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The Chesapeake Bay is the nation's largest estuary
The Chesapeake Bay has a 64,243 square mile watershed that drains portions of six jurisdictions
(New York, Pennsylvania, Virginia, West Virginia, Delaware and the District of Columbia)
For just a moment, imagine yourself in the Chesapeake Bay Watershed in the early 1800s
during a period of great change
As human population has increased in the Chesapeake Bay watershed over the past centuries,
land use has changed from primarily undeveloped (forested)
to agricultural and developed land.
These changes have led to increases in nitrogen inputs.
Nitrogen and phosphorus are major macronutrients necessary for plant growth,
but in excessive amounts create increased algae growth,
decay and low oxygen for aquatic life.
In 2011, this increased algal productivity from nutrient enrichment
often termed eutrophication resulted in the most expansive low oxygen
or hypoxia conditions, also known as dead zones,
in the Chesapeake Bay
In fact, hypoxic volumes in the Chesapeake Bay have increased since the 1950s.
In 2010, the largest Total Maximum Daily Load (TMDL) in the Nation was developed
for the Chesapeake Bay for nitrogen, phosphorus, and sediment.
These pollution allocations were further divided by major river basins and States.
Management practices expected to eventually reduce nitrogen by 25 percent are set to be implemented in the Bay watershed by 2025.
The proportional sources of nitrogen load delivered to the Chesapeake Bay
varies by state.
The Chesapeake Bay watershed consists of six major river drainage regions that contribute varying streamflow to the Bay.
Land use in the Chesapeake Bay watershed is predominantly undeveloped, along with agricultural and developed areas.
Before we talk about the various drivers of change that affect the delivery of nitrogen to the Chesapeake Bay,
lets define some important terms and units of measurement
Watershed - a land area that drains rainfall to a waterbody (such as a stream)
Concentration - the mass of nitrogen in a known volume of water, often measured in milligrams per liter
Load - mass of nitrogen delivered for a given time
often measured in kilograms per year
Yield - the load of nitrogen relative to contributing landscape (watershed)
often measured in kilograms per square kilometer per year
Flow-Normalized Load - adjusted sediment loads for flow and season, often measured in kilograms per year
Let's also review, the major sources of nitrogen
and how it’s is cycled around the Chesapeake Bay watershed
Lastly, its important to understand the magnitude of those sources as inputs to the watershed and exports to the Bay
Those responsible for management decisions
have varying degrees of influence on the changes that control the movement of nitrogen.
We are now ready to discuss the various factors that affect the delivery of nitrogen to the Chesapeake Bay
Local resource managers have less control over the climate and hydrology
So lets talk first about climate and how increasing concentrations of greenhouse gases
like carbon dioxide that trap heat close to the Earth’s surface have caused increases in mean annual temperature
Climate scientists use four major emission scenarios (representative concentration pathways [RCPs]
of 2.6, 4.5, 6 or 8.5 watts per square meter) for how increasing amounts of greenhouse gases could trap heat.
The mean annual temperature across the entire Chesapeake Bay watershed is expected to increase by 2.0 °C by 2050
compared to 1995, with larger differences likely in the northernmost part of the watershed.
Projected precipitation changes from 1995 to 2050 for the Chesapeake Bay watershed show a
6.3% increase in the average annual precipitation
with the largest increases occurring in the southern states
Overall, the Chesapeake Bay is experiencing a warming trend with shifts to drier summer
and fall seasons followed by wetter winter and spring seasons.
The components of the hydrologic cycle
(precipitation, evapotranspiration, runoff, recharge,
groundwater discharge, and streamflow)
store, process, and ultimately transport water and nitrogen to the Chesapeake Bay.
Continued warming temperature trends are projected to produce less snowpack and earlier runoff in the winter
versus the spring
followed by less change in runoff
during the summer and fall
These seasonal shifts in runoff will affect nitrogen export to streams.
Let’s also remember the groundwater transport of nitrogen
The time it takes for groundwater and nitrogen to reach streams (lag time) can range from days to years.
This means that, in some areas, the effect of management actions to reduce nitrogen may not be realized for decades to come.
At the regional level, local resource managers also have less control over atmospheric deposition
and land use changes, driven often by population growth.
Emissions can lead to the atmospheric deposition of nitrogen (wet and dry)
downwind from the sources after being transformed and transported in the atmosphere.
Atmospheric oxidized nitrogen deposition, across the Bay watershed has shown a declining trend since the 1990s
largely due to the air quality management actions required by the Clean Air Act.
Whereas, chemically reduced nitrogen deposition, across the Bay watershed has shown a slight decline or no trend.
About half of the oxidized nitrogen emissions that contribute to atmospheric deposition nitrogen loads to the Chesapeake Bay
originate from an airshed that is much larger than the actual watershed
Atmospheric deposition of nitrogen is widespread across the Bay watershed.
