Tag Archive for ‘eutrophication’ at Resilience Science

Tag Archive for 'eutrophication'

Algal Bloom along the Coast of China

Published by Garry Peterson on July 9, 2008 in Regime Shifts and Visualization. 0 Comments

There has been a lot of news coverage of the large coastal algal bloom at China’s Olympic sailing site in Qingdao. The Chinese government claims the bloom is now under control.

NASA’s Earth Observatory has published some remote sensed images of the bloom from MODIS:

On June 28, 2008, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured these images of Qingdao and the bay of Jiaozhou Wan. The top image is a natural-color image similar to what a digital camera would photograph. The bottom image is a false-color image made from a combination of light visible to human eyes and infrared light our eyes cannot see. In this image, vegetation appears vibrant green, including the strips of algae floating in the bay and in the nearby coastal waters.

These images show the bay at the beginning of a local cleanup effort. (Daily images of the area are available from the MODIS Rapid Response Team.)

Regime shifts in the Gulf of Mexico

Published by Garry Peterson on June 25, 2008 in Regime Shifts. 1 Comment

Eugene Turner, Nancy Rabalais, and Dubravko Justic’s recent article Gulf of Mexico Hypoxia: Alternate States and a Legacy (Env. Sci. Tech., 2008 42(7) 2323–2327) suggests that benthic carbon in the coastal benthic may be a critical slow variable regulating coastal hypoxia. As organic matter accumulates in sediments it demands increasing amounts of oxygen, making the area more vulnerable to nutrient driven hypoxia.

The Gulf of Mexico is one of the most studied coastal hypoxic zones in the world, but it is not the only one. The number of these zones has greatly increased, primarily due to agricultural expansion and intensification (one of the many ways that agriculture has been driving ecological regime shifts). The authors compare changes in coastal hypoxia in the Gulf of Mexico to that found in the Baltic Sea, which has also been suggested to have undergone a regime shift. The authors conclude:

… there has been a system-wide response to the combination of organic buildup in the sediments and higher nitrogen loading, which has increased the area of hypoxia generated for a given nitrogen load and has increased the opportunity for hypoxia to develop. The results discussed above demonstrate that the average [nitrogen] loading of the 1980s would result in a hypoxic zone that is twice as large in the past decade.

…Hypoxia has well-documented catastrophic consequences to the benthos, including animals with multiyear life spans, and creates large areas without commercial quantities of shrimp and fish. The changes in the Mississippi River-influenced continental shelf over the last 30–40 years should be considered to a shift to an alternate state in the sense that (A) the threshold for hypoxia development has been exceeded on a continuing basis and the size of the hypoxic zone has increased and may be approaching its maximum size, given physical constraints on shelf geometry (e.g., width, depth, and length); and (B) the return to a previous system state is more difficult the longer that the current level of nutrient loading is stable or increasing.

… respiratory demand in the sediments remains a legacy influencing water quality of the eutrophied continental shelf in the northern Gulf of Mexico. …The goal of reducing the size of the hypoxic zone to 5000 km² thus becomes more difficult to achieve for every year without a significant reduction in nutrient loading. Each year without reducing the nutrient loading rates means that it will take longer to realize the Action Plan goal, because the legacy of accumulated organic matter and its respiratory demand increases with time.

Climate change amplifies eutrophication

Published by Garry Peterson on April 8, 2008 in Greenlash. 1 Comment

Hans Paerl and Jef Huisman have a perspective article in Science that reviews how climate change may promote blooms of cyanobacteria Blooms Like It Hot (320 (5872): 57 ):

Nutrient overenrichment of waters by urban, agricultural, and industrial development has promoted the growth of cyanobacteria as harmful algal blooms (1, 2). These blooms increase the turbidity of aquatic ecosystems, smothering aquatic plants and thereby suppressing important invertebrate and fish habitats. Die-off of blooms may deplete oxygen, killing fish. Some cyanobacteria produce toxins, which can cause serious and occasionally fatal human liver, digestive, neurological, and skin diseases (1-4). Cyanobacterial blooms thus threaten many aquatic ecosystems, including Lake Victoria in Africa, Lake Erie in North America, Lake Taihu in China, and the Baltic Sea in Europe (3-6). Climate change is a potent catalyst for the further expansion of these blooms.

Rising temperatures favor cyanobacteria in several ways. Cyanobacteria generally grow better at higher temperatures (often above 25°C) than do other phytoplankton species such as diatoms and green algae (7, 8). This gives cyanobacteria a competitive advantage at elevated temperatures (8, 9). Warming of surface waters also strengthens the vertical stratification of lakes, reducing vertical mixing. Furthermore, global warming causes lakes to stratify earlier in spring and destratify later in autumn, which lengthens optimal growth periods. Many cyanobacteria exploit these stratified conditions by forming intracellular gas vesicles, which make the cells buoyant. Buoyant cyanobacteria float upward when mixing is weak and accumulate in dense surface blooms (1, 2, 7) (see the figure). These surface blooms shade underlying nonbuoyant phytoplankton, thus suppressing their opponents through competition for light (8). Cyanobacterial blooms may even locally increase water temperatures through the intense absorption of light. The temperatures of surface blooms in the Baltic Sea and in Lake IJsselmeer, Netherlands, can be at least 1.5°C above those of ambient waters (10, 11). This positive feedback provides additional competitive dominance of buoyant cyanobacteria over nonbuoyant phytoplankton.

