The Ocean of Life

by Callum Roberts

  Now the water of the mountains will not flow down to the valley forever

  The first experiment in major dam building took place forty-six hundred years ago, when ancient Egyptians tried to control Nile floodwaters at Sadd el-Kafara.9 Their construction was destroyed by a flood before it was completed, but this early failure didn’t hold us up for long. We have since raised over thirty-five thousand major dams, transforming freshwaters. Nearly half of the world’s largest rivers are dammed, and over 40 percent of river flows to the sea are intercepted by reservoirs.10 Collectively they exert a growing influence over life in the sea. The attractions of low-carbon energy and greater water security (at least in the medium term, before reservoirs clog with silt) are unstoppable, even if we now have a far better understanding than we once did of the costs of blocking rivers.

  Before the Aswan Dam was built, when sediment-laden Nile floods reached the Mediterranean they triggered plankton blooms that fed huge shoals of anchovies and a lively fishery. The dam now blocks most of these nutrients, but a new source has been found in the nourishing broth of effluents that flow into the sea from almost every river and stream in the country. The anchovy fishery, which floundered after the dam was built, is back in business, since today the Nile carries the waste of a population of eighty million as well as fertilizer runoff from the delta—not that this thought would tempt me to a feast of anchovies the next time I visit Alexandria!

  Dams and crop irrigation cut off sediments and divert water, reducing nutrient flows into coastal seas. Their impact may in fact be more far-reaching than one would imagine. The Yangtze River carries vast quantities of soil from the Chinese highlands. But even this great river supplied less than a tenth of the phosphorus and a third of the nitrogen—two key plant nutrients—to the East China Sea.11 The rest upwelled from deep water beneath the river plume. (Buoyant plumes of river water flowing offshore at the surface can drag deeper water in the opposite direction, sucking nutrients into sunlit waters where phytoplankton thrive.) After the Three Gorges Dam was built across the Yangtze River, peak primary production by phytoplankton in the East China Sea fell by 86 percent, due to loss of nutrients.12

  Dead zones form most readily where water movement is sluggish and it is stratified into layers, cutting deeper water off from life-giving oxygen. If water slops around for long enough, stagnant pools near the seabed lose their oxygen as the sinking “snow” of decomposing plankton builds into drifts. Despite their massive nutrient flows to the ocean, the Orinoco and Amazon rivers have no dead zones, because water is pushed offshore too fast for oxygen to be lost. Where the flows of once great rivers have been staunched to a trickle after dams, cities, and crops have drunk their fill, estuaries may hold water long enough for dead zones to form. This is the case, sadly, for rivers like the Loire in France and Po in Italy, when their lack of oxygen chokes life from thousands of square miles of seabed every summer.

  Enclosed seas like the Baltic, Adriatic, and Black also suffer problems of stagnation. Shaped like a deep bowl that is cut off from the Mediterranean by a shallow sill in the Bosporus Strait, the Black Sea was a freshwater lake when sea levels went down fairly dramatically during the last glaciation. It refilled eventually, with saltwater, in a catastrophic flood seven thousand years ago, when the Mediterranean rose high enough to break through the Bosporus. Massive floods triggered a tremendous human migration, and the event may be recorded in the ancient tale of Gilgamesh, and perhaps Noah’s flood.13 Today only a warm, less saline, surface layer is well oxygenated and able to support abundant life. This low-density layer sits over the cooler, more saline waters of the deep basin like a lid and has suffocated life below. Deeper than about five hundred feet, the Black Sea is devoid of oxygen.

  This lack of oxygen means the Black Sea conceals some spectacular shipwrecks, as the National Geographic explorer Bob Ballard has revealed. He discovered one wreck off Sinop, the ancient tuna fishing port, that dates back fifteen hundred years. Spared the jaws of shipworms, the hull and deck are intact, and even the mast still stands, a ghostly relic of a vanished age. The ship offers an extraordinary peek into the seafaring life of the ancient world, but at over a thousand feet deep it remains tantalizingly beyond the reach of archaeologists, so its secrets, like its timbers, are untouched.

