Tag Archives: nitrogen

Aquatic Dead Zones

      I’ve published several links to global maps of coastal hypoxia. Now, NASA has produced a new map of global hypoxic zones, based on Diaz and Rosenberg’s . Spreading Dead Zones and Consequences for Marine Ecosystems. in Science, 321(5891), 926-929.  NASA’s EOS Image of the Day writes on  Aquatic Dead Zones.

      Red circles on this map show the location and size of many of our planet’s dead zones. Black dots show where dead zones have been observed, but their size is unknown.

      It’s no coincidence that dead zones occur downriver of places where human population density is high (darkest brown). Some of the fertilizer we apply to crops is washed into streams and rivers. Fertilizer-laden runoff triggers explosive planktonic algae growth in coastal areas. The algae die and rain down into deep waters, where their remains are like fertilizer for microbes. The microbes decompose the organic matter, using up the oxygen. Mass killing of fish and other sea life often results.

      Over fertilizing the world

      Three faces of global over fertilization from agriculture in China and the USA, and its complex effects on food webs.

      1) Chinese farmers are acidifying there soil by over applying fertilizer.  Acidic soils impede crop growth and amplify the leaching of toxins.  Since the early 1980s, pH has declined from 0.2 to 0.8 across China, mostly due to overuse of fertilizer.  This is shown in a new Science paper, Significant Acidification in Major Chinese Croplands (DOI: 10.1126/science.1182570) by JH Guo and others.

      Topsoil pH changes from 154 paired data over 35 sites in seven Chinese provinces between the 1980s and the 2000s. The line and square within the box represent the median and mean values of all data; the bottom and top edges of the box represent 25 and 75 percentiles of all data, respectively; and the bottom and top bars represent 5 and 95 percentiles, respectively. (From Guo et al)

      Reporting on the paper Mara Hvistendahl writes, “Beginning in the 1970s, Chinese farmers applied ever-increasing amounts of fertilizer with the hope that it would lead to bigger harvests. Instead of high yield, however, they got water and air pollution. Today, agricultural experts estimate that in many parts of China fertilizer use can be slashed by up to 60%.”  In another issue of Science she also reports on current Chinese efforts to reduce fertilizer use.  In the Wall Street Journal, Geeta Annad reports on overfertilization in India “Pritam Singh, who farms 30 acres in Punjab, says the more desperate farmers become, the more urea they use. Overuse is stunting yields.”

      2) The Washington Post reports on how in the US large feed lots are causing water quality problems in Manure becomes pollutant as its volume grows unmanageable

      Animal manure, a byproduct as old as agriculture, has become an unlikely modern pollution problem, scientists and environmentalists say. The country simply has more dung than it can handle: Crowded together at a new breed of megafarms, livestock produce three times as much waste as people, more than can be recycled as fertilizer for nearby fields.

      … Despite its impact, manure has not been as strictly regulated as more familiar pollution problems, like human sewage, acid rain or industrial waste. The Obama administration has made moves to change that but already has found itself facing off with farm interests, entangled in the contentious politics of poop.

      3) Fertilization of ecosystems can have complex ecological consequences. In a paper in PNAS, John Davis and others show that in a Long-term nutrient enrichment decouples predator and prey production DOI: 10.1073/pnas.0908497107.

      Relationship between primary consumer and predator secondary production for the reference stream (gray circles), the treatment stream (black circles), and previously published data (open circles). The arrows represent the temporal trajectory of the treatment stream starting with the 2 years of pretreatment (P1 and P2) and ending with the fifth year of enrichment (E5). The data labels correspond to the sampling year for the reference and treatment streams. The previously published data include 5 years of production data from the reference stream (C53) and a similar Coweeta stream (C55) that had experimentally reduced terrestrial leaf inputs during 4 of those years (21). It also includes previously published data from an unmanipulated year that compared our current reference (C53) and treatment (C54) streams (22). AFDM is ash-free dry mass.

