by Tammy Domanski, PhD from the Environmental Center at Anne Arundel Community College
Ever wonder what those numbers and terms mean when you want to know if Spa Creek water is safe to swim in? What about a good habitat for fish or other animals? Here is a primer on what the numbers mean.
Water temperature gives us an indication of how water is warming with summer season and will affect many of the other measured parameters. For example, higher water temperatures are generally associated with lower dissolved oxygen levels and increased occurrence of algal growth.
Salinity is a measure of salt. The waterways around the Chesapeake Bay are brackish with historical values in Spa Creek varying from just under 7 ppt through greater than 11 ppt. Salinity can be affected by temperature and rain. Runoff after a heavy rain can decrease salinity significantly. Salinity also generally increases as water temperature increases.
Conductivity is a measure the ability of the water to conduct electricity or heat. We measure in mSiemens per Celsius with value corrected to 25 degrees Celsius for all measurements. This value is directly correlated with salinity but measures total ions. Ions commonly found in water include calcium, magnesium, potassium and sodium cations and bicarbonate, carbonate, chloride, nitrate, and sulfate anions.
Secchi Depth is a measure of water clarity. We measure in cm. Clarity is important for submerged aquatic vegetation survival. The higher the Secchi depth, the more light penetration occurs. A recommended Secchi depth for SAV growth can vary depending on water depth and salinity. For the Severn River, a Secchi Depth of 100 cm (1 m) is the cutoff, although in the shallower water in the creeks, that value may be lower.
Dissolved oxygen is measured in mg/L. A standard cutoff for DO is 5 mg/L. Dissolved oxygen is necessary for the survival of many marine animals. When DO falls below 5 mg/L the area is considered degraded. Values below 2 mg/L are considered very hypoxic and below is 0.2 mg/L is considered anoxic. Temperature, wind, and algal blooms will affect DO.
pH is a measure of hydrogen ions in water. A range of 6.5 – 8 is considered healthy for the marine life in our rivers. Runoff, car exhaust, acid rain can all affect pH.
Total Suspended Solids is measured as mg/L and values below 15 mg/L are needed for SAV growth and survival.
Chlorophyll A is measured as micrograms/L. The Chesapeake Bay Program has determined that chlorophyll A concentrations below 15 µg/L are required for SAV growth. Things like algal blooms will significantly increase levels, which in turn affect clarity and DO levels.
Nutrients, Nitrogen and Phosphorous. Both of these nutrients are measured in µg/L. There are multiple forms of each that can be measured. These assays will be conducted at the Chesapeake Biolabs in Solomons, the total value of each, which includes multiple forms, will be measured. High levels of either contribute significantly to algal blooms. The Cheaspeake Bay Program has set a high limit of 20 µg/L of phosphorous and 150 µg/L of nitrogen.
Enterococci, measured as colony-forming units/ 100 ml are indicators of fecal contamination by warm-blooded animals including birds and mammals. The bacteria that fall into this group are not themselves, typically infectious, but their presence is strongly correlated with the presence of other bacteria that can cause both gastrointestinal and skin infections. The high limit for water that is regularly used for swimming is 104 cfu/ 100 ml. In addition, the county, and many other agencies, recommend not swimming in the water for 48 hours after a rain event of 0.5 inches or more, especially for those that are immune-suppressed and those with open wounds.