The last time marine life was subjected to seas as corrosive as those anticipated by 2100 was in a period geologists call the Paleocene-Eocene Thermal Maximum, fifty-five million years ago. Sea-surface temperatures spiked around nine degrees Fahrenheit to sixteen degrees Fahrenheit higher than today’s due to a massive release of methane, and possibly of carbon dioxide.16 We aren’t sure exactly where these gases came from or why they were released. Theories include volcanic eruptions, offgassing of peat, or a massive belch of methane hydrates from the seafloor. A methane belch seems the most likely explanation, given the chemical signature of sediments laid down at the time. Methane would have been rapidly oxidized to form carbon dioxide, which in turn acidified the seas. The amount of carbon released at that time is about the same as that which would come from burning all of the world’s fossil fuel reserves today, but it took place more slowly—over a few thousand years rather than the few hundred years in which we are doing it today.17 You can see this event written in sediment cores drilled from the deep sea. Chalky sediments give way suddenly to a brown layer of clay, which almost as quickly returns to chalk. The whole event is over in just four inches to eight inches of core, corresponding to the hundred thousand years or so it took the Earth to swallow back the excess carbon it had released.18
The Paleocene-Eocene ocean acidification spike does not seem to have led to any great cataclysm for marine life. Looked at against the sweep of geological time, it might be reported as “Minor disaster strikes: not many dead.” Yes, the few coral reefs around seem to have suffered badly,19 but the only outright extinctions were of deep-sea foraminifera. These tiny single-celled organisms—neither animal nor plant—succumbed when corrosive water swept over them as the oceans acidified.20 All the bottom-living foraminifera went extinct, while those from surface waters survived.21 Nonetheless, conditions for their existence and that of other calcareous plankton became harsh, and sediments from the time are full of stunted and malformed shells. If any creatures are capable of adapting to higher acidity, we would expect to find them among single-celled plankton, for whom a generation can be measured in hours or days. They do seem to have toughed it out pretty well. Although there were major changes in their composition, there was no apparent shutdown in the construction of chalky body structures.22 Some species fared well under conditions of higher acidity. We can take some comfort from this but have to remember that the rate at which we are adding carbon dioxide to the sea is much faster today, so the impacts will likely be far more severe. Carbon dioxide will accumulate in the surface layers of the ocean that are home to most marine life long before it is mixed into the deep seas. It takes a thousand years or more to flush carbon dioxide into the deep sea, where it is eventually neutralized by dissolving chalky sediments.
Coral reefs, like polar bears, have come to symbolize the perils of carbon dioxide emissions and climate change. They are sentinel habitats that forewarn of dangers ahead. If they perish, millions of people will be left homeless. But I have another concern that gets to the very heart of the planetary life-support system: What will acidification do to phytoplankton? Phytoplankton, or plant plankton, produce roughly half of the oxygen that we breathe and are the foundation of nearly all ocean food webs. Life itself depends on them. We depend on them.23
Many phytoplankton, such as coccolithophores, produce chalky skeletons. These ball-shaped microscopic plants armor-plate their bodies with exquisitely ornamented disks of chalk that overlap like porcelain shields. Spring blooms form when the days lengthen, fed by nutrients stirred from deeper water by winter storms. Countless trillions of coccolithophores then turn the sea a milky white, producing swirls that can be seen from space. A single bloom can produce a million tons of calcium carbonate in just a few weeks.24 These blooms do their bit for our climate by reflecting heat back into space from the oceans. The effect of acidification on coccolithophores and other photosynthetic plankton is not straightforward. Carbon dioxide boosts photosynthesis, just as it does with land plants. In experiments at higher levels of ocean acidity many coccolithophores seem to produce just as much carbonate, or sometimes more than under normal conditions.25 Another plus is that not all phytoplankton produce calcium carbonate skeletons. Perhaps diatoms, with their silica shells (the basic ingredient of glass), will take up any slack caused if carbonate-producing phytoplankton struggle—or cyanobacteria, or dinoflagellates. We had better hope so.
