How can the impacts of changes in coastal ecosystems be predicted to help prepare for potential impacts on natural resources and human communities?
One or two 45-minute class periods, plus time for student research
Classroom style or groups of 3 – 4 students
Coastal ecosystems are constantly changing environments. Some changes are barely detectable (such as a gradual build-up of sediment over a period of twenty years), while others can have major impacts on natural resources and human communities (such as hurricanes or chemical spills). NOAA’s National Ocean Service (NOS) has developed a series of ecological forecasting projects to predict these impacts and help prepare for them. The goals of ecological forecasting are to:
Ecological forecasts vary in time and spatial scales that range from hours to decades and local to global. Current ecological forecasting projects include:
Ecological forecasting of sea nettle abundance in Chesapeake Bay is a good example of how physical and chemical environmental factors can be used to make predictions about biological organisms. Sea nettles in Chesapeake Bay belong to the species Chrysaora quinquecirrha, and are members of the phylum Cnidaria which also includes hydroids, anemones, corals, and other jellyfish. Like all jellyfish, sea nettles have an umbrella shaped body called a bell. Several dozen yellow tentacles and four long, frilly, white feeding arms hang down from the bell. The tentacles are equipped with rows of stinging cells called nematocysts. These cells are used to capture and immobilize prey, which can be almost any small creature that blunders into the tentacles. Small fish, microscopic crustaceans, larvae of many species, and comb jellies (another Cnidarian) are typical food. Studies in Chesapeake Bay tributaries have shown that in one day sea nettles are able to consume as much as 40% of bay anchovy eggs and 94% of the copepods present, demonstrating that sea nettles can have a significant impact on individual species and possibly on other organisms that use these species for food. While they are generally regarded as a nuisance or worse, sea nettles may actually benefit the oyster industry since they consume large quantities of comb jellies which are a major predator on oyster larvae. Nematocysts can also deliver a painful sting to humans, and in large numbers can pose a serious danger to people in the water. Knowing where and when to expect sea nettle “outbreaks” can help swimmers and fishermen avoid this risk.
Large numbers of sea nettles are associated with a specific range of salinity and temperature. This association is the basis for an ecological forecasting project that uses information on salinity and temperature in the Chesapeake Bay to create maps that describe the probable distribution patterns of sea nettle populations. A mathematical model of the Chesapeake Bay (the Chesapeake 3-D Physical Oceanographic Model, or C3PO) as well as satellite imagery are used to predict salinity and temperature conditions in the Bay. Every Friday, maps of probable sea nettle presence are created based on locations where current environmental conditions are favorable to sea nettles. In addition, the sea nettle forecasting project uses volunteer observers (such as high school and college students, boaters, fishermen, mariners, and individuals from organizations that use the Bay) to confirm or reject these predictions by reporting the presence or absence of sea nettles in all regions of the Bay.
The mathematical model to predict the probability of encountering sea nettles from estimations of sea-surface temperature and salinity was developed from data on Chrysaora quinquecirrha abundance collected between 1987-2000. Temperature, salinity, and depth were also measured at the sites where abundance data were collected, and a statistically significant correlation was found between these factors and Chrysaora quinquecirrha abundance. In this lesson, students will seek a correlation between abundance and physical factors, and make inferences about conditions that are most likely to be associated with large populations of sea nettles. In addition to acquainting students with the basis for ecological forecasting of sea nettle outbreaks, this lesson also provides an introduction to some of the data sources that can be useful for this type of project.
Students have several options for calculating average January-June river discharge from monthly stream flow data. The easiest is to import the relevant data into a spreadsheet and perform the calculations with a simple sum function divided by 6. Alternatively, the appropriate data could be added using a calculator, or even paper and pencil.
Students should realize that years having the greatest abundance were also years with the highest sea surface temperatures (27°C-29°C), and years in which January-June discharge from the Susquehanna River averaged 1,594-3,637 ft3/sec. Students should note that the abundance of was zero in 2000, even though the discharge rate was also in this range, and should infer that the sea surface temperature of 24.5°C (the lowest recorded during the study) was sufficiently low to outweigh potentially favorable salinity conditions. Following discussion of students’ results and inferences, you may want to tell them that the technical journal article on which this activity is based reports that the majority of Chrysaora quinquecirrha are observed in shallow (< 15 m) surface waters with temperatures ranging from 26 to 30°C and salinities ranging from 10 to 16 parts per thousand.
