The duration of the Chesapeake Bay’s annual “dead zone” has declined over time, according to research published last month in the scientific journal “Limnology and Oceanography.”
First reported in the 1930s, the Bay’s dead zone, or conditions of low dissolved oxygen also known as hypoxia, results from excess nutrients, which fuel the growth of algae blooms. As these blooms die, bacteria decompose the dead algae. This decomposition process removes oxygen from the surrounding waters faster than it can be replenished, suffocating marine life. While intensified agriculture and development continue to push nutrients into rivers and streams, research by Yuntao Zhou and others shows the duration of the Bay’s dead zone decreased from five months to four months between 1985 and 2010, and the end of the hypoxic season moved up from October to September. This could suggest that efforts to manage nutrient loads—through upgrades to wastewater treatment plants, cuts to vehicle and power plant emissions and reductions to runoff from farmland—are working.
This same research showed no change in the average onset of the Bay’s dead zone or for its average volume, whose peak has moved from late to early July. In other words, while the duration of the Bay’s dead zone has declined, its size and severity have not.
While Zhou points out that nutrient pollution is the foremost factor that fuels the development of our dead zone, his research also shows that weather patterns can act as an additional driver. Northeasterly winds, for instance, can create conditions that reinforce the separation between the Bay’s fresh and saltwater, leading to larger hypoxic volumes.
The Chesapeake Bay’s dead zone measured near average in size this past summer, coming close to scientists’ June prediction of a smaller than average hypoxic zone in the nation’s largest estuary.
Dead zones, or areas of little to no dissolved oxygen, form when nutrient-fueled algae blooms die. The bacteria that aid in algae bloom decomposition suck up oxygen from the surrounding waters. The resulting hypoxic or anoxic conditions can suffocate marine life, shrinking the habitat available for fish, crabs and other critters.
Each summer, the Maryland Department of Natural Resources (DNR) and the Virginia Department of Environmental Quality (DEQ) collect water samples to measure the hypoxic volume of the Bay. Since 1983, this number has ranged from 15.3 to 33.1 percent. In 2013, it measured 22.1 percent: 5.6 percent higher than the previous year and just above the 21.9 percent average.
Dead zones are impacting the distribution and abundance of fish that live and feed near the bottom of the Chesapeake Bay, according to new research from the Virginia Institute of Marine Science (VIMS).
Dead zones, or areas of little to no dissolved oxygen, form when nutrient-fed algae blooms die and decompose, and are most pronounced in the deep waters of the Bay’s mainstem during warm summer months. During a decade-long study of the bottom-feeding fish that inhabit this portion of the Bay’s water column, scientists noticed drastic declines in species richness, diversity and catch rate as dead zones restricted habitat and displaced the fish toward more hospitable waters.
So-called “demersal” fish—which include Atlantic croaker, white perch, spot, striped bass and summer flounder—avoid dead zones because a lack of oxygen can place stress on their respiratory and metabolic systems. While the fish often return to their former habitat when oxygen levels improve, dead zones can also wreak havoc on their forage grounds, stressing or killing the bottom-dwelling invertebrates the fish need for food.
“Once oxygen levels go up, we do see the average catch rate go up,” said Andre Buchheister, Ph.D. student and author of the VIMS study. “That’s a good sign. It indicates that once those waters are re-oxygenated, it’s possible for fish to move back in. But the availability of food is compromised, and studies have shown that the productivity of benthic biomass—or the critters that live in and on the bottom of the Bay—is stressed.”
The impact that demersal fish displacement could have on Bay fisheries is unclear, Buchheister said. Commercial fishermen who work outside of the mainstem might not be affected. But recreational anglers searching for striped bass could struggle if their forced move out of cool, deep waters is shown to contribute to poor health among the population.
In June, a forecast from researchers at the University of Maryland Center for Environmental Science (UMCES) and the University of Michigan predicted a smaller than average dead zone for the coming summer, thanks to lower than average nutrient loads that entered the Bay last spring. But to return the Bay’s mainstem to its former health, “one or two good summers won’t make that much of a difference,” said Buchheister. Instead, benthic habitat must be rebuilt, as long-term improvements boost Bay health from the bottom up.
Images courtesy Virginia Institute of Marine Science (VIMS)
Dead zones, or areas of little to no dissolved oxygen, form when nutrient-fueled algae blooms die. As bacteria help these blooms decompose, they suck up oxygen from the surrounding waters. The resulting hypoxic or anoxic conditions can suffocate marine life.
The Chesapeake Bay Program tracks dissolved oxygen as an indicator of water quality and Bay health.