Coccolithophore blooms are often stopped in their tracks by follow-on explosions of viruses that eat them. Viruses are critical to nutrient cycling and phytoplankton production in the sea, liberating nutrients to be used again by other plants and animals. Probably the most astonishing fact I have ever heard about the sea concerned viruses. A two-pint bottle of crystal clear seawater contains some four billion viruses.26 Added together there are something like 4,000,000,000,000,000,000,000,000,000,000 viruses in the sea (4 nonillion, if you want an easier way to say it). They outnumber all other marine life-forms combined by 15 to 1. To get an idea of what that enormous number means, imagine placing all the viruses end to end. They would form a thread less than one two-hundredth the thickness of the finest spider gossamer that would stretch for two hundred million light years. You read that right: The thread would reach so far across the universe it would pass by sixty galaxies and countless millions of stars. The microbial world of our ancient seas is still alive and well. In all the research I have read on ocean acidification I haven’t come across a single study of the effects it might have on viruses. As it happens, viruses have slowed the rate at which the oceans are acidifying. Because they are so good at recycling nutrients in sunlit surface waters, less carbon is exported to the deep sea via the sinking bodies of animals and plants. This means less carbon dioxide is sucked from the atmosphere by the ocean; good for acidification, bad for climate change.
Most scientists’ attention to date has concentrated on what will happen to the production of chalky carbonate body parts in more acid seas. But pH change in itself can alter the availability of micronutrients that phytoplankton need to thrive. Iron is a key nutrient that limited phytoplankton productivity in ancient oceans and still does throughout much of the high seas. Iron is less willing to combine with organic matter under more acid conditions, and will therefore be less available to organisms that need it.27 Ocean acidification could expand areas of the sea where iron limits productivity, which would ultimately reduce the amount of food the sea could produce for us.
The more we look at the effects of acidification, the more ways we find in which it will disrupt life at sea. Dozens of experiments on captive invertebrates show increased acidity affects almost every aspect of life, from growth and behavior to reproduction. One surprise is that predator avoidance in larval fish can turn into fatal attraction.28
The orange clownfish lives among the stinging tentacles of large sea anemones on coral reefs. Clownfish hide their eggs under the anemones’ frilled edge to keep them safe. After a few days, newly hatched larvae head for the open sea. Weeks later they return under cover of darkness to find an anemone of their own. The most dangerous moment of their lives comes as they thread their way through a thicket of predatory mouths in search of a new anemone. Larval clownfish can smell their predators, which helps them avoid being eaten. But clownfish larvae raised under levels of ocean acidity that will be common in 2100—if we fail to curtail carbon dioxide emissions—lose this ability, and instead they are attracted to their predators. Other experiments have shown that they also lose the ability to smell their anemone hosts.29 Perhaps they will be able to spot the anemones when dawn arrives, but by then they will be seen by a bevy of other predators that take over from the nightshift.
Unless orange clownfish can adapt quickly, acidification will likely bring their time on this planet to a close. We can only guess at how common such responses are among the many hundreds of thousands of other species with planktonic larvae. These larvae may be more sensitive to acidification than adults, which have been the
subjects of most studies to date. What we do know is that the use of smell to detect habitats and predators is widespread among marine life. Once again Ischia offers a glimpse of what the future may hold. Hardly any larvae of species with chalky skeletons set up home or survived near the underwater carbon dioxide vents.30 Experiments on sea urchins have shown reduced sperm vitality and fertilization at acidity levels that could be reached by century’s end.
Oyster farms on the Oregon coast give one indication of how hard life will get.31 Oysters there periodically experience a complete failure of reproduction linked to more acid water. I described previously how upwelling recently flooded the continental shelf with deep water so low in oxygen it caused mass mortality of animal life. Upwelled waters are also more acidic than surface water. When farms were flushed by these more acid waters, larval oysters died at the point when they had to form their first shells.
