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March 3, 2019

Learn About Water Quality Measurements!

The Federation prides itself on science-based restoration. To enhance our understanding of the river, the South, West, and Rhode Riverkeeper collects precise water quality data from almost over 50 different sites around the tidal portions of the Rivers by boat on a weekly basis from April-October using a HydroLab sonde that takes readings every quarter meter in depth to create a vertical profile. The data collected includes: temperature, pH, dissolved oxygen, salinity, conductivity, and water clarity. We are grateful to our amazing corps of volunteers who help us take these measurements.

The Federation used to collect data at over a dozen sites in non-tidal portions of the river every other week all year, but stopped in 2018. We still use a YSI ProPlus to take stream measurements at our restoration sites as necessary. Since most of our streams are less than a foot deep, we only take one set of readings per site.


Oxygen is a critical necessity for life. In aquatic systems oxygen is found in the form of dissolved oxygen (DO) and without it, the fish, crabs, and oysters cannot survive. Low dissolved oxygen concentrations can lead to reduced growth, reproduction rates, a change of the distribution and behavior patterns of the aquatic organisms, as well as death.

Dissolved oxygen is most commonly measured in milligrams per liter (mg/l) and can enter the water through photosynthesis from aquatic grasses, phytoplankton, or algae as well as by the physical process of wind mixing. Dissolved oxygen levels are usually better on those days, or seasons, when there is greater wind mixing, however, the ecosystem quickly regresses back to being dominated by low dissolved oxygen in the absence of wind if nutrient levels are otherwise excessive. Wind, rain, water temperature, and tide are constantly affecting dissolved oxygen readings. Understanding the trends in dissolved oxygen requires many readings, in all seasons, all weather, and in both rainy and dry years to start to see a pattern.

Why We Care about DO

Low dissolved oxygen levels can be caused by algae blooms which are fueled by excessive nutrients (usually from fertilizer or sediment—The South River is surrounded by sediment that naturally has Phosphorus bonded to it). Algae is short lived and the bacteria that decompose the algae use up immense amounts of dissolved oxygen. These pockets of no, or very low dissolved oxygen levels are known as Dead Zones, due to their lethal nature to aquatic life. The South River regularly has dead zones in the spring and late summer, which is very disruptive to the river’s ecosystem.

Impact on Aquatic Life

Dissolved oxygen concentrations of 5.0 mg/L or greater will allow marine creatures to live and thrive. 2mg/l or less is considered Hypoxic, and 0.2mg/l or less is considered anoxic. The oxygen requirements vary from species to species, the complexity of the species and where the animal resides in the South River. Worms and small clams living in the South River’s muddy bottom, where oxygen levels are naturally low, only need dissolved oxygen concentrations of at least 1 mg/L. Fish, crabs and oysters that live or feed along the bottom require oxygen concentrations of 3 mg/l or greater, and spawning migratory fish, their eggs and larvae need up to 6 mg/l.



Water Clarity is the measurement of how far sunlight can pass through the water column. Sunlight is one of the key elements needed for underwater grass to grow. When the water column’s clarity is reduced, the underwater grasses do not receive the sunlight necessary for growth. Excess sediment is the leading factor for the South River’s poor health.

Because of their small size, the particles of sand, silt and clay that we call “sediment” often float through the water rather than settling to the bottom, and can be carried long distances during rainstorms. When there are too many sediment particles suspended in the water, the water becomes cloudy and muddy-looking. Cloudy water does not allow sunlight to reach the plants that grow on the bottom of the Bay’s shallows. Without sunlight, these plants—including underwater grasses—die, which affects the young fish and shellfish that depend on them for shelter.

Impacts of Excessive Sediment

  • Nutrients and chemical contaminants can bind with sediment, spreading through the Bay and its waterways with particles of sand, silt and clay. Fish and shellfish that live and feed on or near contaminated sediment can become contaminated themselves, triggering fish consumption advisories in various portions of the watershed.
  • Excess sediment can smother oysters and other bottom-dwelling species.
  • Accumulating sediment can clog ports and channels, affecting commercial shipping and recreational boating.

How Do We Measure Water Clarity?

Despite advances in science, researchers still use a technique for tidal waters that is hundreds of years old to measure clarity: a secchi disk. This black and white sectioned disk is lowered into the water until the user can no longer see the division between the colors. The disk is then slowly raised until the user begins to make out the distinct sections again. This depth is used as a measure of the transparency of the water. The penetration of sunlight is crucial for the growth of underwater grasses, which acts as a filter, dissolved oxygen producer, and a vital nursery habitat for countless aquatic species. Scientists predict that underwater grasses can survive when the water clarity averages 1.0 meters during the growing season.

For freshwater streams, measuring clarity is more complicated. The Federation traditionally used a conductivity probe on a hand held YSI sonde that is placed in the stream. However, conductivity doesn’t directly measure clarity, but rather the electrical conductivity in a solution to monitor the amount of nutrients, salts or impurities in the water. Thus, if there is road salt dissolved in the stream, the stream may appear clear, but still have a high conductivity reading. The Federation uses a turbidity probe, which does measure the amount of suspended particles by measuring how the light is refracted, but the particles could be sediment, algae, bacteria or other. Both of these methods only provide the percentage of contaminates in the water. Without knowing the volume of water going past the probe, you cannot get the total amount of contaminates or load. With all our modern science, we do not know the amount or sediment pouring into the South River within an order of magnitude. We can only tell whether it is improving or worsening.  Yet sediment remains our largest source of pollution for the South River, both by volume and impact.


