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Browse History: Influencing Factors
Indicator Icon Influencing Factors

Influencing factors include those parameters that affect the dissolved oxygen conditions in the Bay. These factors help determine if the amount of low dissolved oxygen in the Bay will be small or large. Examples of factors that influence dissolved oxygen conditions are river flow, winds, and chlorophyll (phytoplankton) biomass.

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Indicator Icon Influencing Factors

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Timing of flow and stratification affected dissolved oxygen conditions in mainstem

In 2011, Susquehanna river flow to the Chesapeake Bay was well above average in March, April, and May, which are the months that most affect summer dissolved oxygen conditions in the mainstem. Average to below average summer river flow in June, July, and August was followed by very high flows in September, following Hurricane Irene and Tropical Storm Lee (see graph below; data from USGS).

2011 and long-term total flow to Chesapeake Bay 

Hurricane Irene came through the Chesapeake Bay region on August 27th and 28th. The storm delivered a significant amount of rain and wind to the region, but the Susquehanna River did not receive as much rainfall as Maryland and Virginia's Eastern Shores and Virginia's lower Western Shore. However, Tropical Storm Lee, which came through the region on September 7th, dumped an even larger amount of rain in the Chesapeake Bay area, including the Susquehanna River. It was the second highest flow amount measured at Conowingo Dam on the Susquehanna River during the period of record (see graph below). For an in-depth account of the storms' impacts on the Bay, read this article at The Bay Journal.


2011 daily and long-term total flow to Chesapeake Bay

The amount of nutrients, such as nitrogen and phosphorus, is directly related to the amount of river flow entering the Bay. Nutrients fuel phytoplankton growth and subsequent blooms, which eventually die and sink to deeper water where bacteria consume the dead phytoplankton. This process consumes oxygen, potentially leading to hypoxic or anoxic conditions. 2011 flow appears to have affected summer conditions by shifting the intensity of low dissolved oxygen conditions to earlier in the summer (see Anoxia and Hypoxia). High flow in spring led to an early appearance of anoxia and hypoxia. By the time the first measurements were taken in early June, there was already over 4 km3 of anoxia and 8 km3 of hypoxia in the mainstem waters.

Fresh water entering Chesapeake Bay from its tributaries can also influence hypoxic or anoxic conditions. Fresh water tends to flow on top of saltier water because fresh water is less dense than salty water. This “layering” of water is called stratification. Because the major source of oxygen for deeper waters in the upper and mid-Bay is from surface waters, stratification reduces mixing of oxygen into deeper waters and increases the likelihood of anoxic and hypoxic conditions.

High spring flows combined with hot summer temperatures led to strong stratification during June and July. If Hurricane Irene had not come through the region in late August, causing turbulent mixing of the Bay's waters, that stratification and associated large volumes of anoxic and hypoxic water could have continued through the remainder of the summer. The winds associated with the hurricane mixed well-oxygenated surface waters with the poorly-oxygenated bottom waters, causing low dissolved oxygen conditions to completely disappear, except in the deepest parts of the mainstem channel. As a result of all these processes, hypoxia was reduced and the anoxia disappeared in late summer. See indicator pages for more detailed information.

2011 average monthly air temperature at mid-Bay
During the spring and summer of 2011, the average monthly temperatures were well above (~2.4 °F) the long-term (1945–2010) average for this same time of year.  April and July, especially, were close to breaking the record for the warmest months over the period of record. Warm air temperatures are important to consider when thinking about dissolved oxygen conditions in the Bay because they can lead to strong vertical stratification of the water column. As the surface waters become warmer, they can become inhospitable for some marine species. These animals may be capable of swimming to deeper and colder waters, however, if those deeper layers are low in oxygen, the animals may become stressed or squeezed out of their habitat.  (Data source: NOAA National Data Center)

True color image of the Chesapeake Bay on September 12, 2011

On September 12, 2011, the effects of the heavy rainfall during Tropical Storm Lee were visible to passing satellites. Dirt and debris flowed down the major rivers (Susquehanna and Potomac are most striking in this image) and into the Bay. The water appears to look like chocolate milk due to the high concentrations of sediments in the water. Some farmers reported losing many feet of topsoil over acres of their land. (Source: Bay Journal, Eyes on the Bay)

Total suspended matter as estimated by the MODIS instrument on the NOAA Aqua satellite

Satellite observations of total suspended matter (TSM) immediately before (above, left) and after (above, right) Tropical Storm Lee. The plume of sediment originates from the Susquehanna River where, in some areas of the watershed, rainfall totals exceeded 32" in the two weeks preceeding September 11. These images show the total amount of mud, silt, and debris (i.e., TSM) that was present in the bay before and after the flood events. While much of the northern main stem of the Bay is off the scale for TSM, if a water sample were collected with a TSM of 35 mg L-1 (the maximum amount on the scale in the image above), it would look more like a cloudy brown than true chocolate milk colored.  (Image from NOAA

High sediment levels in the Bay can temporarily decrease water quality by lowering light levels, introducing pollutants, and by coating the seafloor with a layer of silt that may harm aquatic grasses and benthic organisms (e.g., oysters). Due to cloud cover during much of September that interfered with satellite measurements, it is not possible to determine how quickly, or where, these sediments settled to the Bay floor.  Scientists from the University of Maryland Center for Environmental Science are currently sampling sites around the Bay to determine the thickness and spatial extent of these sediments from Tropical Storm Lee. In a system that is already stressed, such events may hamper some ongoing restoration efforts.