Overfishing has greatly increased our reliance on shellfish from the wild as well as from farms. Prawns, lobsters, clams, and scallops are relatively more resilient to overfishing than fish and have come to dominate intensively fished seas. All lay down carbonate shells. In 2006, half the value of U.S. fisheries came from shellfish. The fishing industry is badly exposed to risk from more acid seas. Not only that, acidification threatens the important role that filter-feeding shellfish play in cleansing ocean waters.
Ocean acidification affects a fundamental aspect of almost all existence: the ability of marine life to process oxygen. When carbon dioxide levels increase, it becomes harder to extract oxygen from water. Dissolved carbon dioxide quickly diffuses across gills and into cells, reducing the amount of oxygen in circulation and suppressing metabolism. Animals stressed in this way expend more energy to maintain basic bodily functions, which means they can’t afford to give as much to growth and reproduction. More acidic oceans may increase the severity of problems resulting from low oxygen, or anoxia, something I will return to in the next chapter.
Coral bleaching used to be what kept me awake at night; now it is ocean acidification. Warming seas have devastated reefs worldwide by weakening the sunlit coalition of corals and algae. Acidification is a punch in the gut to reefs that are already on their knees. It is chilling to think that within the space of one hundred years humanity could reverse a process of coral reef formation that has flourished since the end of the last ice age.
More acid seas will not be devoid of life; there will be winners as well as losers. But life will change. The seabed around Ischia’s champagne hotsprings is covered by lush stands of sea grass, their vivid green leaves unsullied by the chalky crusts and tubes of coralline algae and worms that cover plants farther away. Brown and red seaweeds elbow sea grasses aside wherever rocks punctuate the sand. These plants benefit from the fertilizing effect of carbon dioxide and the lack of sea urchins to eat them. Good news, at least, for the Mediterranean’s hard-pressed green turtles. They love sea grass.
CHAPTER 8
Dead Zones and the World’s Great Rivers
British parliamentarians enjoy a long summer break that lasts from July to October. What could be so important to draw them away from the business of running the country for so long? The Houses of Parliament squat in gothic splendor on the bank of the River Thames, and it is to this river that we must look for the answer. Like many cities, London grew organically over the centuries, with little or no central planning. By 1815 it was the largest city in the world, inhabited by 1.4 million people, but it was a modern city with a decidedly medieval approach to sewage and refuse, much of which was dumped in the street or in cesspits. Ironically, it was an invention intended to improve hygiene, the flush toilet,1 that helped create the problems in the Thames for which ninteenth-century London became notorious, and which culminated in the Great Stink of 1858.
In the late eighteenth century, flush toilets began to replace chamber pots. They greatly increased the volume of waste discharged into cesspits, which regularly overflowed into the street. A law was passed in 1815 to remedy the problem by allowing the discharge of sewage into the Thames through a sewer network then under construction. By mid-century the Thames had become choked with sewage and refuse that sloshed back and forth with every tide past the houses of Parliament. As bacteria went to work on the sludge they sucked up dissolved oxygen and turned the river into a stinking sewer that belched poisonous gases into the heart of London. In 1858 the stench became so bad that the windows of the Houses of Parliament had to be hung with sheets soaked in bleach to drive off the smell. Eventually summer sittings of Parliament were abandoned.
