Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture.

Transcription

1 LECTURE 13 - HYPOXIA Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Hypoxia, slide 1 here One of the more important chemicals parameters in natural waters is the amount of dissolved molecular oxygen, often abbreviated as DO. Oxygen strongly affects the chemical redox potential, and is vital to many forms of life. Hypoxia, slide 2 here Physical factors affecting the amount of oxygen dissolved in water are temperature, pressure, and, for marine waters, salinity. Dissolved oxygen is often expressed as the amount of oxygen dissolved in water relative to the maximum amount of oxygen that can dissolve in water at the existing temperature, pressure and salinity. This level is the saturation value, so such levels are referred to as percentages of saturation value. Healthy waters generally have a minimum of 75-80% of the saturation limit. Hypoxia, slide 3 here Biological factors can also influence the level of dissolved oxygen. Bacteria and other organisms in water use up dissolved oxygen, contributing to Biological Oxygen Demand, or BOD. Also contributing to BOD is sewage or organic discharges into bodies of water, which are then oxidized, consuming oxygen. Nutrient runoff from terrestrial sources contributes to increased number and size of aquatic organisms, which use oxygen. Weather can affect oxygen levels. During bright, sunny weather, photosynthesis will occur at high rates, and water will be well oxygenated. If it is cloudy for prolonged periods, photosynthesis will diminish, but respiration will continue, using up oxygen, and causing low DO levels. Static bodies of water, such as lakes or rivers with low current flow rates, usually have far higher densities of rooted plants than rivers with swift currents. High plant densities result in large DO swings. During daylight, large amounts of oxygen are produced. At night, photosynthesis shuts down, but respiration continues, and DO levels may plummet. Hypoxia, slide 4 here Hypoxia occurs when dissolved oxygen in aquatic systems is reduced to the point where it becomes detrimental to aquatic organisms living in the water. This level will vary with species. Fish generally require a minimum of 30% DO in the water. For other species, hypoxia is often defined as between 1 and 30% DO. The term dysoxic is sometimes used to refer to waters which are between hypoxic and anoxic. 1

2 Hypoxia, slide 5 here Oxygen levels can also be measured in ppm DO. The graphic shows levels that are necessary for fish. The temperature is critical for aquatic organisms. As temperature goes up, metabolic activity increases, and more oxygen is needed. For example, trout need 5-6 times more oxygen at 24 than at 4 C. (Oram, 2010). Hypoxia, slide 6 here Anoxia refers to water in which the DO levels are less than 1%. Terms related to this are anaerobic or reducing, depending on context. Hypoxia, slide 7 here Oxygen solubility decreases as temperature increases, so the ocean s least oxygenated waters are found near the equator, and the most oxygenated are found in the open ocean areas of the Arctic. The figure, from Wiki Commons, shows the general level of saturation in the world s oceans. Oxygen saturation levels are available in tables from USGS (2013). If the water is saline, a simple correction formula is used, as shown at the beginning of the document National Field Manual for the Collection of Water-Quality Data, Section 6.2 Dissolved Oxygen. Hypoxia, slide 8 here Before considering the oceans, lets look at lakes, a somewhat simpler system. Much of the oxygen in lakes, and in the ocean, comes from the atmosphere by diffusion. Recall that diffusion means that a substance is moving from a region of higher concentration to a zone of lower concentration. In lakes, a thermal stratification often develops, with a thermocline separating the dense, cold bottom waters of the hypolimnion from the warmer, less dense waters of the epilimnion. Since warm water holds less DO than cold water, the epilimnion is often less oxygenated than the hypolimnion. Hypoxia, slide 9 here The figure shows the oxygen and temperature concentrations observed in Cobbosseecontee Lake, Maine on August 16, Since oxygen is more soluble in colder water, it is clear that most of the oxygen here is coming from the atmosphere by diffusion. Little mixing occurs, so the upper layer remains better oxygenated despite the lower solubility of oxygen in warm water. Hypoxia, slide 10 here The figure shows a plot of conditions observed in Jordan pond, Maine on September 9, The epilimnion is less oxygenated than the hypolimnion. Photosynthesis by phytoplankton below the 2