      Their research showed that there were differences in how predators and prey responded to fertilization, but these only emerged over time.  Increases N and P entering a stream increased populations of both predators and prey, however later on prey populations continued to increase but predator populations declined,because fertilzation shifted the streams prey to larger, predator resistant species, which reduced the efficiency with which energy flowed through the food web.

      Nitrogen deposition making lakes more regulated by phosphorus

      Nitrogen deposition is increased the extent to which lake algal populations are regulated by phosphorus, shifting lake food webs.  Because, the patterns of human amplification of nitrogen and phosphorus trasport are different this should drive different patterns in lakes in different regions.

      James Elser and other write in Science Shifts in Lake N:P Stoichiometry and Nutrient Limitation Driven by Atmospheric Nitrogen Deposition (2009 326 (5954):835).  From the abstract:

      Human activities have more than doubled the amount of nitrogen (N) circulating in the biosphere. One major pathway of this anthropogenic N input into ecosystems has been increased regional deposition from the atmosphere. Here we show that atmospheric N deposition increased the stoichiometric ratio of N and phosphorus (P) in lakes in Norway, Sweden, and Colorado, United States, and, as a result, patterns of ecological nutrient limitation were shifted. Under low N deposition, phytoplankton growth is generally N-limited; however, in high–N deposition lakes, phytoplankton growth is consistently P-limited.

      They conclude:

      Our findings show that, despite the potential of watershed vegetation uptake and sediment denitrification to buffer lakes against elevated N loading, increased inputs of anthropogenic N have accumulated in receiving waters. As a result, shifts in lake N:P stoichiometry have altered ecological nutrient limitation of phytoplankton growth. Phytoplankton in lakes that are less influenced by anthropogenic inputs experience relatively balanced or N-deficient nutrient supplies, but enhanced N inputs from the atmosphere during the past several decades of human industrialization and population expansion appear to have produced regional phytoplankton P limitation.

      Producer diversity is likely to be low when resource supply ratios are skewed in favor of one particular nutrient relative to others (11, 18). Thus, increased N loading from the atmosphere may reduce lake phytoplankton biodiversity, similar to anticipated effects of N deposition on plant diversity in terrestrial ecosystems (19, 20), by possibly favoring those relatively few species that are best able to compete for the limiting P.

      … Thus, sustained N deposition that generates stoichiometric imbalance between P-limited, low-P phytoplankton and their P-rich zooplankton consumers (12) may result in reduced production of higher trophic levels, such as fish. Projected increases in global atmospheric N transport during the coming decades (24) are likely to substantially influence the ecology of lake food webs, even in lakes far from direct human disturbance.

      Nitrogen transfer from sea to land via commercial fisheries

      Roxanne Maranger an ecologist at the University of Montreal and other have a neat paper in Nature Geoscience Nitrogen transfer from sea to land via commercial fisheries that shows that commercial fishing removed substantial amounts of nitrogen from coastal oceans. They show that while fertilizer run-off into the ocean and fishery removal of nitrogen have increased over the past forty years, the increase in nitrogen inputs has been faster. Consequently the proportion of nitrogen removed from coastal zone has dropped from a global average of about 60% in 1960 to about 20% in 2000. This trend as well as the spatial pattern of nitrogen withdrawal are shown in figure 1 of their paper:

      Nature GeoScience

      Figure 1. a, Total amount of N in fertilizer run-off (Tg N yr-1=1012 g N yr-1) delivered to the global ocean (left axis, blue line) and N returned as fish biomass (left axis, red line) per year over time. The orange line (right axis) is the proportion of fish N removed relative to fertilizer N exported (ratio fish N:fertilizer N) reported as a percentage. b, The ratio of fish N removed to fertilizer N entering 58 different large marine ecosystems (LMEs) for the year 1995.

      The paper shows that fishing can help reduce the impacts of nitrogen pollution. But that nitrogen pollution that destroys fisheries, through the creation of anoxic “dead zones”, can make nitrogen pollution even worse by removing a major source of nitrogen withdrawals. Similarly, overfishing the reduces the amount of fish biomass that can be removed from a system will make the system more vulnerable to eutrophication.