T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 1

Name: Blooms Unit (3 pts) Section:

Blooms

A bloom is the rapid increase in the population of an organism (a “population explosion”). Blooms occur when something needed for growth and reproduction, something that was holding back or “limiting” the size of the population, becomes more abundant. Phytoplankton blooms typically occur when the amount of sunlight or nutrients increases. (Sunlight and nutrients are the two things that phytoplankton need for photosynthesis that there may not be enough of in ocean water.) A bloom will stop and the population will shrink when the resource runs out or goes away.

Zooplankton bloom when phytoplankton bloom, because more phytoplankton means more food for them. However, most animals cannot reproduce as fast as phytoplankton, so their population grows more slowly and is not large enough to prevent the growth of the phytoplankton population (until the phytoplankton bloom begins to slow down on its own).

Humans can cause blooms of phytoplankton by adding lots of additional nutrients to ocean water. (We cannot affect the amount of sunlight, can we?) Typically, humans add nutrients by dumping untreated sewage or when rain washes fertilizers and animal wastes off farmland and into rivers. (Animal wastes are the feces or manure from cows, pigs, and so on.) In developed countries like the United States, sewage is treated to a very high standard and accidental spills of untreated or partially treated sewage are rare, so in the United States farming is the primary human activity that causes or contributes to blooms of algae (phytoplankton) in lakes and the ocean.

1. What is a bloom?

2. How do humans cause blooms of phytoplankton? In other words, what do humans add to ocean water that cause phytoplankton to bloom?

3. Why do the things that humans add to ocean water cause phytoplankton to bloom? In other words, how or why do they benefit phytoplankton? T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 2

Storm Drains and Sewage

Storm drains carry anything on the surface of the land into rivers, lakes, and the ocean when it rains. Yard waste and animal feces in storm drain runoff are sources of nutrients. However, it takes a while for them to be decomposed by bacteria in the ocean. The nutrients are released slowly, a little bit at a time. Thus, storm drain runoff from a city typically does NOT have enough nutrients in it to cause a harmful bloom of phytoplankton. It is the bacteria that grow on the fecal matter that typically cause the worst problems: the bacteria can make beachgoers sick. The ocean helps the bacteria get into our bodies through our eyes, ears, mouth, and so on. The water offers support to the bacteria so that they can drift to us. Government authorities will shut down a beach (close the beach) when too many bacteria are found in the water. (When a beach is closed, signs are put up telling people that they should not get in the water, that getting in the water is prohibited.)

Note that it is bacteria from land animals that typically make human swimmers sick. Most ocean bacteria are NOT well adapted to invade the bodies of land mammals like humans.

Sewage comes from all the stuff that goes down the drains in your house: the kitchen sink, toilets, bathroom sinks, the bathtub, the dishwasher, the washing machine, and so on. (Sewage is also called wastewater.) Sewage is a soup of dead, decaying material, wastes, and chemicals that flows out of our homes and other buildings. Before sewage is released into the ocean or other bodies of water, the government mandates that it must be treated. (It must be cleaned.) This treatment typically focuses on killing disease-causing bacteria and removing excess nutrients. Nowadays, sewage is treated to a very high standard in developed countries like the United States. The treated wastewater is practically drinkable.

Thus, both rainwater runoff from storm drains and water coming out of sewage treatment plants are NOT typically the causes of harmful blooms of phytoplankton in the United States, certainly not the biggest and longest lasting ones. In the United States and other developed countries like T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 3

Great Britain, France, Germany, Japan, Australia, and so on, it is the fertilizers put on the fields of farms that cause the worst harmful blooms of phytoplankton. (Note: Farmers put lots of fertilizers on their fields to grow the food we eat. Using lots of fertilizers helps them grow more food per square foot of field, which makes food cheaper for us to buy.)

There are so many humans doing so much farming (and other activities, like releasing untreated sewage in some parts of the world) that we are now responsible for a significant fraction of the nutrients that enter the ocean. This is a somewhat recent development. Plenty of nutrients were entering the ocean naturally before we existed. Earlier we learned that rain water washes nutrients naturally found in soil on land into rivers, lakes, and the ocean, and this is still true. (I think it would be fair to say that a lot of the nutrients entering the ocean each year are due to human activities, but you should NOT simply say that nutrients enter the ocean due to human activities. Natural sources remain important.) I think it is fair to say that humans ARE responsible for unnaturally large amounts of nutrients entering the ocean in some places and at some times. It is unnaturally large amounts of nutrients that are likely to result in harmful blooms.

