Drought and Deluge Change Chesapeake Bay Biology


If the Chesapeake Bay were a high school student, it might be afraid to show its parents its report card. For the past seven years, the environmental monitoring group the Chesapeake Bay Foundation has rated the health of the Chesapeake Bay ecosystem a paltry 27-28 on a scale of 100. The 2004 “State of the Bay” report revealed widespread pollution with nitrogen, phosphorus, mercury, and sediment. Underwater grasses—the nurseries of Maryland’s famous blue crabs—followed a dramatic crash in 2003 with another small decline in 2004. The rate of development targeting wetlands, forest buffers, and other natural landscapes shows no sign of slowing down. The status of the rockfish population earned the Bay’s fisheries only A; blue crabs earned a C; oysters and shad each received an F.

  Page 2 Photograph of the Chesapeake Bay

Although the Chesapeake Bay appears serene at the water’s surface, changes in the surrounding landscape threaten the complex ecosystem. (Photograph courtesy USDA Agricultural Research Service)

  Blue Crab

The blue crab is one of many aquatic species in the Chesapeake Bay that is threatened by poor water quality. NASA research is revealing how rainfall in the surrounding watershed affects the health of the Chesapeake and its inhabitants. (Photograph courtesy United States Food and Drug Administration)


Multi-billion-dollar federal restoration efforts are underway, and watershed states Pennsylvania, Maryland, Virginia, and District of Columbia signed an agreement in 2000 that committed each state to doing its part to get the Chesapeake Bay off the Environmental Protection Agency’s list of impaired U.S. waters by 2010. One of the main targets of restoration is the reduction of nitrogen from aging and over-capacity wastewater (sewage) treatment plants and stormwater runoff. Nitrogen flowing to the Bay from rivers and streams fertilizes algae and other single-celled plants in the Bay, and they bloom explosively. When the plants die, they are decomposed by bacteria. Bacterial activity can use up all (or nearly all) of the oxygen dissolved in bottom waters, creating conditions where other aquatic life can’t survive.

In 2004, ocean and remote-sensing scientist James Acker generated some evidence supporting the idea that reducing the flow of nitrogen into the Bay would improve Bay health. A Chesapeake-watershed resident, Acker works at NASA’s Goddard Earth Sciences Data and Information Services Center (GES DISC) in Greenbelt, Maryland. In 2004, Acker was seeking a research project he could use to test-drive an online, data analysis program recently developed by scientists and technical staff at NASA. The program had been designed for rapid analysis of satellite observations of ocean color, the visible light that penetrates water surfaces and reflects off what is floating or dissolved there.

  Satellite image of the Chesapeake Bay

Acker was looking for a project he could use to demonstrate to scientists and natural resource managers how easy the new data tool was to use, and how quickly it could give them information about biological processes in a particular body of water. Ocean color depends on what is in the water. When large numbers of plants are growing in the water, the chlorophyll and other plant pigments affect the water’s color, making it greener, sometimes even with shades of red. Acker decided to compare satellite-based chlorophyll observations of the Chesapeake Bay from the previous two years.

“In 2002,” Acker explained, “the entire mid-Atlantic region was gripped by a severe drought. When the drought relented, 2003 then became one of the rainiest years in [recorded] history.” The contrasting rainfall and runoff patterns were bound to influence Bay biology, he reasoned. Acker expected that the changes in the Chesapeake Bay environment due to the 2002 drought and the 2003 deluge would create differences in satellite ocean color observations. He didn’t expect, however, that his quick proof-of-concept demonstration project would provide convincing evidence that one of the main goals for improving the health of the Bay—reducing the input of nitrogen from streams and rivers—was right on target.


Researchers use satellite measurements of ocean color to estimate the amount of microscopic plant life that lives in the Chesapeake Bay and other bodies of water. The kinds and amounts of plant life are indicators of the health of marine ecosystems. (NASA image courtesy Jeff Schmaltz, MODIS Rapid Response)


The Two Faces of Nitrogen: Nutrient or Nemesis

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“The main problem with nitrogen,” says Acker, “is that too much is definitely not a good thing, particularly for the aquatic environment.” Water running off farm fields and neighborhood lawns carries nitrogen with it, and treatment plants rarely remove all of the excess nitrogen from wastewater before returning it to the flow. The problems are worse for treatment plants that have outdated technology, or that are operating over capacity. Human wastewater isn’t the only nitrogen waste problem for the Chesapeake Bay; concentrated production of pork and chicken creates huge volumes of nitrogen-rich manure, and some of that nitrogen invariably ends up in the Bay.