Discuss the concept of “limiting factors.” Be sure students understand that the habitat of a species is defined by many physical and biological parameters, and that one or more of these limits the size that populations can attain. Point out that while salinity and temperature are important physical factors that influence the presence of sea nettles, other factors (such as food availability) become limiting even when salinity and temperature are optimal.
Navigate to http://www2.vims.edu/bridge/ and in the Search Box type in Chesapeake Bay.
Have students write a brief essay describing how the concept of ecological forecasting might be applied to human populations, and what limiting factors might be used as the basis for a forecasting model. You may also want to ask them to speculate on what might happen when one or more of these limiting factors are removed.
Visit http://jellieszone.com/ for more information about jellyfishes, including the deadly cubomedusae of Australia, as well as how jellyfish can be kept in aquaria (with a lot of work!).
J.E. Purcell and M.B. Decker. 2002. Effects of climate on relative predation by scyphomedusae and ctenophores on copepods in Chesapeake Bay during 1987-2000. Limnology and Oceanography 50(1):376-387 – The technical article on which this lesson is based; available online at http://onlinelibrary.wiley.com/doi/10.4319/lo.2005.50.1.0376/pdf
http://www.nodc.noaa.gov/dsdt/index.html – Interactive Data Access and Retrieval System Web site, including access to real-time and archival AVHRR data
http://nwis.waterdata.usgs.gov/nwis – U.S. Geological Survey National Water Information System Web site
Answer the following questions after reading “Jellyfish in Chesapeake Bay and Nearby Waters” by Dr. Jennifer E. Purcell:
The mathematical model that is used to make ecological forecasts of sea nettle abundance in Chesapeake Bay was developed from data collected during 1987-2000 on Chrysaora quinquecirrha abundance, temperature, salinity, and depth. Forecasters use satellite data to obtain real-time estimates of sea surface temperature. Another mathematical model that describes water circulation in the Chesapeake Bay is used to estimate salinity distribution in various parts of the Bay. Because salinity is strongly influenced by freshwater input from major rivers, variations in the discharge rates of these rivers correlate closely with salinity changes in the Bay. In this exercise, you will use online databases to obtain information on sea surface temperature and river discharge for comparison with data on abundance in samples collected during summer months from 1987-2000.
Abundance data were obtained by towing plankton nets (1 m diameter, 1.6 mm mesh) equipped with a flowmeter. The volume of water filtered by the nets ranged from 60 m3 to 160 m3. Captured jellyfish were separated and counted by hand. Table 1 summarizes Chrysaora quinquecirrha abundances determined by this method.
for Sample Month
7 Jul, 1987
28 Jul, 1988
28 Jun, 1989
15 Aug, 1990
27 Jul, 1995
20 Jul, 1996
11-13 Jul, 1997
11 Aug, 1998
29 Jun, 1999
28 Jul, 2000
Sea surface temperature (SST) is measured by satellites carrying the Advanced Very High Resolution Radiometer (AVHRR), which measures infrared radiation from Earth’s surface in six spectral ranges. AVHRR data have been provided in Table 1.
The U.S. Geological Survey provides access to water data from approximately 1.5 million sites throughout the United States through the National Water Information System (NWIS). To obtain water discharge data for the Susquehanna River:
Compute the average monthly discharges from January through June for each of the sample years (1987-1990 and 1995-2000). Use these data to fill in the stream-flow discharge column in Table 1. Plot the data in Table 1. You will have to use three different scales for the y-axis since values for abundance, temperature, and river discharge differ by several orders of magnitude. What conditions appear to correlate with higher abundance of Chrysaora quinquecirrha?
