Environmental impacts on lake Azué௠in Haiti due to degradation of its watershed

3.8 Treats on Lake Azuei

Deforestation associated with heavy agricultural pressures on marginal farms, accelerating soil erosion, which increases the volume of sediment carried by streams into the lake (MDE, 2001) Lake Azuei receives water of watersheds of 110,000 ha. Deforestation and erosion has accelerated the formation of gullies, the flow capacity of the watershed has increased from 30 to 40% compared to traditional capacity which was 15 to 20% in the past. Some references give the watersheds of Haiti following characteristics: Evaporation 60-70%, 20-25% infiltration, runoff from 25 to 30%. If one assumes that the infiltration and runoff are very strong in the karst area, it will deduct about 20 to 25% of the watersheds finish their race in the Lake Azuei (Michel, 2011). According to Patrick Louis, a geologist, the bathymetric surveys conducted by his institution at the lake during the last thirty years, reports that the level of sedimentation, for the period from 1983 to 1990, more than 2 meters.

Retrieved : October 31 , 2011 from www.haitiwebs.com/showthread.php?t=48496 .

In December 2010, hundreds of fish were dead in the Lake Azuei. According to initial findings of the investigations, the death of fish was caused by oxygen deficiency and increasing the level of acidity (pH) in water following the use of calcareous materials in the work of raising the stretch of road (linking Haiti to the Dominican Republic) affected by the rising waters of Lake Azuei. Land use and land cover changes can affect the physical, chemical and biological characteristics of the soil. Changes in land use can specifically affect the infiltration capacity: changes from natural or seminatural vegetation to agriculture or pasture can decrease infiltration rates (Yimer et al., 2008).

3.9. Lake water Quality

Understanding a lake's physical, chemical, and biological properties is essential to determining the lake's condition and in making informed lake management decisions.

According to USEPA (1998), the water quality measurements or "parameters" most frequently tested in lakes are:

Ø Physical measurements such as water clarity,
dissolved oxygen, and temperature;

Ø Chemical measurements such as nutrients, suspended
solids, pH, and conductivity; and

Ø Biological measurements such as chlorophyll and
bacteria.

3.9.1 Physical Parameters

a. Water clarity or Transparency: is commonly measured using a Secchi disk, an eight-inch diameter, weighted disk that is painted black and white in alternate quadrants and attached to a calibrated rope. The disk is lowered into the water and the depth at which it is no longer visible is recorded. Secchi disk measurements reveal how deep sunlight can reach into the water and thereby indicate general water quality conditions. Hence, Secchi transparency is considered an indirect measurement of how much algae and sediment is in the water.

b. Dissolved Oxygen measurements determine the amount of oxygen in the water available for fish and other aquatic life.

c. Temperature measurements are used to characterize the presence or absence of thermal stratification (the forming of "layers" of water with distinct temperature differences. Both dissolved oxygen and temperature measurements are an integral part of a basic lake assessment.

A lake's temperature variations are important in influencing what types and how many fish will live and reproduce in that lake. Low oxygen levels may restrict where fish can go within a lake and limit the types and numbers of fish in the lake's bottom waters. Generally, warm water fish (e.g., bass, bluegill, catfish) need at least 5 milligrams per liter (mg/L) of dissolved oxygen to survive, while coldwater fish (e.g., trout and salmon) require 6-7 mg/L.

3.9.2 Chemical Parameters

Nutrients in lakes serve the same basic function as nutrients in your garden: they help plants grow. In a garden, lush growth and high productivity are considered beneficial, but this is not necessarily so in a lake. While some algae and other aquatic plants are essential parts of a healthy lake ecosystem, elevated nutrient levels can stimulate nuisance plant growth and lead to an out-of-balance ecosystem and impaired recreation.

a. Phosphorus is usually the nutrient that regulates algae growth the most. For many lakes, additional phosphorus loadings will stimulate additional algae growth. Because phosphorus is so important to the growth of algae and aquatic plants, many lake and watershed management activities focus on reducing phosphorus availability in the lake water. Measuring phosphorus provides an indication of the fertility or "nutrient enrichment" of a lake. Lakes with low nutrient concentrations are termed "oligotrophic." Those with intermediate nutrient levels are called "mesotrophic." Lakes with high nutrient concentrations are termed "eutrophic." Lakes with extremely high nutrient levels are labeled "hyper-eutrophic." TP concentrations above 0.030 mg/L are enough to stimulate nuisance algae growth.