4. True or false? “Storm drain runoff is a common cause of harmful blooms of algae.”

5. How is sewage treated before it is released into the ocean? In other words, what is removed from the wastewater?

Harmful Blooms of Phytoplankton

You might expect that causing blooms is a good thing: more nutrients means more phytoplankton and thus ultimately more animals for us to eat. However, unnaturally large numbers of phytoplankton can cause big problems. We will examine two major examples: dead zones (regions of water with low levels of oxygen) and toxic algal blooms (phytoplankton who make poisonous substances). Our example of a toxic algal bloom is red tides. T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 4

Red Tides: An example of a harmful bloom

Red tides are a bloom of red-colored phytoplankton (typically dinoflagellates); there are so many phytoplankton in the water that they make the water look red (or brown or yellowish). Some red tide algae – but not all! – produce chemicals like domoic acid that are toxic to vertebrates like birds, mammals, and humans. Filter feeders like clams and other shellfish strain the algae out of the water during a bloom. They eat lots of algae, and the more they eat, the more toxic they become. (This is called bioaccumulation.) If we or other vertebrates (like seals and sea gulls) eat the shellfish, we can become sick. Illnesses caused by toxins being passed up the food chain in this way include ciguatera fish poisoning (CFP), diarrhetic shellfish poisoning (DSP), and paralytic shellfish poisoning (PSP).

In general, it is safe to swim in red tides; you won’t swallow nearly enough water to become poisoned. However, the algae do leak toxins into the water, and these cause some people to develop rashes. As long as you get out of the water when you notice the rash and wash it off, you should be fine. ) Breaking waves can catapult the toxins into the air, and some of these toxins cause respiratory (breathing) problems for people who live near the coast. (This discussion is NOT true for all blooms of algae, especially those in lakes. It can be extremely dangerous to drink or even touch water in a lake filled with toxic algae.)

Note that the phytoplankton are NOT poisonous because of toxic chemicals absorbed from the water. Like many land plants, the red tide phytoplankton naturally produce chemicals that are harmful to humans.

The most dangerous animals to eat after a bloom of red tide phytoplankton are filter-feeders like clams, mussels, oysters and so on. When people see a bunch of dead fish floating in the water, they are usually smart enough to guess that something might be wrong with the fish. (They do not say to themselves “Yum, a free lunch.”) However, the toxins in red tide phytoplankton do not kill clams, mussels, oysters, and other invertebrates. Toxins build up more and more in the bodies of shellfish like clams, mussels, and oysters as they eat phytoplankton. The more phytoplankton they eat, the more toxins are entering their bodies.

It should be safe to eat clams, mussels, oysters, and other shellfish in restaurants, because the United States has good regulations and does a good job of inspecting seafood. Restaurants will import shellfish if the local ones may be poisonous. T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 5

6. What are red tides? In other words, why does the water become “red”?

7. How can red tides harm (but do not kill) humans?

8. How can red tides kill humans?

9. In general, is it safe to swim in a red tide? Why or why not?

10. In general, is it safe to eat in local restaurants when there is a toxic algae bloom in the nearby ocean? Why or why not? T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 6

“Dead Zones:” An example of a harmful bloom

Another example of a harmful algal bloom is a dead zone. Dead zones occur at the end of a bloom. A bloom typically ends when there are few nutrients left in the water or it becomes too hard to get sunlight.

During a bloom, phytoplankton reproduce more and more, and each offspring absorbs nutrients from the water. Each newly “born” phytoplankton needs enough nutrients to make chlorophyll and other molecules, so that they can carry out photosynthesis and make food for themselves. At some point, there will be few nutrients left in the water, and when phytoplankton reproduce, their offspring will not be able to find enough nutrients in the water. In other words, phytoplankton that are “born” will die, because they cannot get enough nutrients so that they can make food for themselves.