3 thermocline is producing oxygen which is readily soluble in the colder water of the hypolimnion. Phytoplankton are capable of photosynthesis. They split carbon dioxide, releasing oxygen into the lake or ocean, and use the carbon to build organic matter. Some of the oxygen remains in the surface waters, and some escapes into the atmosphere. Phytoplankton are the largest source of oxygen in the earth s atmosphere. The decrease in oxygen at the lower levels of the hypolimnion is due to an excess of respiration over photosynthesis as light levels diminish. (Senator George J. Mitchell Center for Environmental and Watershed Research, 2006) Hypoxia, slide 11here Before investigating hypoxia further, we need to consider sources and sinks of oxygen in the oceans. This video, featuring Dr. David Shormann, discusses the most important sources and sinks. Hypoxia, slide 12 here There are numerous regions in the world s ocean which are known as dead zones. These are regions which are anoxic and where life is limited. The name is a bit misleading, in that some life forms may be present. Often these are anaerobic. Dead zones may occur naturally, but most have a least an anthropogenic influence or are directly caused by man. There are hundreds of dead zones in the ocean. In a 2008 article in Science, Diaz and Rosenberg report over 400 dead zones in the world s oceans, affecting a total area of more than 245,000 square kilometers. The size of these regions varies considerably. Hypoxia, slide 13 here The cause of many of the world s dead zones is eutrophication. Eutrophication is derived from the term eutrophic, which means well-nourished. Art (1993) defines eutrophication as, The process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates. These typically promote excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish. Eutrophication is a natural, slow-aging process for a water body, but human activity greatly speeds up the process. Eutrophication is often natural in fresh waters, such as lakes, but it usually isn t natural in coastal waters. Eutrophication is often visible in satellite imagery. Hypoxia, slide 14 here The image shows large blooms of phytoplankton extending from the mouth of the Mississippi River all the way to the Texas coast. Reds and oranges represent high concentrations of phytoplankton and river sediment. Phytoplankton are sometimes described as suspended, microscopic algae. 3

4 Hypoxia, slide 15 here Oxygen does not mix downward into the lower layers very well. The Mississippi River water is fresh water, and is considerably less dense than the salt water in the Gulf of Mexico. The river water sits on top, and is separated from the salt water by a boundary called a pycnocline. As phytoplankton die, they begin to sink to the bottom. Death often occurs because the phytoplankton bloom uses up all available nutrients. In the case of the Gulf of Mexico dead zone, the primary nutrient supply is agricultural runoff carried by the Mississippi River system to the Gulf. Hypoxia, slide 16 here The Mississippi drains 41% of the area of the continental United States. Its watershed includes large areas used for agricultural, accounting for 55% of all U.S. agricultural lands. As a result, it delivers an immense amount of nutrients, principally nitrogen and phosphorus, to the Gulf. Hypoxia, slide 17 here The figure, from the USGS, shows that the annual amount of N delivered to the Gulf generally ranges from one to two million metric tons. As the figure shows, the amount delivered is strongly influenced by the streamflow. Hypoxia, slide 18 here The figure, also from the USGS, shows that the annual amount of orthophosphate delivered to the Gulf generally ranges from thirty to sixty thousand metric tons. There is more variation in the phosphorus input than the nitrogen input, and the phosphorus input is even more strongly influenced by streamflow. Hypoxia, slide 19 here This huge input of nutrients causes very large blooms of phytoplankton. Natural inputs of nitrogen and phosphorous would be several orders of magnitude less than what is observed. It is expected that there should be natural variability in phytoplankton mass, primarily due to climatic changes and seasonal affects. This is observed in the open ocean. Human-caused disturbances in the near-shore coastal environments is intense. According to Cloern and Jassby (2008), who compiled time series of phytoplankton biomass (chlorophyll a) from 114 estuaries, lagoons, inland seas, bays and shallow coastal waters around the world, our synthesis reveals no comparable structure in the seasonal variability of phytoplankton at the land sea interface where biomass can peak during any season. Distinct monthly climatologies are not evident but, instead, the data reveal continuous distributions in the timing and amplitude of biomass cycles and high variability of seasonal patterns within geographic regions and even within ecosystems. 4

