The Sibun River Watershed

The Sibun River represents one of the major watersheds of central Belize.  Sometimes referred to as the Xibun River in Mayan culture,  the Sibun originates at about 2600 feet (800 m) in the Maya Mountains, located on the border of Guatemala and Belize and travels almost 100 miles (100 km) through central Belize before emptying into the Caribbean just south of Belize City. Along its way, the river passes through a variety of terrains, including the limestone (karst) geology of central Belize and the mangrove forests of the coastal regions. Although the exact role of the Sibun in Mayan culture is not well recorded, its close proximity to the caves of central Belize, and its connection to the ocean, probably meant that it served as an important means of transport for the Mayans through central Belize.

Map of Belize showing the location of the Sibun River (green highlight).

Our class projects monitoring the water quality of the Sibun River have several important goals:

  • To educate students and tourists on the importance of water-quality monitoring to the health of a watershed.
  • To establish a data base of water quality data that may be used for long-term monitoring of this important aquatic ecosystem.

An Introduction to Water Quality Monitoring

Water is the compound that unites all living organisms. On average, our bodies are about 60 percent water by weight, and through our metabolic processes cycle approximately 2.5 liters of water per day from the external environment. The quality of our water source, be it from private wells or municipal water supplies, depends on a large number of variables. One of the biggest factors in water quality relates to the growth of the human population in all areas of the world. Population growth and the subsequent changes in land use and resource cycling place an intense demand on our water systems.

When one thinks of water quality, one typically thinks of pollution. Water pollution is a major concern of environmentalists. However, the word pollution is typically associated with industrial sources such as waste, reactor cooling, or waster-water treatment. In reality, scientists divide water pollution into categories. The first is called point sources. These are typically industrial or commercial sites that have filed a request with the state or federal government to release waste material into a body of water. While historically there have been problems with point sources, modern regulatory systems keep a fairly accurate inventory of waste materials being released from registered facilities. The second source is non-point sources. These may be from non-registered waste producers, such as agriculture, or from unidentified sources. Unidentified sources are primarily residential sources (lawn pesticides), runoff from parking lots, contaminants in the air, etc. In many parts of the world, non-point sources are considered to be the biggest threat to water quality because they are difficult to identify and regulate.

There are three different levels of water quality analysis. They are physical (Type I) and chemical (Type II) and biological sampling (Type III). Each of these provides a different, yet important, perspective on the water quality of a given watershed.

Physical Analysis (Type I)

The physical characteristics of a stream play a major role in the quality of water in it by influencing the chemical composition. For example, streams that pass over limestone deposits can be expected to have a high alkalinity and an acidic pH. Streams that have a course substrate (rocks, boulders) that mix up the water will have a higher dissolved oxygen concentration than slow-moving deep streams. Thus it is important to note the physical characteristics of a stream while conducting water quality analysis.

Physical analysis is perhaps the simplest method of water quality monitoring. Often it can provide a quick indication of potential problems, such as increased sedimentation or abnormal fluctuations in water levels. The State of Georgia’s Adopt-A-Stream program has developed an easy method of assessing the physical characteristics of a stream:





Chemical Analysis (Type II)

There are many different means of  performing a chemical analysis of water quality. In some areas,  commercial labs and local health agencies often provide services. However, in remote areas, including our field locations in Belize, monitors must rely on using a series of prepared tests, each of which will analyze a specific component of water quality. Chemical analysis is not the only mechanism to assess water quality. These are often referred to a colorimetric tests since they involve the use of a chemical agent that detects the presence of a specific compound in the water. The agent changes color depending on the quantity of the monitored compound in the sample

pH Measurement

Most of you are probably familiar with pH as a measure of how acidic a sample is. The pH scale ranges from 0 to 14. Values between 0 and 7 are considered to be acidic, while values from 7 to 14 are considered to be alkaline (basic). A pH of 7.0 is considered to be neutral. Recall that the pH scale is actually a logarithmic measurement of the hydrogen ion concentration in a solution. Since it is logarithmic, any whole number change actually represents a 10-fold change in hydrogen ion concentration or acidity. This a pH of 4.0 is 100 times more acidic than a pH of 6.0. Aquatic organisms require a pH range of about 6.5 to 8.2. As the pH deviates from this range, it begins to impact the food chain, with smaller organisms perishing first followed by larger species. Many larger fish can survive a pH range of 6.0 to 9.0.


Alkalinity is the measure of the amount of buffering compounds in a water supply. These buffering agents, carbonates and bicarbonates, help maintain the pH of the water system. The compounds originate within the rocks and soils surrounding the water supply. Some materials, such as limestone, supply a significant amount of buffering agents. Unlike pH, which measures the actual acidity of the water, this test measures the ability of the water supply to resist changes in pH over time.

From an environmental perspective, water sources that have alkalinity levels between 100 and 200 parts per million (ppm) are considered well buffered against pH change. Therefore, these water systems would resist the influence of acid rain. Most freshwater streams should have an alkalinity level between 20 and 200 ppm.


Temperature is a relatively simple measurement to make, yet it represents an important factor in water quality. Aquatic organisms prefer a stable temperature. Sudden changes in temperature may create stress for the organisms. Changes in temperature may be the result of many factors, including the depth of the water. To environmentalists, sudden changes in water temperature are an indication of thermal pollution. This is frequently caused by industrial activities that utilize water as a coolant, e.g. in nuclear power plants. However, thermal pollution may occur simply because of the removal of trees and shrubs from the banks of waterways. The metabolism of many aquatic organisms, including those the larval stages of many insects (the macroinvertebrates) that serve as a component of the aquatic food web, do not tolerate warm water or sudden changes in water temperature. For these reasons, thermal pollution is now recognized as a serious threat to water quality in many areas, not just industrialized cities.