b. Nitrogen forms of greatest interest to lake studies are nitrate+nitrite nitrogen (NO₃⁼+ NO₂^-), total ammonia nitrogen, and total Kjeldahl nitrogen (total ammonia plus organic nitrogen). The inorganic nitrogen forms (nitrate and ammonia) are readily utilized by algae for growth. Researchers have found that inorganic nitrogen concentrations above 0.30 mg/L are able to stimulate algae growth. Additionally, high ammonia concentrations are toxic to fish and other aquatic organisms.

Suspended solids: Suspended materials influence a lake's transparency, color, habitat quality, recreational values, and overall ecosystem heath. Total suspended solids (TSS) and turbidity reveal the amount of solids suspended in the water, and include both inorganic forms (such as soil particles) and organic forms (such as algae). TSS measures the actual weight of material per volume of water. Turbidity measurements depend on how much light is scattered or absorbed by a water sample (more suspended particles cause greater scattering; tinted water absorbs more light). Turbidity is reported in nephelometric turbidity units (NTU).

Ions:The amount and types of ions (positively or negatively charged dissolved minerals) in the water provide general information on the lake's overall ionic chemistry. Parameters that describe the ionic makeup of lake water include alkalinity, conductivity, and pH. They also generally reflect the lake's watershed geology and soil characteristics.

1. Alkalinity is a measure of the acid-neutralizing or "buffering" capacity of water. The higher a lake's alkalinity, the greater its résistance to a decline in pH. Alkalinity is commonly influenced by bicarbonates and thus is reported as the concentration of calcium carbonate (CaCO₃) in the water. Natural alkalinities fall within a general range of 20 to 200 mg/L CaCO₃. Illinois lakes typically have high alkalinities and therefore are well-buffered from the effects of acid rain.

2. Conductivity (specific conductance) is a measure of the water's ability to conduct an electric current. It is useful for estimating the concentration of total dissolved solids (TDS) in the water (TDS equals about 60 percent of conductivity). Because the measurement is made using two electrodes placed one centimeter apart, conductivity is generally reported as micromhos per centimeter (umhos/cm). Lakes with high alkalinity often have high conductivity and vice versa.

The water's pH is a measure of its hydrogen ion (H⁺) concentration and reflects the water's acidity. The lower (more acidic) the pH, the more "free" or "unattached" hydrogen ions in the water. The pH scale ranges from 0 to 14. A pH of 7 is neutral, less than 7 is acidic, and greater than 7 is basic or alkaline.

A lake's pH will fluctuate somewhat each day and from season to season in response to photosynthesis by algae and other aquatic plants, watershed runoff, and other factors. Like dissolved oxygen concentrations, pH may change with depth, primarily due to various chemical reactions and a decrease in photosynthesis.

3.9.3 Biological Parameters

a. Chlorophyll

Chlorophyll is the pigment in plants that allows them to create energy from light (photosynthesis). There are several forms of chlorophyll and each absorbs a different wavelength of light. Chlorophyll a is found in all photosynthesizing plants. For this reason, the amount of planktonic (suspended) algae in a lake is commonly estimated using the chlorophyll a concentration. Chlorophyll a should be "corrected" for phaeophytin a, a common breakdown product of chlorophyll a that can interfere with its measurement. Analysis for chlorophyll b and c provides information on the various types of algae present (blue-greens, green algae, diatoms, etc.).

b. Fecal Coliform Bacteria

Analysis of bacteria levels at swimming areas is a good public health practice and it may be required by state, county, or local health department regulations. Analyses typically are made for fecal coliform bacteria, or for the particular fecal coliform Escherichia coli (E. coli). Fecal coliforms are microscopic organisms that live in the intestines of warm-blooded animals and are excreted in their waste. Coliform bacteria themselves usually are not disease-causing. However, they are good indicators for the presence of sewage and animal wastes that may contain disease-causing organisms and pathogens.