Last Thursday, I sat in a canoe explaining to a group of paddlers how
dissolved oxygen levels were in constant flux in the Hudson
River. During the daylight hours, aquatic plants produce and release
oxygen and levels increase. At night, production stops and levels
decline as oxygen is consumed by animals, plants, and bacteria. One
savvy paddler later asked, "How does this change with the seasons?" I
decided this was an excellent question to explore using the HRECOS data
By plotting all the dissolved oxygen measurements taken in 2009, we can see that the difference between daily high and daily low dissolved oxygen levels increases as we transition from winter to spring and summer. This is likely a result of increasing activity among both the producers and the consumers.Very small changes in dissolved oxygen can be seen in the winter months. Although submerged plants do not survive the winter, microscopic floating plants called phytoplankton can be found even in the coldest waters photosynthesizing and producing oxygen. Similarly, many oxygen consumers such as fish, aquatic insects, and bacteria are less active but still present and consuming oxygen in the winter.
As the spring temperatures and light levels rise, the activity of plants, animals, and bacteria increases. More oxygen is produced and more oxygen is consumed resulting in dramatic fluctuations in daily dissolved oxygen levels.
Although every station displays an increasing spread between high and low oxygen levels from winter to summer, there are significant differences among the stations.
Oxygen consumption is very high at the Tivoli Bays stations. These stations are located at the mouths of the north and south Tivoli Bays (see map). Each incoming tide provides a fresh supply of oxygen that is rapidly consumed by the fish, bacteria and insects that populate this bay. The most abundant plant, the water chestnut, does little to replenish the supply since it releases the oxygen it produces into the air and not the water (for a more detailed description see: In Water Chestnut Beds, Oxygen Levels are Tide Dependent). Because oxygen consumption is very high in these waters, we see a greater spread between daily high and low oxygen levels.
Dissolved oxygen concentrations at the Piermont Pier and the George Washington Bridge stations are very strongly influenced by the tidal cycle. Both stations are located below the salt front - the furthest reach of marine waters. Because of this, salinity levels increase at these stations with high tides. This is especially true during spring tides - large tides that occur every new or full moon. Since oxygen is less soluble in salty compared to fresh water, these spring tides result in lowered dissolved oxygen concentrations (for a more detailed description see: The Breathing Tide). It is for this reason that we see pulsing dissolved oxygen concentrations at Piermont Pier and George Washington Bridge.
Schodack Island and Norrie Point stations have less dramatic dissolved oxygen fluctuations. These stations are monitoring waters from the river's edge and not from a bay or marsh (see maps for Norrie and Schodack). Because of this they have less dramatic oxygen consumption rates compared to the stations at the Tivoli Bays. Both Schodack Island and Norrie Point stations are located in the freshwater section of the river so they are less significantly influenced by the tide compared to the Piermont Pier or George Washington Bridge stations.
If you are interested in examining and exploring this data for yourself, you can download data one parameter at a time from our live data page which can be access by clicking "Current Conditions" from the menu to your left. Alternatively, you may download the Excel file I used to process this data by clicking here: Data for this HRECOS Story.
When examining this data on your own, be aware that saturation levels will affect the ammount of fluctuation seen in dissolved oxygen % saturation compared to milligram/liter concentrations. Dissolved oxygen % saturation is calculated by dividing dissolved oxygen concentrations by the total dissolved oxygen the water is able to hold (the saturation level). Warmer water is able to hold less oxygen than colder water. With a smaller denominator, changes in dissolved oxygen will be more dramatic when expressed in % saturation in the summer compared to the winter.