Chesapeake Bay

Chesapeake Bay provides our surrounding communities with countless ecosystem services including economic (fisheries, tourism, etc), environmental (water filtration, bird flyways, etc), and recreational (beaches, scenic views, etc). For all these reasons, scientists, decision-makers, and concerned citizens are questioning how changes in land use and climate will affect the ecosystems, and the connected economy, of Chesapeake Bay. This is why managers, researchers, and educators from The National Oceanic and Atmospheric Administration, The University of Maryland Center for Environmental Science, The Chesapeake Bay National Estuarine Research Reserves of Maryland and Virginia, and Chesapeake Environmental Communications have partnered to investigate this large question!

114 years of data collected here in our backyard, show clear evidence that physical climate changes are occurring and that species and habitats are responding to these changes:

Climate Change Evidence

• The length of the growing season has increased by more than 30 days over the last century.
• Over the past century, we have about 30 fewer frost days per year
• There are 30 more warm summer nights per year now than 100 years ago
• The Chesapeake region receives about 4.5 more inches of rain per year than it did a century ago

The Data

A recent study by scientists at the University of Maryland’s (UMD) Horn Point Environmental Laboratory examined climate data at Chesapeake Bay National Estuarine Research Reserve system (NERRs) locations. There are 3 Reserve sites in the state of Maryland and 4 in Virginia. The goal of the study was to determine if there is evidence of climate change impacts at the NERRs locations.

Unfortunately, climate data at the Chesapeake NERRs only dates to the 1990s - not far enough back in time to see conclusive evidence of climate change. The National Climatic Data Center (NCDC) has data going back to the year 1900 at several locations in Maryland and Virginia that are close to Chesapeake Bay. These data definitely do go back far enough in time to detect changes in climate in the Chesapeake region. These data match up well with the NERRs data and the scientists feel that gave them a good idea of climatic conditions at the NERRs locations and the Chesapeake Bay in general going back to 1900.

Extreme Climate Indices

Scientists can look at several factors that characterize a regions climate such as how long the growing season is, how many days per year are cold enough for frost to occur, and rain fall intensity to name a few. These factors are called Extreme Climate Indices. When you have a long data record and calculate these indices for each year of that record, you can determine if some or all of them are changing over time. If they are changing, then there is a high probability that climate for that region is changing.

Growing Season Length

Growing Season Length (GSL) is the number of days in a calendar year that plant growth can occur. This means that trees, shrubs, and crops have an additional month each year where temperatures are warm enough for them to grow. The GSL index for Chesapeake Bay is telling us that the region is getting warmer. Use the arrows (< & >) to scroll through the graphs.

  • The length of the growing season has increased by more than 30 days over the last century.
  • Growing season is starting earlier and ending later than in the past.
  • Increases in the GSL are larger in the Chesapeake than those observed over the whole east coast.

Frost Days

Frost Days is the number of days in a calendar year when daily minimum temperature is less than 0° C (32° F). Looking at the number of Frost Days over time, it is apparent that there has been a decrease in the number of Frost Days each year over the period 1900 to 2014. Comparing the early 1900s to the early 2000s, the Chesapeake experienced a decrease of 33 Frost Days per year in the northern portion and 28 days in the south. Use the arrows (< & >) to scroll through the graphs.

  • Over the past century, we have about 30 fewer frost days per year
  • This may prevent agricultural pests from dying off in winter

Tropical Nights

Tropical Nights refers to the number of days in a year where the minimum temperature does not go below 68°F (20°C). Comparing the early 1900s to the early 2000s, there are about 30 more Tropical Nights annually throughout the Chesapeake region. And 4 out of 5 years with the largest number of Tropical Nights occurred within the last decade (2005, 2010, 2012, and 2014). Use the arrows (< & >) to scroll through the graphs.

  • There are 30 more warm summer nights per year now than 100 years ago
  • Hotter nights can have human health impacts, especially in urban areas
  • Hotter nights are linked to decreases in crops.

Total Precipitation

The Total Precipitation Index is the total amount of precipitation that falls in a year. The measurement is made on precipitation as water so precipitation falling in other forms such as snow, sleet, or hail is melted before it is measured. The amount of precipitation that the Chesapeake Region receives each year is increasing. Since 1900, the Chesapeake has received 5.2 to 16.8 mm (0.2 – 0.7 inches) more precipitation each decade. This equates to about a 12% increase over the time period. Use the arrows (< & >) to scroll through the graphs.