The latest NOAA-funded forecast from researchers at the University of Maryland Center for Environmental Science (UMCES) and the University of Michigan predicts an average summer hypoxic zone of 1.108 cubic miles, lower than last year’s mid-summer hypoxic zone of 1.45 cubic miles.
This predicted improvement should result from the lower than average nutrient loads that entered the Bay this spring. According to the U.S. Geological Survey (USGS), 36,600 metric tons of nutrients entered the estuary from the Potomac and Susquehanna rivers, which is 30 percent lower than average.
The Bay’s dead zones are measured at regular intervals each year by the Maryland Department of Natural Resources (DNR) and the Virginia Department of Environmental Quality. While the final dead zone measurement will not take place until October, DNR biologists measured better than average dissolved oxygen on its June monitoring cruise, confirming the dead zone forecast.
In 2011, monitoring data collected by the Bay jurisdictions and other partners showed that dissolved oxygen concentrations in the Chesapeake fell to their lowest level in the last four years with 34 percent of the waters meeting the established DO standards for the summer months. This represents a decrease of 4 percent from the 2010 figures according to the Chesapeake Bay Program (CBP) partnership and is almost half of the higher DO values recorded a decade ago.
In spite of lower levels and in the face of many weather challenges, various Bay habitats and creatures that have been the target of restoration efforts showed resilience last year. In CBP news this March, scientists from Virginia Institute of Marine Sciences (VIMS) reported that despite a decrease in Bay grasses overall, the restored, healthy grass beds at Susquehanna Flats remained intact, widgeon grass beds grew (likely due to seed germination stimulated by lower salinities) and new grass beds were found in Virginia’s James River. In terms of fisheries, preliminary data by oyster scientists from Maryland Department of Natural Resources and NOAA showed good news, too. Experts estimate last year’s oyster survival rate was at its highest since 1985, oyster biomass increased 44 percent and oyster disease was at an all time low.
“Last year’s heavy rains and even this year’s early algae blooms and fish kills reinforce the critical importance of controlling polluted runoff reaching the Bay’s waters,” said Nick DiPasquale, Director of the Chesapeake Bay Program. “The survival rates of some oyster and grass beds in 2011 shows us that our efforts are working. By actively restoring and protecting valuable resources we can build a stronger, healthier Bay ecosystem that can withstand the forces of nature. Clearly, while we can’t control the weather, we can restore the watershed’s ability to survive its more extreme events. We know what works; we just need to do more of it.”
Experts were not terribly surprised by the final information on the Bay’s 2011 “dead zones” given the extreme weather. Between the very wet spring that sent excessive nutrients downstream, a hot, dry, early summer and more heavy rains accompanying Tropical Storm Lee and Hurricane Irene, conditions in the Chesapeake were bound to be affected.
Peter Tango, CBP Monitoring Coordinator and U.S. Geological Survey scientists explains, “The Bay ecosystem functions most effectively when fresh and salt water can mix, just like oil and vinegar need to mix to form salad dressing. A large fresh water influx such as that in 2011, along with intense heat, can result in vast differences in quantities of warm fresh and cool salt water in the Bay. These variables make it more difficult for water to mix vertically in the water column.”
In addition to vertical mixing, the dissolved oxygen levels in the Bay are also affected by what happens at the edges. Tango continues: “By the fall of last year, the Upper Bay became mostly fresh water due to rain. The Lower Bay became a hot tub due to heat,” illustrates Tango. “While the initial effects of the Tropical Storm Lee’s arrival was to mix the Bay more than usual in late summer, this combination of salinity and temperature conditions resulted in minimal levels of oxygen in bottom waters that lasted well into the fall. The delay in autumn vertical mixing and the persistent summer-like water quality conditions at the northern and southern boundaries pushed on the mid-Bay waters, resulting in what we scientists call a dissolved oxygen or ‘DO squeeze.’”
All of the Bay's living creatures – from the fish and crabs that swim through its waters to the worms that bury themselves in its muddy bottom – need oxygen to survive, although the amounts needed vary by species, season and location in the Bay. A DO squeeze challenges the health of fish, crabs, and other Bay creatures since they become compacted together – predator and prey, from north to south and bottom to top – in significantly smaller sections of water where and conditions are less-than-ideal for their survival.
A new study analyzing 60 years of water quality data shows that efforts to reduce pollution from fertilizer, animal waste and other sources appear to be helping the Chesapeake Bay’s health improve.
The study, published in the Nov. 2011 issue of Estuaries and Coasts, was conducted by researchers from The Johns Hopkins University and the University of Maryland Center for Environmental Science (UMCES).
The research team found that the size of mid- to late-summer low oxygen areas, called “dead zones,” leveled off in the Bay’s deep channels during the 1980s and has been declining ever since. This is the same time that the Bay Program formed and federal and state agencies set the Bay’s first numeric pollution reduction goals.