London’s problems (mercifully since solved) have been repeated across the world and continue to plague most cities in developing countries today. Many of the world’s most populous cities straddle mighty rivers on which people depend to bring water and carry away waste. By the time these rivers reach the sea they are loaded with sewage, toxins, and trash from cities, as well as industrial and agricultural runoff.2 The problem increased sharply after World War II, when farms became dependent on chemical fertilizers to boost yields. Fertilizers now fuel coastal plankton blooms that can cover thousands of square miles and are often visible from space. Satellite photographs of the Gulf of California in Mexico show the telltale green of phytoplankton growth around the mouth of the Yaqui River soon after coffee growers spray their crops with fertilizer.3
Plankton blooms are a natural seasonal feature in spring and early summer, when the days lengthen, and must have occurred ever since the seas were first populated with microbes more than two billion years ago. They are mostly seen in temperate and polar seas, where nutrients are stirred from the bottom by winter storms, and at the mouths of large rivers. When plankton die their decay takes oxygen from the water. As a result, beneath pollution-fueled plankton blooms the water often becomes anoxic, spreading a shroud of death. Dead zones are now permanent or seasonal features at over four hundred places worldwide, all of them coastal or in enclosed seas, like the Baltic.4 At the last count, in 2008, collectively they covered a hundred thousand square miles, or about 1 percent of the area of the world’s continental shelves. This may not sound like much, but these are some of the most productive and biologically rich areas of the sea. We can ill afford their loss.
One of the world’s largest dead zones forms every year off the mouth of the Mississippi delta in the Gulf of Mexico.5 The Mississippi River catches the runoff from 40 percent of the landmass of the lower forty-eight United States and discharges an average of 140 cubic miles of water into the Gulf of Mexico every year, enough to fill Lake Mead, the largest reservoir in the United States, sixteen times over. Its water is loaded with particles of organic matter and dissolved nutrients that plankton need to thrive. In the past, this annual nutrient bonanza sustained rich fisheries around the delta, but in recent years nutrient overload has tipped the balance from benefit to blight.
The dead zone begins to form in the spring, after winter snow melt and rains swell the mighty Mississippi and flush its nutrient-rich water into the Gulf of Mexico. When stultifying summer heat anaesthetizes human life in Louisiana, Alabama, and Tennessee, the plankton begins to decay and oxygen levels plummet. Those that can flee do so, as a suffocating blanket engulfs the ocean. Many don’t make it, and the corpses of fish, crabs, and shrimp litter the seabed. Others, like clams, snails, worms, and starfish that cannot leave, die where they are. As the shroud of death expands, fishermen are forced to travel further offshore to make a catch. They pass through waters that are eerily empty—no leaping fish, nothing showing on their fish finders, nothing to interest the birds that pass overhead. Researchers like Melissa Baustain from Louisiana State University dive in the dead zone. She said, “The deeper we go down in the water, it gets kind of scary, because there’s nothing there. There’s no fish, there’s no organisms alive, so it’s just us.”6 At its peak the dead zone off the Mississippi River chokes life from nearly eight thousand square miles of sea.
Some people believe that the dead zone is a
natural feature of the Gulf of Mexico, but much of the Mississippi watershed is farmland, and the evidence points firmly to agricultural fertilizers. The history of the dead zone can be read in the bodies of microscopic animals trapped in sediments laid down on the seabed near the mouth of the Mississippi and far beyond.7 The relative numbers of species that depend on oxygen-rich water compared to those that thrive when oxygen is scarce tell the story of when the dead zone first formed and how it has grown with time. Chemical signatures in the sediments add detail to this history.
The Mississippi watershed has seen drastic changes since the colonization of the Midwest and the conversion of prairie to farmland. That led to a steep change in the rate of soil loss around the beginning of the nineteenth century, which can be seen in core samples through a corresponding rise in the productivity of diatoms, a type of phytoplankton. Sediment loads fell again through the early twentieth century, as dams were built and trapped mud. The application of artificial fertilizers to cropland soared after 1950, along with the number of sinking diatoms, and has not yet peaked. The dead zone took hold only after this, although it was too small to attract much notice until the 1970s. Since then it has formed nearly every year like a spreading bruise. Inexorable and inescapable, it chokes life from a place that was within living memory the most vigorous fishing ground in the Gulf.
A Sumerian creation myth inscribed on clay tablets five thousand years ago appears to describe an early effort at dam building:8
Ninurta, the son of Enlil, did great things
He built a big mass of stones in the mountains…
He constructed a barrier at the horizon…
The stones competed with the powerful water
The Ocean of Life Page 13