  Photograph of chicken farm on the Easter nShore of the Chesapeake

One source of excess nitrogen in the Chesapeake Bay is chicken manure. Animal waste, fertilizers, and incompletely treated sewage deliver more nutrients to the Bay than the native ecosystem can handle. (Photograph courtesy USDA Agricultural Research Service)


For plants, nitrogen is an indispensable nutrient. As any homeowner in search of a lush green lawn knows, fertilizing with nitrogen enhances the thickness and greenness of the grass. Nitrogen fertilizers allow a smaller number of farmers to produce the crops that feed the rest of the human population. Unfortunately, explains Acker, excess nitrogen in the water makes algae and other single-celled plants (phytoplankton) grow excessively. As the excess algae die, bacteria that decompose the plant matter may use up virtually all the dissolved oxygen in the water, creating bottom-hugging, low-oxygen “dead zones.” In summer 2004, a dead zone spanned more than a third of the Chesapeake Bay floor. Around the world, similar dead zones are occurring with increasing frequency in estuaries and near the mouths of major rivers.

Immersed in the Subject

Acker has been involved with the Chesapeake Bay ever since moving to the Chesapeake watershed from the shore of another bay—Tampa Bay, where he studied chemical oceanography at the University of South Florida’s Department (now College) of Marine Science. His first studies of the Bay were about the influence of acid rain on Bay chemistry and biology. Since 1996, Acker has been working as a data support scientist at NASA’s Goddard Space Flight Center. Throughout his career as a data support scientist, Acker has helped oceanographers in 80 different countries to get and use remote-sensing data for oceanographic research.

  Map of dissolved oxygen in the Chesapeake Bay, late July 2004

The decay of algae and other phytoplankton lowers the levels of dissolved oxygen in the Bay—creating dead zones where most animals cannot survive. This map shows measurements of dissolved oxygen for July 15–30, 2004. The graph on the right shows dissolved oxygen levels between the surface and a depth of 40 meters through the center of the Bay. Orange and red colors correspond to the dead zone. (Map copyright Chesapeake Bay Program)


While living in Virginia and Maryland, Acker also became familiar with the Chesapeake Bay in a more direct fashion—by competing in a local event called the Bay Bridge Swim, a 4.5-mile crossing of the Bay. As an oceanographer, a resident of the Bay watershed, a researcher on the chemistry of streams and oceans, and even an occasional semi-aquatic part of the Bay ecosystem, Acker is interested in the vital importance of estuaries such as the Chesapeake Bay. At his job, Acker works to increase the awareness among oceanographers of how to use ocean-color data to investigate the oceans. Those two interests intersected when his colleagues at the data center developed a new data analysis tool they called “Giovanni.”

  Photographs of the Chesapeake Bay swim

Giovanni is a Web-based software program that helps scientists who aren’t experts in remote sensing to overcome some of the technological and practical hurdles of working with satellite data. Analyzing year-to-year changes in the plant productivity of coastal waters was just the type of project its designers had envisioned for Giovanni.


Jim Acker has first-hand experience in the Chesapeake: he has participated in the annual Great Chesapeake Bay Swim. (Photographs copyright Jeff Hall (left) and Cheryl Wagner (right).


Color the Bay Green (for Chlorophyll)

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Through the Chesapeake Bay “health bulletins” that appeared in the local news, Acker knew that the Bay had experienced low phytoplankton productivity in 2002 and high productivity in 2003. Acker wondered if it would be possible to use Giovanni to detect these changes in chlorophyll concentration. He also wondered if the changes in chlorophyll would be detectable not only near the mouth of the rivers where the nitrogen and other excess nutrients were flowing in, but also farther away at the mouth of the Bay where it opened into the Atlantic Ocean.

  Maps of Chlorophyll Concentration, April 2002 and April 2003

“In other areas where rivers have high nitrogen levels,” explains Acker, “dead zones occur adjacent to the river outlet. The most famous example is the large dead zone that occurs in the Gulf of Mexico near the mouth of the Mississippi River each spring and summer.” But chlorophyll observations are more accurate in the open ocean than in shallow water like the Chesapeake Bay, which contains many things that can interfere with the detection of chlorophyll, including sediment, organic material, and even the bottom of Bay itself. In addition, summertime haze caused by air pollution and water vapor is worse closer to land. The haze interferes with satellite observations of the water.

In other words, the very place where the Chesapeake dead zone occurs is also the most challenging area for satellites to monitor. If Acker could detect phytoplankton changes farther from the river mouth, he could have more confidence in the accuracy of the results. So Acker decided to use Giovanni to examine the variation of chlorophyll concentration in a study area at the Bay mouth, just east of the Chesapeake Bay Bridge-Tunnel.