“This study shows that our regional efforts to limit nutrient pollution may be producing results,” said Don Boesch, president of the University of Maryland Center for Environmental Science. “Continuing nutrient reduction remains critically important for achieving bay restoration goals.”
The study also found that the duration of the dead zone – how long it persists each summer – is closely linked to the amount of nutrient pollution entering the Bay each year.
For more information about the dead zone study, visit UMCES’s website.
Hurricanes, earthquakes, a freezing cold winter and a blistering hot summer – 2011 has been an interesting year for weather in the Chesapeake Bay region. Scientists with the Smithsonian Environmental Research Center have written some preliminary thoughts about the bizarre weather and its link to conditions in the Bay in a post on SERC’s blog, Shorelines.
While SERC tends to focus on the long-term picture rather than brief snapshots, this year has prompted more than a few raised eyebrows among our scientists. What does it mean for the environment? What does it mean for Chesapeake Bay? And can any of it be linked to climate change?
Visit SERC’s blog to read more about the link between 2011 weather and the Chesapeake Bay.
Image courtesy Iris Goldstein/Flickr
The early summer dissolved oxygen forecast (called an “anoxia forecast”) is based on nitrogen loads to the Bay during winter and spring, as well as high river flow in May due to heavy rainfall. According to scientists, the Bay’s 2011 low-oxygen area – commonly called the “dead zone” – could be the fourth-largest since 1985.
The annual summer ecological forecast uses data such as nitrogen loads, wind direction and sea level to predict dissolved oxygen levels in the Bay’s mainstem. The forecast is split into early summer (June to mid-July) and late summer (mid-July to September) because scientists have observed a significant change in oxygen levels following early summer wind events.
The forecast is supported through research at the Chesapeake Bay Program, Johns Hopkins University, Old Dominion University, and the University of Maryland Center for Environmental Science Horn Point Lab.
For more information about the dissolved oxygen forecast, visit Chesapeake Eco-Check’s website.
There was a smaller low-oxygen “dead zone” and fewer fish kills and sea nettles in the Chesapeake Bay this summer, according to an annual review of summer conditions by scientists with Chesapeake Eco-Check.
The Bay’s summer health is influenced by the amount of water that flows from streams and rivers in winter and spring. This fresh water carries nutrient pollution, which fuels the growth of algae blooms that eventually break down in a process that robs the water of oxygen.
River flow was above average in winter and early spring but below average in late spring and summer. This shifted the intensity of low-oxygen conditions to earlier in the summer. A large, dense algae bloom in the upper to middle Bay in March combined with high temperatures early in summer led to the worst low-oxygen conditions of the summer appearing in late June. After that, below-average flows combined with favorable winds allowed conditions to improve.
Some algae blooms can be toxic to fish, causing large numbers of fish to die in events called “fish kills.” There was only one recorded fish kill linked to toxins from a harmful algae bloom. Three fish kills were the result of low oxygen caused by algae blooms, and seven fish kills were due to low oxygen alone.
The summer review was developed through Chesapeake Eco-Check by scientists with the Chesapeake Bay Program, University of Maryland Center for Environmental Science, Johns Hopkins University, Old Dominion University, University of Michigan and Maryland Department of the Environment.
For more information about the summer review, visit the Chesapeake Eco-Check website.
We've all read the stories about the Bay's “dead zones”—areas of the Bay that become devoid of oxygen during the Chesapeake's hot summer months and cannot support most forms of life. But how do parts of the Bay get that way?
Dissolved oxygen, or DO, refers to the amount of oxygen that is present in a given quantity of water. We measure it as a concentration using units of mg/l (i.e., the milligrams of oxygen dissolved in a liter of water). Keeping track of the Bay's oxygen levels is important because everything that swims or crawls in the Bay—from prized striped bass to the worms crawling at the bottom—requires oxygen to live.
Temperature determines the amount of dissolved oxygen that water can hold. Yet, even at the warmest temperatures that we typically see in the Bay—around 91 degrees Fahrenheit—the water is still capable of having DO concentrations of about 6 to 7 mg/l, which is enough oxygen for striped bass and most other Bay species to survive.
On average, the Bay area experiences the warmest weather of the year between mid-July and early August. But high temperatures are only a small part of the reason why oxygen levels drop in parts of the Bay's mainstem each summer.
The causes of the Bay's low DO begin on the land and in the air.
Residents of the Bay watershed can help give the Bay's crabs, fish and other critters some relief from low DO by taking simple actions to reduce nutrient pollution, including driving less, picking up pet waste and reducing the use of lawn fertilizers.