Why are nutrients (nitrogen and phosphorus) important?
Just as nutrients are important for land based plants and animals, phytoplankton and algae need nitrogen and phosphorus to grow. Aquatic organisms, such as menhaden eat only phytoplankton/algae for food. However, too much of a good thing is bad. If you overfertilize your garden, it dies. The same is true for the Rivers; too many nutrients cause an over production of phytoplankton and algae. The phytoplankton and algae uptake the nutrients, causing them to grow, and as they grow, they expand into a giant bloom, and the plants consume oxygen. Then when the phytoplankton and algae die, the biological decay consumes oxygen.

What causes high levels of nutrients?
Nitrogen comes from atmospheric sources, such as burning coal and other fossil fuels, as well as from human and animal waste, fertilizer, and the breakdown of organic material. Phosphorus is found in fertilizer, but it is also bound to sediment, so erosion of soil from construction sites or stream banks can often result in both sediment and phosphorus pollution.

Where do nutrients come from?
The nutrients in the Rivers watershed mostly come from stormwater runoff, leaky septic and sewer systems, eroding stream banks, and fertilizers. Some nutrients are also recycled from
bottom sediments during low oxygen events.

When does this occur?
Rain events “wash” the nutrients into the streams and into the Rivers. Conventional septic systems are designed to “leak” nutrient into the groundwater, which eventually makes their way to the river. In some cases, sewer lines are antiquated and eventually break and leak, discharging nutrients into the groundwater as well.


Salinity measures the amount of dissolved salts in the water. The Chesapeake Bay has a wide range of salinity. Up north where the Bay is fed by freshwater rivers like the Susquehanna, the salinity is as low as 0.5. By the time you reach the mouth of the Bay down in Virginia, the salinity can be as high as 30. The middle of the bay is considered brackish which is used to describe waters that are a mixture of fresh and salt water.

Salinity not only varies by location, but by time of year and depth as well. The Bay has a higher salinity during drier months and a lower salinity during the wetter months, especially after the winter snow melts and the spring rains. Salinity also increases with depth because the less dense fresh water remains on the surface.

Many plant and animal species are limited by the different salinity ranges so depending where you are in the Bay, you will be able to see different flora and fauna.


Temperature is more than just a number! Temperature is an important water quality parameter because it influences both biological and chemical characteristics of the water. Temperature impacts dissolved oxygen levels, rates at which algae and other plants photosynthesize, metabolic rates of organisms, and bacterial growth. Warmer temperatures tend to be associated with the most problems which is why summer often brings a decline in water quality.

Temperature directly affects dissolved oxygen levels. The relationship is inverse; as temperature goes up, dissolved oxygen goes down. Chances are you have heard the term “dead zone.” Dead zones occur when the dissolved oxygen levels become too low to sustain life. These occur most frequently in the summer because warmer water is less capable of holding dissolved oxygen than cooler water.

The metabolic rate of aquatic organisms is also increased when temperature increased. As their metabolic rate increases, their bodies need more oxygen. At the same time, the increased temperatures are decreasing the water’s capability to hold dissolved oxygen creating stress on the organisms. Additionally, temperatures that are too low or too high can make an organism more susceptible to pollutants and diseases caused by bacteria, parasites, and viruses.

In addition to the changing seasons, other factors can influence temperature. Turbidity increases the amount of heat that is absorbed from sunlight which is one of the reasons we care so much about water clarity. Thermal pollution also influences temperatures. In the South River watershed, most of the thermal pollution comes from stormwater runoff whereas other rivers may be impacted by thermal pollution from power plants. On hot days, stormwater heated up by impervious surfaces such as roads and roof tops before being carried directly into our waterways.


Why is pH important?
This year’s new indicator is pH. Measuring pH and alkalinity in the South River and throughout the South River watershed are important to understanding its overall health. Since the pH of water is critical to the survival of most aquatic plants and animals and alkalinity is a measurement of the water’s capacity to neutralize acids, monitoring pH is vitally important. Substances with a pH of less than 7 are acidic, and those with a pH greater than 7 are basic.

What causes pH changes?
Biological activity can significantly alter pH in the South River. Through a process called photosynthesis, plants remove carbon dioxide (CO2) from the water and expel oxygen (O2). Since CO2 becomes carbonic acid when it dissolves in water, the removal of CO2 results in a higher pH, and the water becomes more alkaline, or basic. When algae naturally begin to increase in estuaries during the spring, pH levels tend to rise. An overabundance of algae (called an algal bloom) may cause pH levels in the river to rise significantly, and this can be lethal to aquatic animals

Where has this been occurring?
The increase in pH peaks shortly after the chlorophyll and/or blue-green algae blooms.

When does this occur?
These increases in pH have generally been occurring in the late spring through the summer, however due to the high nutrient concentrations and climatic events, blooms have also been occurring in the fall and early winter. The impact is that aquatic animals which can only live in a narrow pH range are stressed or killed by the fluctuations.