by Dr. Jennifer E. Purcell, PhD
Shannon Point Marine Center, Western Washington University
In the summertime in ocean waters, for example, off Ocean City, two types of large jellyfish often occur. The Moon Jelly (Aurelia aurita) is a clear, flattened disk, with numerous small tentacles around the edge, and a pink four-leaf clover design in the middle. This species has a very mild sting and poses no threat to swimmers. The Lion’s Mane jellyfish (Cyanea capillata) is brown in color, and has longer tentacles hanging from the edge of its swimming disk and from the middle. This species has a potent sting, and while it is not dangerous to swimmers, it is very unpleasant to encounter. Some other jellyfish are seen less often in the coastal waters. In the autumn, the Mushroom Cap Jellyfish (Rhopilema verrilli) may enter the Bay. It has a deep swimming bell without tentacles, and is creamy white with darker markings on the sturdy central tentacle structures. The Cabbage Head or Cannonball Jellyfish (Stomolophus meleaqris) is like a white softball, or larger, and has a short, hard tentacle structure in the middle. Neither of these jellyfish sting swimmers. The Portuguese man of war (Physalia physalis) very seldom is seen off beaches north of Cape Hatteras. It is a relative of jellyfish that floats at the water’s surface by means of a gas-filled blue float. Beneath the float are extremely long tentacles which have a powerful sting. They should be avoided by swimmers, because a serious sting would cause extreme pain, and could result in hospitalization. The Moon Jelly also occurs in southern Chesapeake Bay during the summer. The Lion’s Mane, or Winter Jellyfish, is found in Chesapeake Bay during the winter (January-April). The jellyfish for which Chesapeake Bay is widely known in the summer is the Sea Nettle (Chrysaora quinquecirrha). It occurs from Cape Cod south along the U.S. East Coast, Caribbean and Gulf of Mexico, yet it abounds in Chesapeake Bay in numbers unequaled elsewhere. It occurs most abundantly in the tributaries of the middle Bay (salinities 10-20 ppt), where it is white in color. In the southern Bay, it often has red/maroon markings on the long central tentacles and on the swimming bell. It has an annoying sting, but is not dangerous to swimmers.
Lightweight protective clothing, like a lycra “swim skin” or panty hose, or a layer of petroleum jelly spread on unprotected skin, will protect a swimmers against stings. If you are stung by a jellyfish, liberally sprinkling a meat tenderizer or baking soda (or vinegar for Physalia) on the sting may reduce the irritation. Severe allergic reactions (anaphylaxis) are uncommon to jellyfish in U.S. waters, but emergency treatment is essential in such cases.
The swimming jellyfish are either male or female. They produce eggs or sperm, which are shed daily into the water during the summer. Fertilized eggs form larvae that attach to hard surfaces, like oyster shells, and grow into tiny polyps. The bottom-dwelling polyps live through the winter in a dormant state. During May through August, the polyps bud off tiny sea nettles about 1/25 of an inch in diameter, that grow rapidly into the visible jellyfish.
Sea nettles capture prey that contact the tentacles trailing behind the swimming bell. These tentacles have millions of microscopic stinging cells that inject toxins to stun or kill tiny animals, and which are responsible for the stings swimmers feel. The prey are transported up the central tentacles to the heart-shaped gastric pouches in the swimming bell where digestion occurs. Jellyfish are prodigious predators because they swim and feed continuously. They do not have eyes, and so do not need light to feed. They feed without interruption because the many tentacles function independently of the others. The tentacles provide a very large surface area for prey capture. Sea nettles feed mostly on microscopic crustaceans called copepods that are very abundant in Bay waters. They also eat young minnows, bay anchovy eggs, worms, mosquito larvae, and comb jellies, so they would seldom be without something to eat.
Sea nettles are made up mostly of water and salts, with organic materials totaling only about 0.2kg of their entire live weight. For this reason, very little food can provide enough organic materials to result in a lot of growth. The jellyfish can get bigger very rapidly, and the amount of food they catch increases directly with their increasing size. Sea nettles are geared for high reproduction. They begin producing eggs when the swimming bell is only about 1 1/2 inches in diameter. The number of eggs increases tremendously as they grow, with a nettle about 4 inches in diameter shedding about 40,000 eggs into the water daily. The polyps can bud to produce more polyps. Each polyp produces up to 45 jellyfish each summer. The Sea Nettle is unusual in its ability to live in water of low salinity (salt content). Most jellyfish species live at ocean water salinity, about 35 ppt (35 parts salt to 965 parts water). The Sea Nettle prefers waters having as little as 12 ppt salinity, and may have estuaries like Chesapeake Bay, to itself without serious competition from most other jellyfish. In fact, sea nettles eat their most abundant competitors in the Bay, the comb jellies. Populations of plants and animals often are controlled by other organisms that feed on them. However, adult sea nettles may have few natural predators in the middle reaches of Chesapeake Bay. Sea turtles, which are known to eat Portuguese men-of-war and some other jellyfish, rarely come far into the Bay. And fish species (harvestfish, butterfish) observed feeding on sea nettles prefer waters of higher salinity.