Chlorophyll concentration (a measure of microscopic marine plant life) in the Chesapeake Bay was much greater in April 2003 (right) than April 2002 (left). Jim Acker discovered the difference using NASA satellite data and the Web-based analysis software Giovanni. In these images, low concentrations of chlorophyll are dark blue, while high concentrations are bright yellow. The data are from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). (NASA images by Robert Simmon)

Map of Chesapeake Bay study area  

Acker used the Giovanni tool to analyze chlorophyll data at the mouth of the Chesapeake in a study area that extended from 75° to 76° West, and from 36.5° to 27.5° North. He chose a location near the Bay mouth because satellite observations of ocean color are more accurate in open water than in shallow, near-coast waters. (NASA images by Robert Simmon, based on data provided by the GSFC Ocean Color team)


Giovanni Gets to Work

Acker provided Giovanni with the latitude and longitude corners of the study area, and then used Giovanni to calculate monthly average chlorophyll concentration for this area in 2002 and 2003, based on satellite observations from the Sea-viewing Wide Field-of-view Sensor and the Moderate Resolution Imaging Spectroradiometer sensor on NASA’s Aqua satellite. Within minutes, he had his results, and they were both obvious and startling: in every month of 2003, chlorophyll was higher compared to the corresponding month in 2002, and the concentrations in some months were nearly twice as high.

Acker had Giovanni plot the variability of chlorophyll with respect to longitude and time. This figure confirmed that chlorophyll at the Chesapeake Bay mouth in 2003 was considerably higher over the entire study area, with the highest concentrations located right where the Bay waters flow into the Atlantic Ocean.

Plot of chlorophyll versus longitude and time at the mouth of the Chesapeake Bay

With monthly streamflow data from the U.S. Geological Survey and the Giovanni results in hand, Acker wrote a short paper describing the apparent effect of 2003’s high streamflow on chlorophyll at the Bay mouth. During the 2002 drought, low streamflow caused a much-reduced delivery of nitrogen to Chesapeake Bay. Algae growth (measured through satellite-observed chlorophyll concentrations) declined. In turn, the Bay experienced a temporary improvement in ecosystem health. The nitrogen, however, didn’t disappear—it simply accumulated on the land surface, awaiting heavier rainfall to release it into streams. When the heavy rains of 2003 occurred, much of the nitrogen stored on land surged into the Bay, causing widespread algal blooms and a worse-than-average year for Bay health.


Chlorophyll levels in the study area were higher in 2003 than 2002. The left-hand graph shows how chlorophyll varied by longitude and time over the course of 2002 and 2003. Degrees longitude appear along the bottom. First letters of months appear along the left side. Chlorophyll levels were highest near the mouth of the Bay (left), where nitrogen was fertilizing the growth of algae. Beginning in March 2003, chlorophyll concentrations jumped dramatically, exceeding 11 milligrams per cubic meter of water in some places. The graph on the right shows the average chlorophyll in the study area. (Figure adapted by Robert Simmon from Acker et. al. 2005)

Graph of streamflow into the Chesapeake during 2002 and 2003

While the basic picture was obvious, the results did raise some questions that Acker, who doesn’t consider himself an expert on Bay biology, couldn’t answer. For example, what did it mean that these changes in chlorophyll concentrations were so obvious at the mouth of the Bay, when the extra nitrogen would have been mostly coming from rivers and streams farther upstream? Acker sent his paper around to Chesapeake Bay experts, hoping for some additional insight into the intricate biological processes that could account for his results.


The monthly inflow of water into the Chesapeake Bay was consistently higher in 2003 (blue) than 2002 (red). The different segments in each column show the amount of water contributed by major watersheds. The high streamflows of 2003 carried excess nitrogen into the Bay, which fertilized algae that then grew explosively. In turn, the decay of dead algae led to the reduction of oxygen levels in the water, and the development of a large dead zone. (Graph adapted by Robert Simmon from Acker et. al. 2005)


Color the Bay Green (for Chlorophyll)

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One of the scientists who examined the paper was Larry Harding of the Horn Point Laboratory of the University of Maryland Center for Environmental Science. Harding has studied the Bay for many years, and he has used remote-sensing data from satellite and aircraft-mounted instruments to investigate the biological behavior of phytoplankton in the Bay.

“Before I talked to Larry,” says Acker, “I wasn’t really aware that the upper and lower parts of the Bay were like two separate systems in many ways. I figured that the relationship between increased nitrogen and plant productivity applied to the whole Bay. I learned from Larry that that wasn’t the case.”

Harding explained that the largest input of nitrogen to the Bay comes from the Susquehanna River, which enters the northern reaches of the Bay at Havre de Grace after flowing through the fertile agricultural regions of Pennsylvania. There is so much nitrogen in the Susquehanna, even when drought reduces streamflow, that the growth of phytoplankton in the upper Bay is not limited by the availability of nitrogen. In other words, drought or no, the upper part of the Bay is probably going to bloom.