  • The Chesapeake region receives about 4.5 more inches of rain per year than it did a century ago.
  • Increased precipitation could potentially lead to increased nutrients and sediments washing into the Bay.

Submerged Aquatic Vegetation

• Some species of underwater grasses in Chesapeake Bay become stressed and die when water temperatures are above 30C for prolonged periods of time
• These warm water periods have been increasing in the Bay and it is impacting grasses
• Forecast models suggest that these warm periods will occur more frequently in the future

Bay Grasses

Submerged Aquatic Vegetation or SAV (ess ay vee) sounds like the latest organic smoothie flavor from your health food co-op. In fact, its just shorthand for a wide variety of flowering plants that root into the sediment of the Chesapeake Bay. If you are a boater or water enthusiast, you may dislike when it gets caught in your boat propeller, or when it washes up on the beach and begins to smell as it decays.

But if you are a small sponge or barnacle, SAV is home. If you are a crab or a fish like spot or croaker, SAV is your daycare. If you are a water fowl, SAV is food. The plants are the glue that binds these communities together, and they are also the glue that binds the sediments together, reducing the ability of waves to stir up and scour the bottom.

Healthy vs. Unhealthy

SAV – like all plants – needs light to grow. Because they are rooted to the bottom, the overlying water must be relatively clear for light to reach the SAV. SAV also need nutrients such as nitrogen and phosphorus (but not too much!) to thrive, and they do best in water with sufficient oxygen. In the Chesapeake Bay, SAV is adapted to our local weather and climate, surviving through cold winters and warm summers. However, not all SAV is well adapted to changing conditions in the Bay.

Over grazing on SAV by waterfowl and invertebrates, hurricanes, or disease can stress the plant communities. Insufficient light due to sediment and algae which can make water murkier may also harm plant communities. And more recently, another stress to SAV beds has been added. High temperatures in 2005 and 2010 reduced the area covered by SAV by stressing the plants with unsuitably warm water.

2005, which experienced one of the biggest SAV losses observed in Chesapeake history, wasn’t an unusually warm summer. But, like humans, SAV have to survive day by day, not average summer by average summer. Hot spells over 86°F occurred frequently in 2005 and 2010, and it was these warm events that stressed plant communities and lead to SAV losses in both years.  

Water temperature records from the Goodwin Island CBNERR site showed that not only did 2005 and 2010 experience many warm water days (more than 25% of the summer), but 2002 also had many warm days (see graph below).

Using records from weather stations, we can go back in time, asking how often Bay SAV had to endure frequent warm water events in the past. And the answer is, not very often – frequent warm events (occuring for more than a quarter of the summer) took place just 10 times since 1900. Surprisingly, six of them occurred in the last 15 years. You are 10 times more likely to draw a royal flush in a friendly poker game, than to have so many warm water events in the past 15 years by random chance. FYI - the odds of drawing a royal flush are 649,739 to 1!

Given what we have learned, can we predict the impact of warm water events on bay-wide SAV? Combining all the information on SAV from across the bay, we made a mathematical model (or a complex equation) that relates warm water events to the loss of SAV in a given year. About 70-80% of the time, it accurately predicts SAV losses on the bay-wide average. This indicates that temperature is a stressor for Chesapeake Bay SAV ranging from the freshwater plants in Maryland rivers to the southern salty reaches of Virginia. It also suggests that we can predict its effects.  

So far, we have looked into the past to understand how SAV is responding to warm weather, but to understand what to do in the future, we need to use climate models to predict how often warm water events might occur. But climate models are not perfect, so we look first at how well they predict the past. The climate models (8 different models) predict an average of 14 SAV stressing events over the past century, so they predicted 4 more warm events than were actually experienced. Eight of the 14 predicted events occurred since 2000, 2 more than were actually experienced. So we know the climate models tend to predict slightly too many warm events overall, but with both model results and actual data, 60% of the warm events occurred in the past 15 years. This is an indication that the likelihood of warm events is increasing. 


So what do these models predict for the future? Between 2020 and 2040, the models predict temperature stressing conditions to occur 85% of the time.