  The Black Sea owes its stagnant depths to nature,14 but other enclosed seas have us to blame. Northern Europe’s Baltic Sea connects to the North Sea only by a shallow, tortuous channel that curls around Denmark. The rivers that feed it drain from highly populous countries and run through croplands and intensive pig farms. They receive effluents from heavy industry and paper-pulp mills and sewage from cities great and small. By the time they reach the coast they are loaded with nutrients and organic waste. Sediment samples from one Swedish bay tell a similar story to those from the Mississippi dead zone.15 Land was cleared and trees felled for agriculture and industry at increasing rates from 1800 onward, which led to an ever-growing rise in nutrient runoff. Problems from plankton blooms and anoxic bottom water emerged after the 1950s, when artificial fertilizers came into widespread use. The number of people in Baltic watersheds has more than doubled since then, and sewage has compounded the effects of fertilizers.

  The Baltic is a huge brackish water lake that occasionally gulps saltwater from the North Sea. When conditions are right, the North Sea pours in over the sill around Denmark hugging the seabed as it creeps along. Brackish water (a mix of salt and freshwater) is less dense and thus flows out at the same time along the surface. This density difference restricts mixing between the salty deep and fresher surface layers and so promotes a loss of oxygen when dead plankton sink to the bottom of the sea. Species that need saltwater to thrive are thus confined to the deep layers with dwindling oxygen. One reason why Baltic cod stocks fell so catastrophically in recent years was that their eggs could not survive in these deep pockets of low-oxygen saltwater, although overfishing was the main reason for their downfall.

  Intriguingly, there seems to be a link between the collapse of cod and other predatory fish and the vigor of seaweed growth. Thick, choking mats of filamentous seaweed have overgrown the bottom in shallow regions of the Baltic like tufts of wool, blanketing areas where you could once gaze from the surface through crystal water at the waving fronds of bladder wracks and kelp below. This filamentous weed spells trouble for fishers, whose nets clog with drifting clumps. The decline of big fish and seals from hunting and disease led to an increase in their prey, which in turn were predators of animals that grazed on the mat-forming seaweeds that now smother the seabed. After these smaller predators decimated the grazers, the algae spread.

  Denmark and many other nations bordering the Baltic have made strenuous efforts to reduce nitrogen runoff to the sea to tackle excess nutrients and the problems they cause. Nitrogen inputs have halved since their peak in the early 1980s.16 Unfortunately, it seems the Baltic is stuck in a vicious cycle. Low oxygen at the seabed releases phosphorus that stimulates plankton blooms, which leads in turn to worse oxygen depletion. This happens because when oxygen declines, life on the bottom shifts from larger species that live deep in the sediments, like burrowing worms and clams, to smaller, more transient species, like threadworms, that live on or near its surface. In well-oxygenated conditions the larger sediment dwellers suck particles of food from the water and deposit their wastes deep in the mud as feces. They draw nutrients from the open water and so are no longer available to fuel further plankton blooms. By contrast, the smaller species that feed near the surface when oxygen is scarce release nutrients back into the water, where they further boost plankton growth, increasing the likelihood of more severe oxygen loss as the plankton decay. Once again we have come to see the value of an intact, functioning marine ecosystem only after it has been damaged.

  The Chesapeake Bay offers another sad example; we rue the loss of the vast maze of filter-feeding oysters whose reefs so challenged the navigational skills of Virginia’s first settlers.17 The reefs were
scraped and nibbled away to feed America’s appetite for oysters. Over the last century nutrient inputs to the Chesapeake have soared, from sewage, industrial effluent, and pig farms. Dead zones were first noticed in the 1930s, but their size and frequency has increased with time. By the 1960s, the declining health of the bay could no longer be ignored. But it was not until the late 1980s that a program was begun to reduce the amount of nutrients washing into the bay. It was supposed to restore good water quality by 2010, but it hasn’t worked.18 Forecasts made in early 2011 for that year predicted the fifth-largest volume of anoxic water seen in any of the previous twenty-six summers of monitoring. While excessive nutrients set the scene for anoxia, abundant filter-feeding oysters might still have prevented it from happening had we not removed them.

  The nutrients that fertilize coastal and enclosed seas create another unwelcome problem, which is that plankton blooms can be toxic. Some marine plants produce potent poisons. Collectively they are known as “harmful algal blooms,” but more commonly they are called red or brown tides for the reddish slicks that appear at the surface when a bloom is underway. Red tides cause filter-feeding shellfish such as oysters, clams, and mussels to accumulate toxins that cause paralytic and amnesic shellfish poisoning, sometimes killing people who eat them. They also poison fish and mammals, causing mass die-offs.