In addition, a bloom makes the murky. All the other phytoplankton in the water block sunlight from reaching the water below them. Thus, it becomes harder and harder to get enough sunlight and make enough food to survive.

Eventually the water is filled with a lot of dead phytoplankton. Phytoplankton die of old age (they only live for days, perhaps a week) or from being unable to get enough nutrients and sunlight. The algae lose their green color as their bodies break down, making the water look brown instead of green.

All the dead phytoplankton bodies in the water encourage the growth of bacteria who get the energy they need to live by decomposing (“breaking down”) the dead bodies and wastes of ocean life. However, bacteria use oxygen in the water to decompose the dead bodies (just as we use oxygen to breakdown food molecules in our bodies).

Animals who live in the areas of the bloom, because they cannot breathe. Of course, some animals can swim away. However, many animals like clams and crabs cannot swim away. Some animals who can swim do not leave, presumably because their instincts tell them that they cannot survive in the open ocean away from the coast. They will not find enough food or will get eaten. (Which would you choose, to be eaten or to suffocate?)

One of the best known examples of a dead zone is the one that occurs at the end of the Mississippi River. The Mississippi drains practically the entire middle of the United States, including a vast amount of farmland. Some of the fertilizers that the farmers put on their land to help them grow more food per acre and animal wastes from huge “factory farms” are washed off the land and into local rivers. These rivers flow into the Mississippi River, which carries nutrient-rich water into the ocean. The resulting dead zone harms the livelihoods of the local fishers and the tourism industry. People stay away from beaches with yucky-looking and smelly water, so they do not patronize coastal businesses like hotels and restaurants and go somewhere T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 7

else instead. Fish, shrimp, clams, oysters, and other seafood die from a lack of oxygen, reducing the food supply. Fishers need to go farther out in the ocean in search of animals to catch, costing them time and money for fuel. A harmful algal bloom can easily cost local communities tens of millions of dollars in lost fisheries and tourism revenues, and many such blooms happen each year in the United States.

“Dead zones” were common in the Port of Los Angeles in the 1950s. The Clean Water Act limited the amount of pollution that could be dumped in the harbor, and since the mid-1980s there have not been any more dead zones.

Scientists use the word eutrophication to describe the dead zone phenomenon. Water with low levels of oxygen in it is called hypoxic. The word anoxic (“no oxygen”) is used to describe a lack of oxygen in the water, levels that are so low that animals cannot breathe and will die.

11. Why do animals die in an ocean “dead zone”? What do they die of? In other words, what kills them?

12. How or why can a bloom of phytoplankton create a “dead zone”?

13. What do humans add to the ocean that causes harmful blooms of algae?

14. How or why does this substance cause algae to bloom?

Wetlands and Harmful Blooms of Algae

The plants of wetlands get in the way of rainwater runoff, slowing down the rainwater washing off the land. This gives the plants and algae of wetlands more time to absorb nutrients from rainwater before it flows into rivers, lakes, and the ocean. Wetlands are a natural and effective way of preventing too many nutrients from entering bodies of water and causing harmful blooms of algae.

Wetlands also prevent bacteria and viruses that can make us sick from being washed off the land into lakes and the ocean. “Super” filter feeders like clams, mussels, and oysters often live in or next to wetlands, and eat bacteria and viruses as well as plankton when they suck in water. In addition, the water flowing through wetlands is no longer moving fast enough to carry many sediments, so many sediments sink and trap water, nutrients, bacteria, viruses, and other things in the water in the mud on the bottom of wetlands. T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 8

In the past, we had more wetlands along the sides of rivers and along the coast next to the ocean. However, many wetlands were destroyed in the United States. They were converted into farmland or filled in to make way for homes and businesses. Wetlands continue to be destroyed in many places around the world for the same reasons.

15. What coastal environment absorbs and prevents excess nutrients washed off the land from causing harmful blooms of algae?

Other Kinds of Harmful Blooms

While we have focused on two major kinds of harmful blooms (dead zones and red tides), I would like you to be aware that other kinds exist too. For example, adding lots of nutrients to ocean water can also encourage the growth of harmful microorganisms. Some attack fish and other organisms directly, causing wounds (like Pfisteria piscicida). Some inhibit feeding or breathing (produce lots of mucus and clog gills, like species of Chaetoceros).