Why does this matter? Three good reasons. First, with a model, we can predict over the course of a summer if SAV might be heat stressed, and plan to increase monitoring when high temperature events are more common. Secondly, because some species of SAV are naturally more resilient to temperature fluctuations than others, we might expect to see these naturally resilient SAV types becoming more common in the Bay, and we need to understand whether they provide the same habitat, food, and sediment retention capabilities as the communities we currently have. And finally, as we continue to work to restore SAV, its important to plan restoration for areas that might be cooled naturally by water flow, or to plan to plant species that are not stressed by higher temperatures.

So what can you do to reduce stressors for the SAV around your home? Use only enough lawn fertilizer to keep your grass healthy, and try to apply it so that it doesn’t wash into the Bay. Consider living with some of those pesky weeds (beautiful yellow dandelions), rather than applying weed killer to the lawn.  Control water runoff from your yard and street with rain gardens. Make sure your septic system is working well, and consider upgrading it to reduce nutrient pollution.

Seasonal Cues

• Delayed onset of winter may be interfering with the migratory behavior of some fish species
• Compared to 100 years ago, spring is starting earlier and fall is starting later in the Chesapeake

What a longer growing season means to Chesapeake Bay

Longer growing seasons are a common manifestation of climate change around the globe and we can see these changes right here in Chesapeake Bay. The graph above shows the change in the length of the growing season over the last century in the Chesapeake Region. The green line indicates the start of the growing season each year and the orange line indicates the end of the growing season. As you can see, moving through time the lines are moving apart which means that the growing season is getting longer because it is starting earlier and ending later.

Let’s consider for a moment what the growing season is. We all know that, around Chesapeake Bay, plants start growing in the spring and stop growing in the fall. However, from a plant's perspective, spring and fall are not dictated by the calendar, but by when temperatures are consitently warm enough to grow or cold enough to go dorment. The period between is the growing season.

If the growing season is getting longer, then spring must be starting earlier or fall is starting later or both. The data show that both are happening: spring is starting about a week earlier in the Chesapeake region and fall is starting a little more than 3 days later.

Why does it matter if spring is starting earlier and fall is starting later around the Chesapeake? To us humans, it may not sound like a bad thing that warm weather comes earlier and stays longer. For plants and animals, it is a different story. Their lives depend on getting the timing right for how and when they react to seasonal changes.

For example, if apple trees bloom too early, a late frost may kill all the flowers, eliminating its ability to bear fruit that year. If an oriole arrives from its spring migration too late, it may find that prime nesting areas may have been taken up by other species of birds. This could interfere with the bird’s ability to find a mate or successfully raise its young.

Cold Snaps

Recent evidence from around Chesapeake Bay suggests that some migratory species that call the Chesapeake home during the spring and summer may be having more of a problem with the later onset of fall than the earlier occurrence of spring. These animals use the gradually cooling temperatures that normally occur in fall as a cue to begin their southerly migration to warmer conditions. Now that the onset of fall and winter is delayed, these animals are missing their cue to migrate and they are staying in the Chesapeake later than they should. Eventually, temperatures do drop to normal wintertime temperatures for the Chesapeake region, and often this can happen rapidly. Animals who did not migrate are not equipped to handle it.

These sudden drops in temperature are commonly referred to as Cold Snaps. Temperatures drop to levels that are very cold but still within the range seen for the region in winter. However, these temperatures are much colder than what these animals experience in winter in the southerly regions they normally migrate to. When an animal is incapacitated by a sudden plunge in temperature it is said to be cold-shocked or cold-stunned. Much like a person with hypothermia, a cold shocked animal cannot function normally and can die if warmer temperatures do not arrive soon.

Over the last few years, a number of migratory species in the Chesapeake have been caught off guard by sudden drops in temperature. In February of 2014, thousands of Speckled Trout (pictured above) died in the Rappahannock River and Tributaries of Mobjack Bay. Speckled trout normally migrate to warmer waters farther south during the fall and winter. However, milder winters over the past few years had enticed a large portion of the trout population to stay in the Bay during the winter. February of 2014 was unusually cold and thousands of these trout ended up cold-shocked and they subsequently died.  

