Reference

Water Quality Parameters

The following information covers general ranges. Summer = May through October; Winter = November through April.

Temperature

Elevated water temperatures are associated with increased mortalities. They also signal possible spawning and can trigger the outbreak of a bacterium, Vibrio parahaemolyticus, which can cause stomach and intestinal illness in people who eat the infected shellfish. In the summer, normal temperatures range from 45 to 80 degrees Fahrenheit. If the temperatures surpass 64 degrees for more than 24 hours, this is cause for concern. In the winter, normal temperatures range from 38 to 50 degrees Fahrenheit.

Salinity

Low salinities over a long duration can weaken and/or kill shellfish. Shellfish’s resistance to low salinities decreases with increasing water temperatures. In the summer, normal salinity levels range from 5 to 32 psu (practical salinity units). If the salinity levels fall below 10 psu for more than 48 hours, this is cause for concern. In the winter, normal salinity levels range from 5 to 30 psu. If the salinity levels drop below 5 psu for more then 48 hours, this is cause for concern.

Dissolved Oxygen

Very low dissolved oxygen levels will weaken or kill shellfish. High levels are reported to cause "gas bubble" disease. In the summer, normal DO levels range from 85 to 140+ percent saturation. If the levels fall below 20% saturation or rise above 115% saturation for more than 24 hours, this is cause for concern. In the winter, normal DO levels range from 45% to 100% saturation.

Turbidity

Resistance to elevated turbidity varies with species, with oysters being most resistant; very high turbidity will reduce feeding rates and growth. Normal levels range from 0 to 200 NTU (nephelometric turbidity units). If the turbidity level rises above 50 NTU 24 hours, this is cause for concern.

In Situ Chlorophyll

Chlorophyll is the main type of photosynthetic pigment that nearly all phytoplankton use to capture energy from the sun. Thus, the concentration of chlorophyll is typically used to indicate the relative amount of phytoplankton available. Bivalve shellfish require sufficient phytoplankton to sustain growth, so very low levels of in situ chlorophyll indicate reduced growth potential. Typical chlorophyll levels range from 2 to 60 µg/l (micrograms per liter) but vary much seasonally. If the chlorophyll levels drop below 5 μg/l for more than 72 hours, this may be a cause for concern.

The most accurate way to measure the chlorophyll concentration of a water sample is through a time consuming lab extraction and analysis procedure. A rough estimate of relative chlorophyll concentration may easily be made, however, using automated in situ sensors that measure the total fluorescence of the water. Chlorphyll emits light (fluoresces) when it is hit with UV radiation. Thus the amount of flourescence in the water can be taken as an index of the local chlorophyll concentration and, most importantly, can be reported in real time.

pH

Low or high pH levels are unusual in marine waters, but can indicate a serious water quality issue associated, for example, with reduced dissolved oxygen, elevated ammonia levels, etc. Normal pH levels range from 7 to 9. If they drop below 7 or rise above 9 for more than 12 hours, this is cause for concern.