Global warming also affects patterns of precipitation and drought. These changes in the hydrological cycle could further enhance cyanobacterial dominance. For example, more intense precipitation will increase surface and groundwater nutrient discharge into water bodies. In the short term, freshwater discharge may prevent blooms by flushing. However, as the discharge subsides and water residence time increases as a result of drought, nutrient loads will be captured, eventually promoting blooms. This scenario takes place when elevated winter-spring rainfall and flushing events are followed by protracted periods of summer drought. This sequence of events has triggered massive algal blooms in aquatic ecosystems serving critical drinking water, fishery, and recreational needs. Attempts to control fluctuations in the discharge of rivers and lakes by means of dams and sluices may increase residence time, further aggravating cyanobacteria-related ecological and human health problems.

Nitrogen transfer from sea to land via commercial fisheries

Published by Garry Peterson on February 1, 2008 in Ecosystem services and Greenlash. 0 Comments

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:

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.

Mapping Coastal Eutrophication

Published by Garry Peterson on January 28, 2008 in Ecosystem services, Greenlash, Regime Shifts and Visualization. 5 Comments

Current industrial agricultural practices produce a tradeoff between agricultural production and the quality of coastal ecosystems, because agricultural fertilizers that increase crop yields lead to the creation of low oxygen hypoxic areas in areas which receive a lot of nutrient rich runoff.

The World Resources Institute and Virginia Institute of Marine Science, has updated Diaz et al’s recent map of coastal eutrophication. They identify 169 hypoxic areas, 233 areas of concern, and 13 systems in recovery.

The WRI Earthtrends weblog writes about the project:

The map shows three types of eutrophic zones:

(1) Documented hypoxic areas - Areas with scientific evidence that hypoxia was caused, at least in part, by an overabundance of nitrogen and phosphorus. Hypoxic areas have oxygen levels low enough to inhibit the existence of marine life.

(2) Areas of concern - Systems that exhibit effects of eutrophication, including elevated nitrogen and phosphorus levels, elevated chlorophyll levels, harmful algal blooms, changes in the benthic community, damage to coral reefs, and fish kills. These systems are impaired by nutrients and are possibly at risk of developing hypoxia. Some of the systems may already be experiencing hypoxia, but lack conclusive scientific evidence of the condition.

(3) Systems in recovery - Areas that once exhibited low dissolved oxygen levels and hypoxia, but are now improving. For example, the Black Sea recovery is largely due to the economic collapse of Eastern Europe in the 1990s, which greatly reduced fertilizer use. Others, like Boston Harbor in the United States and the Mersey Estuary in the United Kingdom also have improved water quality resulting from better industrial and wastewater controls.

Given the state of global data, the actual number of eutrophic and hypoxic areas around the world is likely to be greater than the 415 listed here. The most under-represented region is Asia. Asia has relatively few documented eutrophic and hypoxic areas despite large increases in intensive farming methods, industrial development, and population growth over the past 20 years. Africa, South America, and the Caribbean also have few reliable sources of coastal water quality data.

A more detailed analysis of this data set will be available in February 2008 in a policy note entitled Eutrophication and Hypoxia in Coastal Areas: A Global Assessment of the State of Knowledge (a list of related publications can be found here.

Mapping anoxic zones - pt 2

Published by Garry Peterson on March 23, 2006 in Greenlash, Ideas, Regime Shifts and Visualization. 1 Comment

Global International Waters Assessment is a systematic assessment of the environmental conditions and problems in large transboundary waters, comprising marine, coastal and freshwater areas, and surface waters as well as ground waters. Involving over 1,500 expert it has assessed 66 of the world’s major river basins and recently published a synthesis report. These publications are freely available online. The synthesis report’s section on pollution provides a map of eutrophication impact.

Fig 14 GIWA

As mentioned in a earlier post on mapping dead zones, eutrophication can produce large coastal hypoxic zones. The GIWA regional assessments reported that dead zones:

… have become increasingly common in the world’s lakes, estuaries and coastal zones, with serious impacts on local fisheries, biodiversity and ecosystem functions. Extensive dead zones have been observed for many years in the Baltic Sea, Black Sea and Gulf of Mexico. The GIWA assessment has compiled information on dead zones in the Southern Hemisphere, including several lagoons in the Brazil Current region, coastal locations in the Humboldt Current region, and in the Yangtze River estuary located in the East China Sea region.

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