Indicator Organisms

What are indicator organisms?

Scientists can help their communities better understand what’s in the water around them. When sewage leaks into lakes and rivers, the public can be exposed to harmful bacteria. Pontchartrain Conservancy monitors for fecal contamination by measuring for indicator organisms, like Enterococcus. Indicator organisms allow for a reliable, safe, and easy assessment for the presence of fecal matter in water. Although these organisms don’t usually make people sick themselves, they are good indicators of other harmful pathogens. 

Risks

Water pollution caused by fecal contamination is a serious public health threat. Fecal contamination can cause nausea, upset stomach, earaches, and rashes in swimmers. In worse cases it can even affect boaters who have limited direct contact with the water. Failing infrastructure, wildlife, and even pets can be a source of fecal pollution.

pH

What is pH?

pH is a measurement used to describe how acidic or basic an aquatic environment is. It is measured on a 0-14 scale, proportional to the concentrations of protons (H+) and hydroxide ions (OH-). A pH of 7 is neutral, with equal concentrations of protons and hydroxide ions. The lower a pH reading is under 7, the more free protons and acidic the sample. The higher a pH reading is above 7, the more free hydroxide ions and basic the sample.

Lemons, milk, and human saliva a few examples of acidic materials. Eggs, soap, and bleach represent basic materials.

Why is pH important?

pH is an important parameter to measure because of its effect on both aquatic life and the toxicity of harmful chemicals.

Most aquatic life prefers to live within the pH range of 6.5 to 9.0 standard units, though some organisms have evolved to thrive in extreme environments. For the most part, if water deviates from this range, organisms will experience stress or die. The further away from this range, the higher the mortality will be within that ecosystem. Macroinvertebrates and fish eggs are often the most sensitive to changes in pH.

Extreme pH levels can also affect both the toxicity of a pollutant and how easily elements and compounds dissolve in water, in turn making it easier for fish and other aquatic life to absorb toxic substances. A small change in pH can make it easier for nutrients like phosphorous and nitrogen to dissolve. Over time, this can lead to an overload of plant production and deplete dissolved oxygen concentrations. This low-oxygen, high-nutrient environment can stress other aquatic life.

For example, ammonia (as NH₃) is toxic to fish at basic pHs (>10). In low and neutral pH aqueous environments, ammonia gains a proton to become an ammonium ion (NH₄⁺), and the toxicity to fish is reduced.

How can pH change?

Natural mechanisms and human actions can affect pH.

Certain carbonate materials, like limestone, can help buffer water against changes in pH and remain closer to neutral. Additional inputs of carbonate materials from sediments like river sand will raise pH towards basicity. The decomposition of excessive plant growth can also affect pH. The decay of organic matter releases CO₂, which can then react with water to create carbonic acid. Although carbonic acid is a weak acid, in large amounts it can lower the pH of an aquatic environment.

Municipal and industrial discharge can alter the pH of a nearby water body. Municipal wastewater tends to contain detergents and soaps, which in large amounts can raise pH. Often, industrial discharges can have extreme pH values, lower than 5 and higher than 9. In these industrial sectors, such as metal production or mining operations, the pH of the discharged water must be adjusted prior to discharge.

Salinity

What is salinity?

Salinity is the measurement of salt dissolved in water. Salinity is usually measured in parts per thousand (reported as “ppt”). The average ocean salinity is 35 ppt: this means that in every kilogram of seawater (1000 grams), 35 grams are dissolved salt. Waters are considered fresh if they have less than 2 ppt.

Because the water in estuaries is a mixture of fresh water and ocean water, the salinity in most estuaries is less than the open ocean. Salinity in an estuary varies based on location in the estuary, the daily tides, and the volume of fresh water flowing into the estuary. However, salinity can also vary with depth: salt water is denser than fresh water. Because of this salt and fresh water may not mix, and salt water ‘wedges’ can form on the bottom of lakes and rivers. In our estuary, salinity levels are generally highest near the Rigolets passes, where flow from the Gulf of Mexico enters, and lowest near the confluence of the rivers on the Northshore, where freshwater flows into Lake Pontchartrain.

How do we measure salinity?

To determine the concentration of salinity, we use a meter that measures the ability of water to conduct a current. This property of water is called conductivity, and it increases with higher concentrations of dissolved ions. Because these conductive ions come from dissolved salts, water with higher salinity will conduct electricity better than water with no dissolved salt.