  Florida is one of the more afflicted regions in the world. It has been troubled on and off by red tides since at least the middle of the twentieth century—when scientists first began to study the phenomenon—and far longer, according to the accounts of sailors and visitors. For most of the last twenty years there has been at least one red tide a year. Sometimes they have persisted for more than a year. Nutrient-rich runoff from intensive agriculture seems to trigger blooms offshore, especially in the southwest, after a heavy rain flushes fertilizers out to sea.19 The limited circulation of Florida Bay concentrates the blooms.

  The dinoflagellate responsible for Florida’s red tides is called Karenia brevis, and it produces a cocktail of potent toxins—brevetoxins—that act on the nervous system. When these algae are churned up by the wind and the waves, the toxins can be inhaled. Manatees, docile sea cows that graze Florida’s shallow sea-grass meadows, are especially sensitive, and red tides kill more of them than any other cause. A particularly severe red tide killed 149 of them in 1996, while over 100 bottlenose dolphins succumbed in 2004. Unsurprisingly, people on land suffer from sore throats and runny eyes, and some develop more serious breathing problems when red tides strike. Hospital admissions for respiratory and stomach problems surge during red tides, and those who live close to the coast suffer the worst.20 Inhaled toxins caused DNA damage, a precursor to cancer, in tests on laboratory rats. Bottlenose dolphins that wash up dead on beaches often contain high levels of brevetoxins. Life in red tide hot spots, such as Tampa Bay, Naples, and Fort Myers, has become bad enough that there is concern it could drive away tourists and depress coastal property values.

  There are many other toxic phytoplankton out there able to make trouble for us in places that are polluted by excess nutrients. A different kind grows in Australia’s Moreton Bay on the Queensland coast, in fibrous tufts attached to the blades of seaweeds and sea grasses. The problem, as with Florida’s red tides, is most severe when storms churn the sea into a spray that can be inhaled, causing lung inflammation and itchy skin. The same algae can be found in Hawaii, where it is eaten by a type of mullet that has become known locally as the “nightmare weke”—when eaten, toxic fish cause hallucinations and nightmares. The toxins aren’t just responsible for short-term health problems. Some promote tumor growth, or produce birth defects. It may be no coincidence that Hawaiians who eat seaweeds have high rates of stomach and intestinal cancers.

  The tumor-promoting chemicals in harmful algae have been blamed for the growth of horrendous tumors on green turtles, although the tumors are triggered by viruses. These animals feed on sea grasses, the blades of which support toxic algae. I once lived in the U.S. Virgin Islands, where I worked as a coral reef researcher, and most of the green turtles gliding around me when I took my daily swim suffered tumors that ranged from apple- to melon-size. Moreton Bay’s turtles have similar problems. Could coastal pollution cause cancer? Now there’s an unpleasant thought that might actually get our attention.

  One group of animals has done better than most from the triple combination of nutrient enrichment, low oxygen, and overfishing: jellyfish. To most of us jellyfish are a nuisance at the beach—half glimpsed but sorely felt. I remember well my childish fascination with the amorphous lumps that sometimes littered the shore. I would prod and chop them nervously with my spade, half expecting they would slide over to sting my feet. When, as a new diver, I first came upon a tiny comb jelly adrift in open water, it dumbfounded me. Comb jellies are about the size of figs and are little more than blobs of seawater wrapped in a transparent glaze. But rows of tiny brushlike cilia beat over that skin, in rainbow waves. Their beauty is fragile, as I soon discovered, for the slightest touch tears them asunder.

  Since the Monterey Bay Aquarium in California figured out how to raise them in tanks, the splendors of jellyfish have been enjoyed by millions of people (the trick is to have a tank without sharp corners and to keep the water moving in gentle loops). It is a testament to their loveliness that jellies can hold the limelight against the more obvious wonders of tuna, turtle, and shark. Their rhythmic pulses have a hypnotic quality, like beating hearts. But the allure of jellyfish conceals a darker side. They are ferocious predators.

  Jellyfish feed on zooplankton; some hunt other jellies. When abundant they compete for zooplankton with other animals, such as herring, sardines, or menhaden. Not much that is edible is off-limits. They hunt eggs and larvae of fish and shellfish, and when jellyfish “bloom,” they can have major impacts on the young stages of commercially important species. A bloom of comb jellies in the Black Sea was implicated in the collapse of several fisheries.21 Blooms can happen quickly, because jellyfish grow fast (it helps that 95 percent of their body weight is water—by way of comparison, only 55 percent to 60 percent of our bodies are water). They multiply with incredible rapidity when the conditions are right, blossoming from a handful to an infestation in a matter of weeks.