Even unnaturally large blooms of phytoplankton that do not result in dead zones can cause problems. Many ocean animals begin life as zooplankton that feed on phytoplankton. When they grow up, they become nekton or benthos that eat other things like seaweed, zooplankton, or corals. If there is an unnaturally large bloom of phytoplankton, fewer animals will die from a lack of food as kids. In other words, more of these animals will survive and become adults that feed on something different. Since there are more adults than normal, the animals eat more food than normal. These adults can disrupt their ecosystems by eating too much. For example, they might eat up all the kelp in a kelp forest or all the corals in a coral reef. This eventually harms them too, because they will run out of food and starve.

Not all harmful blooms are caused by human activities

Blooms can happen naturally, including harmful ones that result in dead zones and toxic red tides. For example, brief, strong winds along the coast of Oregon often enhance upwelling, leading to a short, enormous bloom of phytoplankton that ends in a dead zone.

16. Are harmful blooms like red tides and dead zones caused by humans, do they occur naturally, or are some caused by humans and some natural events? T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 9

Seasonal Blooms in the Open Ocean (Far from Land)

Phytoplankton are more abundant during some seasons than others, depending upon the availability of sunlight and nutrients. The pattern varies a lot from place to place, especially along coasts, but there is a consistent pattern that oceanographers have found in the open ocean, far from land.

I will now discuss the seasonal bloom cycles in the open ocean, far from land. It can help us understand the natural causes of blooms in the ocean.

As winter ends, the number of phytoplankton increase in the mid-latitudes and high latitudes (closer to the Poles). Numbers increase throughout the spring, but go back down during the summer in the mid-latitudes. At higher latitudes closer to the Poles, the numbers keep going up during the summer, reaching enormous levels. Then, during fall phytoplankton populations start going down at the Poles. In the mid-latitudes, though, phytoplankton numbers go up a little, but then go back down as winter arrives.

Notice that I have not discussed the low latitudes, the tropics. I have not discussed the tropics, because there is no bloom cycle in the tropics. The phytoplankton population is fairly steady. It does not go up or down very much. In addition, the phytoplankton population is smaller than at the other latitudes. In other words, the open ocean in the tropics has little life compared to other parts of the ocean. You might say that the tropical open ocean is an ocean desert.

Next, let us turn our attention to explaining the observed bloom patterns that I just described. T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 10

To understand the seasonal bloom cycles at each latitude, pay attention to two key factors during each season: (1) the amount of sunlight available during each season, and (2) the temperature of the water at the surface. If the water gets too warm, waves cannot bring up many nutrients from deeper in the ocean. (Waves bring up nutrients from the thermocline, the layer below the surface mixed layer.)

In the mid-latitudes (the temperate zone) far from land, phytoplankton bloom during the spring and fall. I think that it is easiest to understand why they bloom during these two seasons if you first understand why phytoplankton do NOT bloom during the summer and the winter. During the winter, there is less sunlight because the days are shorter and the Sun does not get as high in the sky: the sunlight hits the surface at an angle, so more reflects off the surface, and the sunlight entering the water does not go straight down, so it does not penetrate as far. Since there is less sunlight available to make food, the phytoplankton population drops. During the summer, there is plenty of sunlight, but the surface water becomes too warm: if the surface water is too warm, waves have difficulty stirring up nutrient-rich water from the thermocline (the layer of the ocean that is below the surface layer). (The deeper water is cold and dense, so when the waves bring the water up, it immediately tries sinks back down.) Of course, warm water has a lower density too, so phytoplankton sink more easily in warm water: the more they sink, the harder it is for them to get sunlight. In summary, during the winter there is not enough sunlight to support a large population of phytoplankton, and during the summer waves cannot bring up enough nutrients.

At the beginning of the spring, sunlight becomes more abundant, but surface water is still quite cold, so waves can easily stir up nutrients from below (from the thermocline). The bloom ends when the surface water becomes too warm, making it difficult for waves to bring up more nutrients. During the fall, sunlight is becoming less abundant (which is bad for phytoplankton), but the real problem is a lack of nutrients, because at first the water is still too warm for waves to bring up many nutrients from below. Once the surface water finally cools down enough, the surface water plunges downwards, and water from below comes up to replace it. This happens again and again, cooling the surface layer of the ocean and bringing up nutrients from below. The cooler surface layer makes it easier for waves to bring up nutrients as well. Thus, the fall bloom eventually occurs when the ocean surface cools down enough for nutrients to be stirred up by the waves and sinking water, but there is still enough light for them to carry out lots of photosynthesis.

