[biofilms & biodiversity]

Water Quality Profile
in the Baltimore Inner Harbor

Here is a brief description of the water quality parameters in general. Water quality tests provide a "snapshot" of the water conditions during the day in a particular area. These parameters change during the day and night, after a thunderstorm and with the seasons. They also influence the biodiversity of the local aquatic community.


The vertical profile of temperature indicates that as depth increases temperature decreases during summer months. Since we are sampling water up to 20 ft. deep the difference in temperature should not be very significant. Weather and seasonal change have the greatest influences on water temperature throughout the year.


This is the measure of undissolved particulate matter, including sediment, plankton, algae, that is estimated by the absorption of light that is passing through the sample. The Lamotte Smart Colorimeter we will use shines a beam of light through the water sample and the % of light absorbed is used as the measure of turbidity.

Secchi depth

This classic estimation of water clarity utilizes a disc that is painted white or white and black suspended at the end of the rope. The rope is lowered into the water and when the disc can no longer be seen clearly the depth is estimated utilizing tick marks in cm along the rope. This is a general estimate of water clarity in the upper portion of water only.


pH is the measure of the (H) hydronium ion content in the water. The concentration of this ion determines if the water is acidic (high H content) or alkaline (low H content). The pH scale ranges from 0-14 and Chesapeake Bay water can have a pH range of 6.5-8.0 depending on the proximity to freshwater or saltwater. Saltwater tends to be more alkaline and will influence the pH.

Dissolved oxygen (DO)

The amount of oxygen gas dissolved in water is critical for aquatic life. The DO is measured in ppm (parts per million) or mg/L (milligrams per liter). An analogy of a ppm would be if you took about 1/20th of a single drop of food coloring (1 microliter) and added it to a liter of water and stirred (donŐt expect a color change). DO levels in the Inner Harbor can range from 7-8 ppm just under the surface and 3-5 ppm at about 20 ft. Fish and other organisms will experience stress when DO levels fall below 5 ppm for extended periods of time.

The organisms inaquatic ecosystems are adapted for oxygen levels much lower than terrestrial organisms. By comparison, the ppm of oxygen in our atmosphere would be approximately 220 ppm! Dissolved oxygen is influenced by temperature, depth, and salinity. The warmer the water gets the more oxygen will be released from water into the atmoshpere. As depth increases the DO typically decreases due to the presence of higher salinity water and as salinity increases the DO will decrease due to the increased density of the water.


The historical salinity measurement was accomplished by allowing a liter of sea water to evaporate completely and weighing the dry material remaining. This mass of salt in grams was used as the measure of salinity in g/L or ppt (parts per thousand). An analogy of a ppt would be to take 1 ml of food coloring and add it to 1 liter of water and stir until the food coloring is evenly distributed. Salinity is the driving force in the Chesapeake Bay ecosystem since it has a major influence on DO levels. Interestingly, many organisms in the Bay love high salinity water found at the mouth of the Bay and are adapted for life at low dissolved oxygen levels. Fortunately, the Bay has a fair amount of turbulence and mixing which helps supply some additional oxygen.

Salinity in the Inner Harbor has ranged from 1-2 ppt during the summer of 1996 to 9-10 ppt during the summer of 1997. The amount of freshwater from rain in 1996 caused lower than usual salinity levels. In 1997 drought influenced the salinity levels during the months of June and July. Stayed tuned for updates on the water quality during the summer of 1998!