  One trick that the familiar moon jellies and their relatives have is a dual life cycle. We know them for their free-swimming stages, but they also live attached to the bottom as polyps a few millimeters across, much like sea anemones. When the time comes these polyps can release dozens of tiny jellies into the water, which can grow from fingernail size to adulthood in a few months. The largest jellyfish in the world by mass—Nomura’s jelly—is found in Asian seas; it grows to a couple of meters across the disk and can weigh 440 pounds or more. Perhaps inevitably, Nomuras are caught and eaten, but even so they have caused great nuisances in Japanese waters. They clog fishing nets, even of the boats that target them. In 2010, one boat sank under the weight of its catch. For boats that target other fish, jellyfish just get in the way and ruin the fishing. Set nets and traps are torn away by walls of jellies pressed against them by waves and currents. They choke the cooling water intakes of power stations, forcing them to reduce output, or sometimes even to shut down.

  Mediterranean resorts have been plagued by jellyfish outbreaks in the last twenty years. The main problem species there is the mauve stinger, whose tentacles inflict slashing welts on the tender bodies of bathers. In the summer of 2004, an estimated forty-five thousand swimmers were treated for stings in Monaco alone. Things got so bad in Spain’s Balearic Islands in 2009 that the local government issued the fishing fleet with bespoke jellyfish nets and put them on notice: Should a bloom occur, sweep up the jellyfish before they hit the tourist beaches. In 2007, Irish salmon farms were overwhelmed by hordes of mauve stingers that slaughtered tens of thousands of salmon in their deathly embrace. Similar mass killings have been reported in Japan, India, and Maryland.

  Despite their fragility, jellyfish are
remarkably resilient, perhaps a legacy of their ancient origins in the pre-Cambrian, when they commanded the waters virtually unchallenged. When nutrients overenrich coastal waters and phytoplankton prosper, there is a shift in the zooplankton from large to small species. Fish hunt by sight and suffer from the loss of easily spotted prey, whereas jellyfish, which hunt by touch, do fine. When the bodies of dead phytoplankton pile up and oxygen levels plummet, most jellyfish are unperturbed. They can survive in water with scarcely a gasp of oxygen, conditions that would kill most fish. These two attributes mean that jellyfish thrive in the low-oxygen nutrient soup of seriously polluted coasts. They bloom even amid the toxic belch of the Benguela upwelling. But there is more. Most jellyfish seem to enjoy a little warming, and experiments suggest they can tolerate the ocean acidification levels predicted for one hundred years hence. The concrete and timber footings of seawalls, harbors, and marinas provide excellent habitat for the bottom-living polyp stages of many jellies. It is probably no coincidence that most recent explosions of jellyfish have taken place in relatively enclosed seas surrounded by densely populated coasts. Overfishing and bycatch have taken a toll on animals that eat jellyfish, like chum salmon, spiny dogfish, and leatherback turtles. In short, the changes afoot in the oceans today could not have been designed better by some great jellyfish megalomaniac intent on global domination. Looking ahead to a future that is gelatinous, some scientists predict a jellyfish joyride all the way through the twenty-first century.22

  CHAPTER 9

  Unwholesome Waters

  Filthy water cannot be washed.

  —West African proverb

  As a boy I lived in a small town set within a deep bay on the North Sea coast of Scotland. I could breathe the sea from my bedroom window, at least on the few days it was warm enough to open. Harbor seals basked on the shore near my front door. It was the 1970s, and there was another smell on the sea breeze in those days—oil. Through my teens I remember measuring the spread of the oil industry across the North Sea in the rising count of bird carcasses that I found piled up on the shore. The tide of bodies swelled over the years: guillemots, razorbills, fulmars, and once even a great northern diver. On land, the quayside that a hundred years before had thronged with the bustle of the herring fleet was now cluttered with pipes, steel cables, and other industrial paraphernalia. At fourteen my biggest regret over the burgeoning oil industry was to see wildflowers buried beneath pipes in an abandoned coastal quarry where I used to entertain myself amid the orchids and insects, dreaming of a future life as a naturalist that I am now lucky enough to have.