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How was the health of Guanabara Bay calculated?

Environmental report cards are used by resource managers to assess and report on the ecosystem health of a region. Developing rigorous, quantitative assessments provides accountability to support environmental protection efforts. A five-step process of developing report cards is used to assess progress: 1) determine values and threats, 2) choose indicators, 3) define thresholds, 4) calculate scores, and 5) communicate results.

This report card provides a transparent, timely, and geographically detailed assessment of health in Guanabara’s Bay and Basin using data from 2013-2015 collected by State Institute of the Environment (INEA). Guanabara Bay health and Guanabara Basin health are defined as the progress of five indicators toward scientifically-derived thresholds or goals. The indicators are combined into two water quality scores, one score for the Bay, and one for the Basin.

Each sample is compared to a threshold to calculate a score from 0 to 100. All sample scores are averaged for each sampling location to reach a station score. All stations in each region are averaged to a region score. All regions are averaged to the overall indicator score.

Overall Health Index

The Guanabara Bay report card compares 5 indicators for the Bay (dissolved oxygen, biological oxygen demand, total phosphorus, dissolved inorganic nitrogen, fecal coliform) and 5 indicators for the Basin (dissolved oxygen, biological oxygen demand, orthophosphate, dissolved inorganic nitrogen, turbidity) to scientifically derived thresholds or goals. These indicators are combined into two Water Quality Indices (one for the Bay and one for the Basin). The data was provided by INEA.

Dissolved Oxygen

The oxygen dissolved in water is critical to the survival of fish and shellfish living in it. The majority of living creatures in the water need oxygen to survive but as dissolved oxygen levels decrease, it becomes harder for animals to get the oxygen they need to survive. Low dissolved oxygen is often the result of eutrophication, which occurs when there are too many nutrients (such as nitrogen and phosphorus) in the water which cause dense algal blooms to grow. When the algae die and decompose, the decomposition process uses up dissolved oxygen in the water, reducing the oxygen available for fish, and other organisms, which may become stressed or even die.

Biological Oxygen Demand

Biological oxygen demand (BOD) is a key indicator of ecosystem health. Nearly all aquatic organisms need oxygen to break down organic material in the water. Organic compounds are naturally found in water, but too many organic compounds indicate polluted water. Organic compounds come from biodegradable organic material such as industrial wastes, agricultural wastes, and human wastes. BOD can be used to determine the effectiveness of sewage treatment systems. Healthy waters will have low BOD levels while polluted waters will have high levels.

Dissolved Inorganic Nitrogen

Nitrogen is a nutrient of great environmental concern, because when in excess it is responsible for eutrophication. All living organisms need nutrients to grow and the nutrients need to be in the water to support the food chain. However, nutrients in excess disturb the natural food chain because some organisms proliferate at the expense of others. This is the case of algae, which grow faster in nutrient enriched environments requiring high demand of oxygen for decomposition when they die. Of all forms of nitrogen, DIN, Dissolved Inorganic Nitrogen (nitrates+nitrites+ammonium) is of great interest because it is the most bioavailable form of nitrogen used by algae.

Total Phosphorus

Total phosphorus is an indicator of too much phosphorus in the water. Phosphorus attaches to sediment particles, so phosphorus and sediment pollution are linked. Phosphorus is an essential nutrient for all plants and animals. But too much phosphorus in the water causes algae to grow in large, dense algal blooms, which depletes oxygen for fish and other marine organisms.

Orthophosphate

Orthophosphate is an indicator of too much phosphorus in the water. Phosphorus attaches to sediment particles, so phosphorus and sediment pollution are linked. Phosphorus is an essential nutrient for all plants and animals. But too much phosphorus in the water causes algae to grow in large, dense algal blooms, which depletes oxygen for fish and other marine organisms. Orthophosphate is the form of phosphorus which is directly ready to be used by the plants, and therefore, immediately acts as fertilizer when discharged in water bodies.

Turbidity

Turbidity is a measure of water clarity which expresses how much light passes through the water column. It is dependent upon the amount of suspended particles (e.g., sediment, algae, bacteria) and colored organic matter present. Clear water is critical for the growth and survival of fish, crabs, and other aquatic organisms.

Fecal coliform

Bacteria such as fecal coliform occur naturally in both fresh and salt water. Bacteria are also commonly found in the intestines of humans and other warm-blooded animals. Most are harmless to humans and animals, but some are pathogenic and can cause illness in swimmers. Pathogens can come from the feces of many animals, including wildlife and pets, or from humans, through leaking septic systems and broken sewer lines.