing control and a bottom-up physiological constraint from low molybdenum availability is the key).

Nitrogen and phosphorus cycling: globally and in marine ecosystems

In addition to our work on nitrogen fixation, the Howarth-Marino lab has intensively studied other aspects of nitrogen and phosphorus cycling in estuaries and coastal waters.

The Howarth-Marino lab has also contributed to many efforts to help elucidate how human activities have changed global cycles of nitrogen and phosphorus.

Nitrogen and phosphorus in large watershed, regions, and countries

A major research emphasis in the Howarth-Marino lab is on the fluxes of nutrients (particularly nitrogen) from large watersheds, regions, and countries to coastal oceans. We have demonstrated that the flux of nitrogen at this large scale is strongly tied to the net anthropogenic nitrogen inputs (NANI) to the landscape, with approximately 25% of NANI exported in rivers to the coast.

Sources of and solutions for nutrient pollution

Nitrogen pollution in the landscape comes from many sources, including sewage, atmospheric deposition (the nitrogen in “acid rain”), and agriculture. Globally and in the US as a whole, agricultural sources dominate, but in some areas and regions, sewage is the largest source while in others atmospheric deposition is the single largest source.

Health and ecological effects of nutrient pollution

The ecological effects of nitrogen pollution are many, including coastal dead zones and other consequences of eutrophication such as seagrass dieback and habitat degradation, reduced production of forests and agricultural crops from the ozone that nitrogen pollution helps create, and acid rain. Human health is effected directly by nitrogen pollution, for instance through ozone exposure and nitrate in drinking water. Nutrient pollution also has indirect effects on health, for instance through increasing pollen production in plants and through favoring the production of some intermediate disease hosts, leading for instance to more schistosomiasis.

Sulfur biogeochemistry

Howarth’s Ph.D. thesis work was on sulfate reduction and sulfur cycling in salt marsh sediments and how this related to the metabolism of marshes, and much of the early work in the Howarth-Marino lab focuses on sulfur biogeochemistry in marshes as well as other ecosystems, including coastal marine sediments and lakes.

Molybdenum biogeochemsitry

Marino’s Ph.D. focused on the bioavailability of molybdenum in coastal marine waters, building on earlier work in the Howarth-Marino lab that suggested the lack of planktonic nitrogen fixation in most estuaries was a result in part of this low availability.

Interaction of element cycles

Much of the work in the Howarth-Marino lab examines the stoichiometric interactions among element cycles, often between nitrogen and phosphorus, and also often involving the cycles of carbon, oxygen, sulfur, and molybdenum.

Role of animals in biogeochemical cycles

The activities of animals can have a profound influence on biogeochemical cycles and their interactions. Several studies in the Howarth-Marino lab have examined how grazing by zooplankton can influence nitrogen fixation in both estuaries and freshwater ecosystems, while other papers have examined how animals affect sediment nutrient fluxes in estuaries.