The spring bloom is larger than the fall bloom due to water’s high heat capacity. It takes the surface water a long time to warm up in the spring, so the water remains relatively cool for much of the spring. Due to water’s high heat capacity it takes the surface water a long time to cool down in the fall, and thus by the time the water has cooled down enough for waves to stir up nutrients from below, there is a lot less sunlight than at the beginning of the fall. T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 11

There is never any bloom in the tropics far from land. As we discussed earlier, the tropical ocean has less life than any other part of the ocean. This is why the water of the tropics is “crystal clear” and “blue:” it is sterile, dead. Green murky water is full of algae and life. Even though sunlight is always abundant in the tropics and the water is clear (so sunlight penetrates well), the surface water is too warm, making it difficult for waves to bring up many nutrients from the thermocline layer below.

The largest bloom in the open ocean, far from land, occurs at the Poles during the summer. Sunlight is abundant. Near the summer solstice, the Sun dips down in the sky but never sets, providing 24 hours of sunlight each day. The water starts out very cold at the beginning of the spring. It takes a lot of heat to melt the ice (due to water’s high latent heat) and it takes a lot of heat to warm up the water (due to water’s high heat capacity), so the surface water never really reaches a temperature that you or I might describe as warm. Thus, waves can always bring up nutrients from below, because water at the surface remains fairly cold, even in the summer.

At the Poles there is one more interesting and important component of the ecosystem: tiny algae that grow on the bottom of the ice. If the ice is not too thick, the algae can get sunlight that goes through the ice. These algae are an important source of food in addition to phytoplankton.

17. (a) When do phytoplankton bloom in the mid-latitudes, in the spring, summer, fall, or winter?

(b) When is the largest bloom in the mid-latitudes, in the spring, summer, fall, or winter?

18. Why do phytoplankton bloom in these seasons and not during the other seasons?

19. When do phytoplankton bloom in the tropics?

20. Why don’t phytoplankton bloom in the tropics?

T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 12

21. When do phytoplankton bloom in the polar oceans, in the spring, summer, fall, or winter?

22. Why do phytoplankton bloom during this season and not during the other seasons?

23. Where is the largest phytoplankton bloom, in the tropics, in the mid-latitudes, or near the Poles?

Stratification and the Seasonal Bloom Cycle

We say that the ocean has become stratified when there are layers of water with very different densities. For example, a layer of warm water might float on top of a layer of cold water, or a layer of fresh water might float on top of a layer of salty water. The lower density water floats on top of the higher density water. The water in one layer does not want to rise up into another layer or sink down into another layer due to its density. If the water in a layer is disturbed by waves, currents, or other phenomena, the water will try to go back below the lower density water and above the high density water as soon as it can, as soon as the waves, currents, or other things stop pushing it. Stratification makes it hard for water in different layers to mix together. For example, in my explanation of the season cycle of blooms, stratification prevents waves from bringing up nutrients from below when the surface layer became too warm.

Examples of stratification (layers of water with different densities)

As is so often the case in this introductory class, my previous explanation for why the warming of the surface of the ocean prevents the growth of phytoplankton is an oversimplification. When the surface water is cold enough, waves not only bring nutrients up but also bring phytoplankton down. This is bad for the phytoplankton, because going downwards carries them away from the Sun. This mixing is inhibited when the surface water warms up: the low-density warm water at the surface does not want to go down into the cold water, and the high-density cold water does not want to come up into the warm water. So, when the surface of the ocean warms, waves no longer bring phytoplankton down and thus the phytoplankton float on top of the cold water just T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 13

below the surface layer. This helps them get sunlight. However, it also means that nutrients cannot be replenished from below. By summertime in the mid-latitudes, the phytoplankton have used up all the nutrients, but they still are able to get a decent amount of sunlight by floating on top of the cold layer beneath the warm surface layer.