5 Hypoxia, slide 20 here Factors important in causing this human domination of the near-shore environments are human landscape transformations, fishing, aquiculture, river damming and diversions, introduced species, and contaminants. As dead phytoplankton sink to the bottom, they are broken down by bacteria in a process known as bacterial respiration. This is the opposite of photosynthesis. Bacteria use oxygen dissolved in the water to oxidize the organic matter, producing carbon dioxide in the process. This depletes the oxygen needed by aerobic life forms, including benthic life like crabs, clams, shrimp, many kinds of mud-dwelling creatures, and even pelagic forms like zooplankton and fish. Hypoxia, slide 21 here Another dead zone that has attracted national attention is one associated with the Hood Canal/Puget Sound Region of Washington State. The Puget Sound area is a system of interconnected fjords. A fjord is a glacially carved valley that was flooded by marine waters as ocean level rose after the Pleistocene Epoch. Freshwater flow into the salt-water estuarine environment complicates dead zone problems in this area. Hypoxia, slide 22 here Hood Canal is a fjord on the western side of Puget Sound, and is one of the four main basins in Puget Sound. This is where many of the dead zone problems occur. There are many factors that influence the Puget Sound dead zone. Two examples will be very briefly discusses. Hypoxia, slide 23 here Puget Sound has limited connectivity to the Pacific Ocean through the strait of Juan de Fuca. Dense, salty water is flows into Puget Sound during the summer. Water flows in but there is no outflow, which causes a stagnant pool during winter months. In the fall and winter (October though March) rainfall causes large inflows of fresh water. During the late winter to spring, snowmelt from nearby mountains adds more freshwater. (Trainer et al., 2007) Hypoxia, slide 24 here At the north end of Puget Sound, Penn Cove is an embayment site, known for its shellfish production. In 2003, commercial shellfish harvest was banned, due to contamination by an algae bloom, capable of producing a potent neurotoxin. A combination of factors led to contamination of the shellfish farm including heavy rain, elevated river flow and substantial southeastern winds. Thermal stratification also caused an increase in toxin producing diatoms. Far-reaching consequences include loss of revenue and destruction of marine life. 5

6 Hypoxia, slide 25 here Bainbridge Island, near the center of Puget Sound, is the location of Wyckoff Point and Eagle Harbor. Wyckoff Point is the site of the former Wyckoff wood-treating facility (operated from ) and a former shipyard. These entities introduced creosote, oil, and other wood-treatment chemicals into the environment. High levels of polycyclic aromatic hydrocarbons (PAH), pentachlorophenol, and dioxins in soil, groundwater, and in seeps on beaches present a substantial continuing contamination problem in this area. Hypoxia, slide 26 here The EPA has a superfund site around the Eagle Harbor area, and is doing a focused feasibility study aimed at understanding the extent of creosote contamination. These sort of problems, and many others are related to, or influenced by, hypoxia. Both of the previous two problems were taken from student Project 2 reports, done for the 2011 class. 6

7 References H.W. Art, Eutrophication, in Art, H.W., ed., A dictionary of ecology and environmental science (1st ed.): New York, New York, Henry Holt and Company, p. 196, J.E. Cloern, and Jassby, A.D., Complex seasonal patterns of primary producers at the land-sea interface, Ecology Letters, 11, no. 12, p. 1,294-1,303, Robert J. Diaz and Rutger Rosenberg, Spreading Dead Zones and Consequences for Marine Ecosystems, Science, 321, , Brian Oram, Dissolved Ox ygen in Water, last modified 2012, last seen October 9, Senator George J. Mitchell Center for Environmental and Watershed Research, PEARL Lakes G u i d e, l a s t m o d i f i e d N o v e m b e r 1 7, , it.htm, last seen March 9, 2011, no lonfer available. Vera L. Trainer, William P. Cochlan, Aleta Erickson, Brian D. Bill, Frank H. Cox, Jerry A. Borchert, and Kathi A. Lefebrve, Recent domoic acid closures of shellfish harvest areas in Washington State inland waterways, Harmful Algae, 2007, Volume 6, Issue 3, available at USGS, National Field Manual for the Collection of Water-Quality Data, Section 6.2 Dissolved Oxygne, 2013,, including Correction Factors for Oxygen Solubility And Salinity last seen October 9, LN13_PP_F16.PDF October 12,