In January of 2011, a cold snap killed about 2 million juvenile Spot, another fish species common to the Chesapeake. In the Chesapeake, Spot are generally spawned in March and spend most of their first year growing in the tributaries of the Bay. They leave the Chesapeake in late fall to spend the winter in warmer coastal waters farther south. Typically, most have migrated by December. In 2011, the juveniles may have missed their cue to head south because of a late start to winter.

In both of these cases, the fish were killed by temperatures that are perfectly normal for the Bay in winter. What was not normal was that these migratory fish were even in the Bay at that time of year. As the Bay region continues to stay warmer later into the year, it increases the likelihood that some migratory species may decide to spend their winters in the Bay. And although winters in the Chesapeake are getting warmer, for the near future we will continue to experience cold snaps. Could winters continue to warm to the point where species that currently migrate south in the winter start staying here all year? If our winters continue to warm, this may be a possibility for animals like Speckled Trout, Spot, and sea turtles since they seem perfectly happy to hang around as long as the water stays warm. This could potentially have negative impacts as they compete for habitat and food resources with the normal winter residents of Chesapeake Bay.

Human Health Impacts

• Warmer winters and springs increase the likelihood of encounter harmful bacteria in Chesapeake Bay
• Compared to 100 years ago, spring is starting earlier and fall is starting later in the Chesapeake

Possible Human Health Impacts in a Changing Chesapeake

So far, we have learned that potential climate change impacts to Chesapeake Bay will be fairly broad and could directly impact many of the species that make the Chesapeake their home. What about humans? Rising sea levels have the potential to damage property and displace thousands of people around Chesapeake Bay. This is a very real concern that many towns and cities around the Bay are trying to prepare for. Possible changes to growing season, temperature patterns, and precipitation patterns could potentially impact many fish and shellfish species and the people that make their living harvesting them. This could affect both the seafood and tourism industries in the Chesapeake Region. While these impacts could be devastating, they will most likely not directly affect the health of humans living around the Chesapeake.

Something that may impact human health is the potential increase in the chance of encountering Vibrio bacteria. The map to the left shows the potential habitat (based on salinity) of the two strains of Vibrio found in Chesapeake Bay that can be harmful to humans. V. cholorae can cause the gastrointestinal illness cholera in humans, usually by eating contaminated shellfish or drinking untreated water. About 80 people are sickened by V. cholorae in the United States every year. In Chesapeake Bay, V. cholorae is generally found in the low salinity regions.

V. vulnificus can cause symptoms similar to V. cholorae by those who consume infected shellfish. V. vulnificus can also infect the open cuts and soars of people swimming or wading in waters where V. vulnificus is present. Only about 100 people a year are infected by V. vulnificus in the United States. However, 85% of those infected require hospitalization. V. vulnificus is generally found in the mesohaline areas of Chesapeake Bay.

The figure below shows the probability of encountering V. cholorae at the Chesapeake NERRs Jug Bay location and V. vulnificus at the Taskinas Creek location. Jug Bay is generally a fresh water location and Taskinas Creek is Mesohaline. These plots show that there is a chance of encountering V. cholorae all year round at Jug Bay, but that this chance increases significantly in the summer when temperatures are warmer. You are unlikely to encounter V. vulnificus at Taskinas Creek in the late fall, winter, or early spring but the chances of it being present increases rapidly as temperatures warm into summer. These graphs tell us that warm water temperature is the primary factor controlling the likelihood of encountering Vibrio in Chesapeake Bay.

Scientists have found that there is a relationship between winter and early spring temperatures and the likelihood of encountering Vibrio in Chesapeake waters the following year. A measurement of the coldest maximum daily temperature in a month (TXn) informs us about how cold it is in a given winter. Lower values would mean a colder winter and higher values mean a warmer winter. The data show that when the average TXn for December, January, and February (winter) is relatively warm, the probability of encountering V. cholorae the following year is higher.

Another userful index is the number of days in a month when the maximum daily temperature is higher than 90% of all the temperature measurements ever taken on a given day at that location (TX90p). It is another index that can tell us about the severity of a given season. The evidence shows that when the average spring (March, April, May) TX90p is relatively warm, there is a higher chance of encountering V. vulnificus in the mesohaline waters of Chesapeake Bay.