Dissolved Oxygen

One of the most important indicators of water quality is a measurement of the oxygen within the system. While there are several methods by which we can measure the cycling of oxygen in an aquatic system, one of the easiest to perform is a test for dissolved oxygen. The oxygen in the system must be adequate to provide for the metabolic activity of all organisms in the food chain. Oxygen may enter a water supply by a number of mechanisms. First, oxygen may diffuse slowly into the water from the surrounding atmosphere. Second, oxygen may enter the water as a byproduct of photosynthesis, especially from any green algae that might be present. Finally, oxygen may enter the water as the result of mechanical action, such as the flow of water over rocks.

A number of factors may influence the amount of oxygen present in the water. A higher temperature results in lower levels of dissolved oxygen. In addition, the overall depth of the aquatic system is a factor, since water at greater depths is less likely to support photosynthetic organisms or receive oxygen from the surface. Altitude and salinity of the water also influence the amount of oxygen available to living organisms.

The acceptable ranges for dissolved oxygen vary greatly between systems. In most cases, 3 ppm dissolved oxygen is the minimum required to support fish. Most biologists recognize a level of 5 to 6 ppm as healthy. Levels above 10 ppm usually indicate the presence of active photosynthetic organisms. High levels of oxygen are not necessarily good, as it may indicate that an overabundance of nutrients (possibly phosphates) has resulted in an algal bloom.img_4114


Phosphates represent an important nutrient for all living organisms. However, the levels of phosphates in an aquatic system are traditionally very low. Thus, phosphates frequently serve as a rate-limiting nutrient for aquatic life. This means that the limited amount of phosphates keeps the population size of many organisms down. Unfortunately, residential, agricultural, and industrial wastes are frequently high in phosphates. When these enter a water system, the result is typically a population explosion of smaller organisms. This upsets the aquatic ecosystem and can result in an eventual population crash after the nutrients are expended.

There are several types of phosphates that may be found in an aquatic environment. Most of these require sophisticated test instruments to detect. However, one of these, the orthophosphates, may be detected using a colorimetric assay. Any levels below 0.1 ppm are considered acceptable, with levels above this typically causing increased plant an algal growth.


Like phosphates, all aquatic systems contain some level of nitrogen. This nitrogen can exist in a variety of forms, including nitrates (NO3), nitrites (NO2), and ammonia (NH3). In addition, a significant amount of nitrogen is incorporated into the structure of living organisms as a component of protein. The test we will use today analyzes the amount of nitrate present in the water. As with phosphates, pollution in the form of human and animal waste may alter the balance of nitrogen in an aquatic system. In general, nitrate levels below 1.0 ppm are considered acceptable.

Not All Water Sources are the Same Chemically

Obviously, all water sources are not the same. Conditions of a single stream can vary considerably based on the season, rainfall, or ambient temperature. Water quality data also varies depending on the type of water source. Wetlands, ponds, and marshes all have their own unique physical, chemical, and biological signatures. Ponds represent a different type of aquatic ecosystem. They are typically much more stagnant than streams and rivers, which reduces the dissolved oxygen concentration. This trend may be reversed by an abundance of photosynthetic algae or other plant material in the water, which serve to increase dissolved oxygen levels. Ponds are often more susceptible to the influence of organic nutrients entering the water, since these tend to accumulate faster than in a stream.

Biological Monitoring (Macroinvertebrates)

The previous labs introduced you to the use of physical and chemical monitoring to evaluate the quality of water in a stream. The chemical and physical data that is collected on the stream reflects the abiotic, or nonliving, characteristics of that body of water. However, as you may have noticed, the physical observations and chemical tests did not examine all of the possible factors that may influence the health of a stream or watershed. The presence of pollution, such as heavy metals or pesticides, has a tremendous effect on an ecosystem. In fact, a stream may look healthy based on the physical and chemical data but actually possess a low level of diversity in comparison to a similar stream. Thus, many ecologists and environmentalists turn to the monitoring of the biotic, or living, components of an aquatic ecosystem as an indicator of water quality. Frequently, these studies focus on the invertebrates living in the stream.

An invertebrate is any animal that lacks an internal skeleton made of cartilage or bone. Approximately 99% of all known animal species are invertebrates. A macroinvertebrate is an invertebrate that is able to be viewed without the assistance of a microscope. Aquatic macroinvertebrates are those that spend at least a portion of their life cycles within an aquatic ecosystem. As aquatic organisms, they are influenced by the physical characteristics of their environment (such as sediment and water flow) and the chemical composition of the water in which they live. Thus, small changes in the physical and chemical aspects of a stream typically have a strong influence on the populations of macroinvertebrates that inhabit the stream. Macroinvertebrates play an important role in stream ecology. They serve as food sources for larger organisms, such as fish and in some cases humans. Voshell (2002) compares the role macroinvertebrates in a steam to that of the earthworm in a terrestrial habitat. You may not see them at first, but their absence leads to the complete collapse of the ecosystem.

Not all macroinvertebrates respond to environmental change in the same manner. Some, such as pouch snails and aquatic worms, are fairly tolerant of change, while others, such as the stonefly, are very sensitive to change. In this lab, we have grouped the organisms into three categories; sensitive, somewhat-sensitive, and tolerant. Since these organisms are all part of an ecosystem, their populations are interconnected. Changes in the physical and chemical characteristics of their habitat may eliminate some species, and this in turn may influence the populations of predators (decrease) and competitors (increase). These changes may be sudden or may take months to manifest themselves. For this reason, water quality monitoring organizations suggest that streams be monitored quarterly, and trends in population structure analyzed in conjunction with physical and chemical data.




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last updated: December 19, 2016