  Landsat image of the Susquehanna River entering the Chesapeake

Only in the mid- to lower Bay, seaward of the large river outlets, is the growth of phytoplankton limited by nitrogen concentrations. In these regions, nitrogen availability varies greatly depending on freshwater flowing in from rivers and streams. When heavy rains—such as those in 2003—increase freshwater and nitrogen flow in the Chesapeake watershed, phytoplankton bloom in the mid- to lower Bay. When rainfall is less, nitrogen levels are lower, and phytoplankton growth is restricted. Under these conditions, Harding has reasoned, the lower Bay acts more like the coastal Atlantic Ocean.

When Acker’s colleague Suhung Shen correlated the freshwater flow data and monthly chlorophyll at the mouth of the Bay for 2002 and 2003, the researchers discovered that there was no correlation between flow rates and chlorophyll in 2002. In 2003, however, the team discovered a significant correlation between the rate of freshwater runoff and the chlorophyll concentration “downstream” at the mouth of the Bay – about one month later.

This observation made perfect sense. It would take a few weeks for the phytoplankton at the Bay mouth to respond to water with increased nitrogen concentrations flowing in from the upper Bay. And when very little freshwater flowed in, as in 2002, waters at the Bay mouth were “decoupled” from the Bay and behaved more like the waters of the coastal Atlantic Ocean. High flows of nitrogen-rich freshwater runoff appeared to reconnect the mouth of the Bay with the biological dynamics of the Bay itself.

Nitrogen in the Bay and Around the World

“This study provided more than I expected,” Acker said. “The observation of how freshwater flow influences phytoplankton at the mouth of the Bay is an indication that sufficient reductions in nitrogen entering the Bay will reduce the influence of the land, at least in the lower Bay. This study helps to confirm the important goal of nitrogen reduction in efforts to restore the Bay.”


The Susquehanna River, which enters the Chesapeake Bay at its northern end, carries 40 percent of the nitrogen that flows into the Bay—the largest single source. There is so much nitrogen in the northern Bay that algae have all the "fertilizer" they need, and changes in streamflow do little or nothing to affect the growth of algal blooms. This satellite image shows brown water flowing from the Susquehanna. (NASA image by Robert Simmon, based on Landsat-7 data provided by the UMD Global Land Cover Facility)

  Map of dead zones around the world

“We also learned how to better use Giovanni for research, and how it can be used with ocean color data to observe chlorophyll concentrations in coastal waters around the world that are not nearly as well studied as Chesapeake Bay,” he added. “There are vital estuaries in many countries that may have problems similar to those of the Chesapeake, and Giovanni could be used to monitor the impact of programs to reduce nitrogen pollution. Giovanni is so easy to use, it can allow many more marine scientists to enhance their current research and knowledge with ocean color data analyses.”

  • References:
  • Acker, J. G., L. W. Harding, G. Leptoukh, T. Zhu, and S. Shen, 2005: “Remotely-sensed chl a at the Chesapeake Bay mouth is correlated with annual freshwater flow to Chesapeake Bay” Geophysical Research Letters, 32.
  • Harding, L. W., Jr., 1994: “Long-term trends in the distribution of phytoplankton in Chesapeake Bay: Roles of light, nutrients, and streamflow,“ Marine Ecology Progress Series, 104, 267–291.
  • Harding, L. W., Jr., and E. Perry, 1997: “Long-term increase of phytoplankton biomass in Chesapeake Bay,” Marine Ecology Progress Series, 157, 39–52.
  • Harding, L. W., Jr., A. Magnuson, and M. E. Mallonee 2005: “SeaWiFS retrievals of chlorophyll in Chesapeake Bay and the Mid-Atlantic bight,” Estuarine Coastal and Shelf Science, 62, 75–94.
  • Malone, T. C. 1992: Effects of water column processes on dissolved oxygen, nutrients, phytoplankton and zooplankton, in Oxygen Dynamics in Chesapeake Bay: A Synthesis of Research, edited by D. Smith, M. Leffler, and G. Mackiernan, pp. 61–112, Univ. of Maryland Sea Grant College, College Park, MD
  • Officer, C. B., R. B. Biggs, J. L. Taft, L. E. Cronin, and M. A. Tyler, 1984: “Chesapeake Bay anoxia: Origin, development, and significance,” Science, 223, 22–27.
  • Development of Giovanni for ocean color data analysis and research is supported by the Ocean Color Time-Series Project, NASA Research, Education and Application Solutions Network (REASoN) CAN 02-OES-01. Principal Investigator for the Ocean Color Time-Series Project is Watson Gregg, with Co-Investigators Wayne Esaias, Charles McClain, Gene Feldman, Steven Kempler, Gregory Leptoukh, and James Acker.

Dead zones are present in estuaries all over the globe, particularly near densely populated or intensively farmed areas. Each red dot on this map indicates a body of water prone to oxygen depletion and the formation of a dead zone. (Map by Robert Simmon, based on data provided by Robert Diaz, Virginia Institute of Marine Science)