Over the last 100 years, the likelihood of encountering Vibrio bacteria in Chesapeake Bay has increased as the changing climate has resulted in improved conditions for them to grow. This likelihood will increase over the next several decades as the Chesapeake region continues to warm. Protecting yourself from these bacteria is fairly straight-forward: don’t eat raw or under-cooked seafood from contaminated waters and don’t swim or wade in the Bay if you have an open cut or sore. Unfortunately, not all residents of the Chesapeake will take these precautions and it is possible that we may see an increase in the number of individuals sickened by Vibrio around Chesapeake Bay and the United States.

Nutrient Pollution

• Increase preciptitation over the last century has likely increased the volume of water flowing out of Chesapeake rivers and the nutrient and sediment loads associated with thes flows
• Forecast models indicate that precipitation (and therefore river flow) will increase through the end of this century

Potential impacts of more precipitation

As we’ve learned, over the last century there has been an increase in the amount of precipitation the Chesapeake receives each year. Annual precipitation in the northern region of the Chesapeake has increased by 0.66 inches (16.8 mm) per decade in the northern region and 0.21 inches (5.2 mm) per decade in the southern Chesapeake region. In other words, the northern Chesapeake region now gets about 6.6 inches more of precipitation per year now than it did in the early 1900s. The southern Chesapeake gets about 2.1 inches more precipitation. Climate change forecast models suggest that by the year 2100, the northern and southern Chesapeake will get an additional 5.3 and 5.0 inches of precipitation per year, respectively.

The most obvious impact of more precipitation is that more fresh water will be flowing into the Bay. The image to the left shows the relationship between annual precipitation and both river flow and nitrogen pollution for four tributaries of Chesapeake Bay. The blue lines show how river flow increases with increasing precipitation and the red lines show how the amount of nitrogen entering each river increases with increasing precipitation.

In Chesapeake Bay, there is a strong relationship between river flow and the amount of nutrient pollution that the Bay receives. In short, precipitation in the form of rain and melting snow washes nutrients and sediment off the land and into the Bay’s streams and rivers where it causes problems ranging from algae blooms and low oxygen concentrations to decreased water clarity. More rain means more water and nutrients coming off the land and higher river flows. Can we expect nutrient pollution in the Chesapeake to increase as the region continues to receive more precipitation moving into the future?

Once scientists understand the relationship between things like precipitation and river flow and river flow and nutrient pollution, they can use these relationships to make estimates about how future changes in one will affect the other. In these relationships, precipitation, river flow and nutrient pollution are called variables because each time you measure them, they can have a different value. In the relationship between precipitation and river flow, the value of river flow changes when the value of precipitation changes. When precipitation is high, river flow tends to be higher and when precipitation is low, river flow tends to be lower. We would say that values for river flow are dependent on the values for precipitation. The amount of precipitation, however, is not dependent on the amount of river flow, which makes sense. Precipitation is therefore the independent variable in this relationship. In short, dependent variables are controlled by independent variables.

Let’s look at another relationship. If we know that nutrient pollution is higher when river flow is high and lower when river flow is lower, which is the dependent variable? It’s nutrient pollution.

Using the relationships between precipitation and river flow and river flow and nutrient pollution, we can estimate how nutrient pollution will be impacted by the forecast increases in precipitation in the Chesapeake region. It is estimated that nitrogen pollution will increase 22% in the Choptank River, 27% in the Rappahannock River, 13% in the Susquehanna River, 23% in the Potomac River, 20% in the Appomattox River, and 20% in the James River by the year 2100. These are only estimates and they assume that land use will remain the same and efforts to manage nutrient inputs will continue at their current level. It also assumes that climate change will continue unchecked.

Land use will most likely not remain the same. The population in the Chesapeake region is projected to continue to grow into the foreseeable future. We will most definitely need to alter the landscape in order to feed and house these additional people. The challenge is to grow smartly using agricultural and development practices that minimize their nutrient loads to the Bay. It is hoped that efforts to manage nutrient pollution will increase, not stay the same. Nutrient and sediment pollution is still the number one issue facing the Chesapeake. Just as important, efforts to reduce green house gas emissions need to be increased. We may be just beginning to see the impacts of climate change in the Chesapeake but if it continues unchecked, these impacts could surpass those of nutrient pollution.