Salinity can also be determined with three other methods: evaporation, titration, and refractometer.

• • Water can be separated from the dissolved solutes by evaporation, which can be weighed to determine mass. However, because large heat inputs are required to accelerate the evaporation, this is a difficult process and cannot be performed in the field.
  • Chemical determination of halide content by titration was for many years the usual routine method for determining the “sea salt” content of water samples. This method, known as the Mohr method, and it requires precipitating salts as silver halide compounds. Titration takes skill and ample amounts of time, so this method has long been surpassed for conductivity measurements.
  • A refractometer measures the change in direction or bending of the light as it passes from air to water. Light moves slower in water than air, and the more salt in the water, the slower the light moves. A refractometer is the best choice for measuring salinity when only approximate values are needed. Additionally, they are easy for young students to use and very affordable.

Salinity in the Pontchartrain Estuary

Over the last 30 years that we have been monitoring water quality, the salinity in Lake Pontchartrain has changed. In 1998, the USGS Environmental Atlas for the Pontchartrain Basin reported a mean salinity of 13 ppt, with an increasing trend at the Seabrook Bridge. Since then, salinity in the lake has dropped. Closure of the Mississippi River-Gulf Outlet and openings of the Bonnet Carré Spillway have contributed to a freshening of the salinity across the basin, which averaged less than 1 ppt in 2018. Since 2017, we have observed that salinity decreases when the spillway opens and increases with strong easterly winds.

Salinity will continue to be an important water quality parameter because it can determine what species of fish and plants grow and thrive in an area. Although freshening salinities are changing the distribution of sport-fish and shrimp across the region, reduced salinity in the basin makes cypress plantings and swamp forest restoration possible. Swamp restoration is vital for rebuilding our defense against storm surge.

Dissolved Oxygen

What is Dissolved Oxygen?

Surprise! There's oxygen in water, and it's not just in the form of H₂O.

We know that oxygen and hydrogen can form a compound called water, but what happens when oxygen is all by itself? Oxygen molecules that are not bonded to any other element are considered "free" or non-compound elements. When free oxygen (O₂) is present in water, we call it Dissolved Oxygen (DO). In its free form, DO is available for fish and aquatic plants to breathe in, much like humans take in oxygen from the air.

Oxygen can be introduced into water through the air or as the byproduct of photosynthesis. From the air, free oxygen can slowly dissolve across the water's surface or enter through aeration (e.g. wind or tide-driven waves, waterfalls, streams). Humans can also increase the amount of DO in a waterbody by aerating mechanically. Such is the case with fish tank air pumps or large dams. Other factors like depth, temperature, and salinity also affect DO concentrations.

Why is measuring dissolved oxygen important?

Because dissolved oxygen is a critical resource for aquatic organisms and a necessary ingredient in decomposing biological pollution such as wastewater runoff, monitoring the changes in DO can help inform us of the health of an aquatic ecosystem. A balanced level of dissolved oxygen is important for the organisms living in our water like fish, invertebrates, bacteria, and plants.

Dangerously high levels of DO can occur in areas where forces of aeration are high, like at the foot of a waterfall or near a hydropower dam. Similarly, algal blooms can produce high levels of DO through rapid photosynthesis. Consequently, fish can develop a deadly gas bubble disease. 

Too little DO can also pose a threat to aquatic life. Dissolved oxygen levels can fall drastically in overproductive waters. For example, an overproductive waterbody may contain more organisms than it can support, or in the case of an algal bloom die off, the rate of oxygen-demanding decomposition may require more DO than available. Likewise, pollution from wastewater and sewage runoff can deplete DO, because it will require free oxygen to decompose. As a result, the area will become anoxic, or without oxygen, and be unable to support most aquatic life.

Temperature

Why is temperature important?

Because temperature influences almost every other water quality parameter, it is one of the most important abiotic factors in an ecosystem. And considered on its own, temperature can affect the way aquatic plants and organisms metabolize and reproduce. Higher temperatures can result in higher metabolic rates, and that can quickly deplete oxygen in an aquatic ecosystem.

Here's how:

Higher temperatures cause metabolic rates to increase, accelerating both growth and decay of aquatic life. Metabolic rate is the amount of energy an organism expends over time, and as it increases so too does the amount of oxygen an organism requires. Decomposition is also demanding of oxygen. Further compounding this effect, higher temperatures make it harder for oxygen to dissolve in water meaning there is less available oxygen for aquatic life. So in an environment where both growth and decay are accelerated, and oxygen is not as readily dissolved into water, the concentration of DO can fall precipitously.