Phytoplankton can stay closer to the Sun if there is a thin layer of warm water on top of a cold layer, because the difference in density between the warm and cold layers makes it difficult for waves to make water move up or down. If the surface layer is thick and cold, waves move phytoplankton up and down easily, which can carry them away from the Sun.

So, the warming of the surface water can actually help phytoplankton get more sunlight, because phytoplankton can float on the cold water beneath the not-very-deep warm surface layer and get lots of sunlight. However, as I emphasized earlier, the warming of the surface layer also prevents nutrients from coming up from below and reaching the phytoplankton.

Stratification can make dead zones occur in the deeper layer or last longer in the deeper layer. Oxygen is made in the surface layer by phytoplankton. Dead bodies tend to sink downwards and decompose in the deeper layer, so oxygen is removed mostly from the deeper layer. Waves and other mixing processes cannot mix much oxygen from the surface layer down into the deeper layer if there is a big difference in density between the layers. If oxygen is removed from the deeper layer and oxygen cannot be brought down from the surface, a dead zone may occur in the deeper layer. And if a dead zone happens in both layers, the surface layer will recover first as phytoplankton begin to grow again and make oxygen at the surface. However, the surface layer cannot give any of this oxygen to the deeper layer if waves and other mixing mechanisms cannot move much water upwards and downwards due to the difference in density between the layers. Thus, oxygen levels remain low in the deeper layer for much longer than the surface layer. T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 14

Animals and the Seasonal Blooms

Animals are well aware of bloom cycles, and their behavior has evolved to take advantage of them. For example, many zooplankton reproduce when phytoplankton begin to bloom, so that their offspring (children) will have lots of food to eat and thus a better chance of surviving and reproducing themselves. In fact, one thing I probably should have mentioned when explaining the open-ocean blooms of phytoplankton: As the zooplankton population grows and grows and they eat more and more phytoplankton, they probably help end the phytoplankton blooms and bring down the phytoplankton population much faster than a lack of sunlight or nutrients would on their own.

Many animals (and phytoplankton) use the temperature of the water as a trigger. When the water reaches a certain temperature, they reproduce. The water temperature represents sunlight: water warms during the spring when the days get longer and the water receives more sunlight. You can see how global warming might mess with this system, warming the water and making it appear like spring too early in the year. Phytoplankton might bloom too early, before enough nutrients are available, and waste their time and energy. Zooplankton babies might hatch before phytoplankton become abundant. Most would starve before the phytoplankton bloom actually happens. Of course, losing most of the next generation would reduce the population of zooplankton and thus food available for small fish and other animals that eat zooplankton.

Larger animals also take advantage of blooms. They migrate vast distances to be at the right place and the right time to feed on zooplankton who bloom when the phytoplankton bloom. (The large animals may also eat small fish who are attracted to the bloom to feed on zooplankton.) For example, gray whales regularly migrate up and down our coast. They head up to Alaska for the summer when the most food is available. They need to eat a lot to build up their blubber (their food reserves), since they feed less frequently during the rest of the year. Then, the whales migrate south, and go past us in the fall so that they can spend winter in the warm, shallow lagoons (estuaries) of Mexico where the females give birth to their calves. In the spring, they migrate north Grey whale migration route. and go past us on the way to Alaska. Grey whales feed during the winter near Alaska, and spend summers in the warm lagoons (estuaries) of Mexico.

T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 15

There are several explanations for why the whales leave Alaska and go down to Mexico instead of living in Alaska all year long. Baby whales don’t have much blubber, so the water may be too cold for them during the winter. In addition, orcas (killer whales) are more abundant farther north (where most of the food is, as we have discussed). Orca’s kill baby gray whales by holding them underwater long enough for them to drown. This would be difficult in the shallow waters of Mexican lagoons. So, gray whales may migrate at least in part to protect their young.

24. When are gray whales found near Alaska, during the spring, summer, fall, or winter?

25. When will gray whales be migrating along the coast of southern California, during the spring summer, fall, or winter?

26. Why do they want to be near Alaska during this season?

T. James Noyes, El Camino College Blooms Unit (Topic 10A-2) – page 16

This page was intentionally left blank.