It is possible that the changes in Chesapeake Bay caused by human activities have led to larger populations of the Sea Nettle in recent times. The waters of the Bay have been enriched with organic materials and nutrients from waste products and fertilizers due to heavy human settlement around the Bay. This kind of enrichment can change the species of microscopic plants suspended in the waters from large to small types. The small plants are more suitable as food for small grazers (copepods), which in turn are more suitable for predators like jellyfish, and less suitable for visual predators like fishes. Dr. Roger Newell proposed that removal of most of the oysters that were once so abundant in the Bay also may leave much uneaten plant material suspended in the water to be consumed by copepods, which in turn could be used by jellyfish and increase their populations. Unfortunately, no early records document the abundance of the Sea Nettle before Man began to seriously change Chesapeake Bay, so there is little with which to compare the present abundance. Mr. David Cargo kept daily records of the numbers of sea nettles at the Chesapeake Biological Laboratory in Solomons, MD for nearly 30 years. He found great annual variation in jellyfish numbers: more occurred in dry years than in rainy years, but no overall increase in abundance was apparent. Such records would need to go back more than 100 years to reveal if human-caused changes in the Bay have affected the abundance of sea nettles.
A lot of effort was spent on jellyfish control in the 1960s, but no method has been very successful. Nets and bubble screens were used to keep them away from swimming areas. The jellyfish tended to clog the nets and to break into pieces that continued to sting. The bottom-living polyp stage also was targeted. Chemicals that killed the polyps also killed many other organisms, and so were unsuitable. A small species of sea slug was found that ate the polyps, but culture methods to produce large numbers of the sea slugs were unsuccessful, and they did not live well at the low salinities favored by the polyps. So far, the greatest reduction of the jellyfish populations occurred after Hurricane Agnes in 1972, but this method of control cannot be recommended.
Research in my laboratory has shown that sea nettles may help oysters. Oysters have a larval stage that spends about two weeks swimming in the Bay before they settle to grow into large oysters. During this swimming stage, they are vulnerable to predators such as sea nettles and comb jellies. Although sea nettles can catch the larval oysters, they spit them out undigested and unharmed. In contrast, comb jellies catch and digest the larvae readily. Comb jellies are a favorite food of sea nettles, and they reduce comb jelly populations to zero in the tributaries during the summer when oyster larvae are most abundant. Therefore, sea nettles appear to protect oyster larvae from a major predator.
In my laboratory, we have been working to determine the importance of sea nettles as predators of copepods. This is of interest because if sea nettles reduce copepod populations, then less food would be available for planktivorous fish, for example bay anchovy and menhaden, which then are food for piscivorous fish like striped bass and bluefish. We have found that sea nettles eat at most 5% of the copepods daily in mid Chesapeake Bay, which is small compared with the growth rate of the copepod population. In the tributaries, however, Sea Nettles can consume 50-90% of the copepods daily, and copepod abundances seem to decline over the summer, possibly due to this predation. Sea Nettles consume up to 50% daily of the bay anchovy eggs and larvae during the peak spawning in July. This research was in collaboration with Dr. Ed Houde of the Chesapeake Biological Laboratory. We also are studying which environmental factors have the greatest effects on the population size of sea nettles. We have done experiments to measure the numbers of sea nettles produced by polyps, and the survival and growth of the baby jellyfish at various comblnatlons of salinity, temperature, and food concentrations. Few sea nettles are produced at low salinities (< 7 ppt) or at high salinities (> 25 ppt). I am beginning a large-scale project to map the distributions of jellies throughout Chesapeake Bay in April, July, and October. In collaboration with several other scientists, I will examine environmental and biological (e.g. food) factors that may determine the distribution and abundance of jellies in the Bay.