The highest values occur in developed areas such as Washington, D.C., and Baltimore, Maryland,
and areas of intensive agriculture such as southeastern Pennsylvania
Regarding land use, since 1950, the population growth has increased exponentially in the Bay watershed
generating activities that have produced excessive nutrient and sediment inputs to rivers and streams
Since 1985, development has occurred mostly at the expense of natural and agricultural areas, such as pasture and cropland.
If the trends of the 1980s through 2010s continue,
then future growth will be focused in suburban and adjacent counties,
with an emphasis on developing large-lot, single-family homes.
Local resource managers have the most control over agriculture and development.
Since 1950, acres in farmland has deceased,
but crop and animal production has remained constant due in large part to the intensification of agriculture.
For example, there has been a substantial increase in the number of broiler chickens sold in the Chesapeake Bay watershed from 1954 to 2012
and selective breeding and nutrition improvements have more than doubled the weight of broilers.
This has led to increases in nitrogen related impacts to water quality in agricultural areas.
For instance, nitrogen concentrations in streams of the Eastern Shore of the Chesapeake Bay
commonly exceed water-quality criteria recommended to protect aquatic organisms
There has also been an increase in the variety and quantity of conservation practices to reduce nitrogen from agricultural areas
An expected eventual nitrogen reduction of 11% from 1985 to 2014
has been estimated for conservation practices and land retirement
Despite these estimates, nitrogen loads to streams in most agricultural areas did not change substantially during this time period
possibly due to the lag time needed to see the effects of nutrient reduction efforts.
More research is needed to better understand the water quality response of conservation practices for planning.
The expansion of developed land is a major driver of nitrogen in the Chesapeake Bay watershed.
Nitrogen from point sources such as wastewater has declined, controlled through regulations and technology,
and will likely continue to be important in controlling nitrogen loads in urban streams.
Management of nonpoint sources such as stormwater of nitrogen has been a recent focus
and will likely continue as urban centers expand across the watershed in coming decades.
Now let's look at some examples of how these factors affect the delivery of nitrogen around the Chesapeake Bay watershed
The Patuxent River demonstrates the clear connection between improved wastewater treatment and nitrogen loads.
Pine Creek: The main source of nitrogen to this forested watershed is from atmospheric deposition,
which has been decreasing since the 1980s.
As a result, Pine Creek has shown a reduction in nitrogen exports.
Difficult Run, an urbanized watershed with increasing nitrogen loads,
most likely as a result of septic discharges and new construction.
The Choptank River, nitrogen loads originating mainly from agricultural fertilizer have continued to increase
despite conservation efforts owing to the long groundwater transit time of legacy nitrate contamination.
Susquehanna River: A large watershed that has shown a promising decline in nitrogen loads since the 1980s
as a result of improvements in wastewater and implementation of management practices
but loads have remained consistent in recent decades.
Overall, total nitrogen loads have decreased from the major rivers to the Chesapeake Bay.
The Susquehanna River, with the largest annual load over time, continues to be a major challenge with respect to reaching the goal of the TMDL
Before you go, lets look at a long-term perspective of the major drivers of nitrogen change up to the present,
and a forecast into the future for the Chesapeake Bay watershed
The sum of all the sources of nitrogen to the Bay increased substantially from 1950 until the 1980s, but varied afterwards.
Nitrogen from atmospheric deposition and wastewater increased substantially from 1950,
but started to decrease in more recent decades owing to the effects of the Clean Air Act and implementation of enhanced nitrogen removal technologies.
Nitrogen from developed areas have increased along with population growth and this trend is projected to continue.
Nitrogen from agriculture has continually increased from 1950 owing to the widespread use of chemical fertilizer
and manure from the intensification of animal agriculture in some areas.
Future scenarios for agriculture, which is the largest source of nitrogen in the watershed,
include both increasing and decreasing amounts from fertilizer and manure sources,
and suggest the export of nitrogen to the Bay presents a challenge to nitrogen load targets.
In summary, the greatest opportunities for future nitrogen reductions to the Chesapeake Bay watershed are in developed and agricultural areas.
There are numerous coastal areas around the world that have been impacted by excess nitrogen.
The science, management, and regulation efforts to save the Chesapeake Bay are unprecedented
and if the Nation’s largest estuary can rebound, it will serve as a model for the world.
For more information on the science provided in this presentation, please reference the following U.S. Geological Survey publication:
Nitrogen in the Chesapeake Bay Watershed: A Century of Change, 1950-2050
An audio described version of this presentation is available at:
This information is being provided to meet the need for timely best science.
The information is provided on the condition that neither the U.S. Geological Survey nor the U.S. Government
shall be held liable for any damages resulting from the authorized or unauthorized use of the information.
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