Measuring Salinity and Dissolved Oxygen in Water

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Scientists, biologists, and anglers all know how salinity and dissolved oxygen concentrations can greatly affect the distribution and survival of finfish and shellfish in tidal and nontidal waters. Being aware of salinity values can help anglers understand how far up the bay and tidal rivers ocean migrant fish might travel and how far down brackish water fish might move down the bay.

Typically, it is easy to understand that during rainy seasons in the spring, salinity values can be low and conversely periods of drought will cause salinity values to rise in the bay and tidal rivers. Water containing salt is more dense than freshwater and during tidal movements it will slide under the more buoyant fresher surface waters in a phenomenon called a salt wedge. Often deep waters in the channels of the tidal rivers will have higher salinity values than surface water.

Fifty plus years ago, tests for salinity were done by a labor-intensive titration method. Today electronic meters used by scientists and biologists measure many water parameters in one easy probe that is lowered into the water. Salinity values are often reported as parts per thousand, which translates to how many grams of salt can be found in a thousand grams of water.

The Bay is a mixing area where 150 major rivers and streams run off its 64,000 square mile watershed to meet the ocean water entering the Bay�s mouth near Norfolk, Virginia. The Bay�s salinity is lowest (0 ppt) at the top of the Bay at Havre de Grace and increases towards the mouth of the Bay (up to 35ppt). Since salinity values in the bay will vary, the NOAA Buoy System is an excellent monitoring system for anglers to see real time salinity values along with other water quality values. (https://buoybay.noaa.gov/) Another good source for Maryland Bay conditions is Eyes on the Bay

Striped bass can tolerate a wide variety of salinity environments but do best in salinities of 10mg/l (p.p.t.) or better. When striped bass are in very low salinity waters they are under a lot of stress and are constantly removing excess water from their bodies. This is one of the reasons catch and release fishing is not allowed when striped bass are spawning in the upper sections of the spawning tidal rivers. An opposing example are many species of salmon that cannot regulate the uptake of freshwater when traveling from the ocean to freshwater spawning areas and they eventually die after spawning.

Dissolved oxygen is the amount of free oxygen contained in a unit of water and is very important to all aquatic life in tidal and nontidal waters. Finfish and shellfish all need oxygen to breathe, and all have various benchmark levels to mark acceptable levels for their survival. Oxygen enters water through absorption at the water surface, wave action, and from photosynthesis from aquatic plants. Dissolved oxygen levels in water are often measured in parts per million in a unit of water (p.p.m.) or mg/l. Cold water will typically hold more oxygen than warm water, so dissolved oxygen levels are usually higher during the winter months and lower in the summer.

Good dissolved oxygen levels promote healthy fish growth, low values cause severe distress and eventual death. As an example, trout need dissolved oxygen levels around 8 to 12 p.p.m., striped bass need a minimum of 2 p.p.m. to survive and 6 to 8 p.p.m. is good, values below .5 p.p.m. causes a fishkill.