Additionally, extreme temperatures above 35°C, can begin to breakdown the biological enzymes necessary for a functioning metabolism. If broken down, aquatic organisms will be less able to process nutrients, and likely die. A drastic change in temperature can also affect the survival of eggs and nymphs.

What can change temperature?

Natural mechanisms and human actions can influence the temperature of an aquatic ecosystem.

Natural sources, such as sunlight radiation and the atmosphere can transfer hear to a water body. While sunlight transfers thermal energy into a water body, the atmosphere can absorb heat from or transfer heat into water, depending on the difference in temperature. If the waterbody is warmer than the air, heat will be pulled out of it and into the atmosphere. If the scenario is flipped and the ambient temperature is warmer than the water body, heat will be transferred into the water. Depending on the amount of their flow, tributaries feeding warmer or cooler water into a lake can also cause changes in temperature.

Human influence on water temperature usually comes as discharge from industrial and municipal facilities. For example, a power plant must cool generation equipment overheating during the production of electricity. Often, they pull water from a nearby source like a stream or river, to absorb heat and cool it down. As a result, the water cools the power plant equipment, but it returns to its original source warmer. The introduction of even slightly warmer water into a cooler aquatic environment can produce the negative consequences mentioned previously.

A similar example includes stormwater runoff from parking lots, or areas paved with asphalt. On hot summer days, afternoon rain showers can carry heat away from these surfaces and into nearby waters.

Turbidity

What is Turbidity?

Turbidity is an assessment of how murky, cloudy, or opaque water visibly appears. Usually, suspended particles (>2 microns in size) are the culprits behind reduced water clarity. Suspended sediment like clay or sand and plant material like algae or other aquatic vegetation decreases clarity by scattering or blocking light.

Clarity itself is a physical characteristic, determining how far sunlight can travel through a water column. The farther light can pass, the higher the clarity. Although clarity can provide valuable observations, it is considerably more subjective than measuring turbidity. We measure clarity by Secchi disk, but this method is prone to error associated with the human eye.

The nephelometric method is used to measure turbidity and requires shining a concentrated beam of light through a water sample. A sensor quantifies the amount of scattered light in a water sample in Nephelometric Turbidity Units (NTUs).

Why do we measure turbidity?

Measuring turbidity is helpful because it can often indicate potential pollution. Suspended solids, either organic or from sediment, can act as a transport for harmful pollution. If hydrophobic pollutants (microbial pathogens, pesticides, mercury, and heavy metals) are present in the environment, they attach to suspended particles in water. The more suspended solids in a water column, the more pollutants can be present.

Additionally, suspended solids block the transmission of sunlight, starve aquatic plants, and consequently, lower dissolved oxygen (DO) concentrations. Plants release an amount of oxygen through photosynthesis that adds to the available DO. With little sunlight to perform photosynthesis by, plants will begin to die and decompose. Decomposition is a process that requires oxygen and will further lower the concentration of DO. As a result, aquatic organisms dependent on the now decaying vegetation will suffer.

Turbidity in the Pontchartrain Estuary

In the past, we've seen increased turbidity play a critical part in destroying the health of Lake Pontchartrain. In fact, the lake's condition was the catalyst for the creation of our organization. Clamshell dredging negatively impacted the ecology of the lake in two ways. First, the act of dredging stirred up settled solids like sand and silt, re-suspending them in the water column and increasing turbidity. Second, it removed the lake's main filter feeder, the Rangia Clam. Rangia clams are crucial for preserving the lake's water clarity because they filter water and scrub it for nutrients. Anything abiotic, like sediment, is expelled from the clam and coated in a slime film. The film coating weighs the abiotic particles down, making them settle faster and reducing turbidity in the lake.

Currently, we are trying to fully understand the effects that opening the Bonnet Carrée Spillway has on the water quality of the lake. When the Army Corps opens the control structure, fresh and muddy Mississippi River water flows into the Pontchartrain Estuary. We know this will cause changes in the composition of the water, that are measurable through the water quality parameters we monitor. We collect water quality data year-round to help surrounding communities better understand the changes seen in the Lake Pontchartrain Estuary.