The Ocean of Life

by Callum Roberts

  Just as consumers in the Western world are developing scruples about the sustainability of the fish they eat, economic growth and increased disposable income in Asia, especially China, have released a surge of demand for seafood. Asian buyers have fanned out across the world in search of supplies, especially targeting poor communities in developing countries. Judging by their actions, most have little thought for sustainability. They buy while the fish last and move on. The Asian market has forced prices for some products sky-high, turning once worthless bycatch species such as blue and silky sharks into valuable targets. In 2004, a single large whale shark fin could sell for over US$50,000.3 The long-term prospect does not look good unless Asian consumers can be persuaded to value sustainability soon.

  Things don’t have to get this bad. Aquaculture could help sustain supplies of fish and better management could maintain or perhaps increase catches from wild stocks recovered to populations far larger than today’s. I will come back to how we could achieve this metamorphosis later on. But let’s return to the other drivers of change first.

  Human population growth and technological development have also driven the rise in pollution. The Industrial Revolution opened the veins from which toxic metals and other chemicals began to bleed into the sea. While time has staunched some of these flows, others have opened, as miners and heavy industry have shifted their dirtiest operations from developed to developing countries. All the while we have invented new ways to make things. Our enthusiasm for novel materials and processes seems always to outrun prudence. Highly toxic chemicals are set free long before we notice their darker sides. The legacy of industrialization is written in no-go zones too toxic for use. But many chemicals have spread so widely they cannot be avoided. They swirl around all of us. They compromise wildlife health and reproduction and contribute to the rising incidence of disease epidemics in the sea. And as people often sit at the apex of the food web, trumping consummate predators like sleek tuna and grinning shark, we too are vulnerable. Seafood lovers the world over are accumulating toxins in their bodies drawn from waters where we thought they had been laid to rest for good.

  Population growth has driven up the demand for food. The world avoided the much anticipated population crash expected in the 1980s and 1990s with the green revolution that industrialized agriculture. We have squeezed ever-greater production from the land by lavish applications of artificial fertilizers and other agrochemicals. They have bought us time, but like fisheries, farmers have won greater production at the cost of the natural capital of their soils.4 Rains and irrigation water carry soils and their chemical burdens to the sea through estuaries and deltas whose filter feeders have all but disappeared and whose wetlands have been stripped and converted to other uses. Once in the ocean, nitrogen and phosphorus fire plankton and microbial blooms that overwhelm the assimilative capacity of food webs decapitated by overfishing and exceed the physical water mixing needed to keep up with demand for oxygen, as dead plankton accumulate.

  Despite the proliferation of dead zones, future seas will not be lifeless. We are creating winners as well as losers. Jellyfish, for example, are great opportunists, and some scientists fear that large parts of our most productive seas will transform into jellyfish empires.5 Few animals other than leatherback turtles and giant ocean sunfish (both in decline) eat adult jellyfish because they lack substance, and they have to eat huge quantities to break even. There are many more predators that eat snack-size juvenile jellies.6 With the wholesale decline of marine life from overexploitation, pollution, and climate change, the ranks of these predators have thinned, so more jellies survive.

  Jellyfish positively thrive in pollution-enriched seas. Given unlimited food, they can reach adult size fast. With their stinging tentacles, they are formidable predators. Here one of the quirks of ocean food webs comes into play, sealing their dominance. Most animals that might eat jellyfish go through tiny egg, larval, or juvenile stages when the tables turn and they are themselves jellyfish prey. Such role reversals of predator and prey are rare on land. In the sea, however, they are prevalent, with surprising effects. The American oceanographer Andrew Bakun invites us to imagine a world in which zebras and antelopes are voracious predators of young of lions or cheetahs. What would the Serengeti look like if this were so?

  The jellyfish joy ride begins when high nutrients combine with a fall in abundance of their predators. When plentiful, jellyfish suppress their predators further by eating more of their young, and so pave the way for a full-blown population explosion. If food runs short, jellyfish don’t just die; instead, they shrink and wait until conditions improve (although, if nutrient levels fall far enough and for long enough, jellyfish blooms can snuff out). In a future with more acidified seas, jellies won’t have troublesome carbonate skeletons to handicap their chances. The altered oceans that haunt our possible future could offer them worlds of opportunity. They have been here before. Enigmatic traces in rocks from the earliest Cambrian period, some five hundred and fifty million years ago, tell of an age of jellyfish that preceded the great radiation of life that established most of the animal groups alive today. Collectively, the modern reappearance of seas dominated by gelatinous animals, microbes, and algae has been dubbed “the rise of slime.”7 It signals a reversion toward conditions that prevailed in the earliest days of multicellular life.

  Climate change adds a third set of drivers of change to life in the sea. Heating will thicken the lower density warm layer of water that sits like a lid over much of the ocean to restrict oxygen transfer to underlying waters and constrain upward flow of nutrients. The result will be reduced productivity over great expanses of sea away from coasts and expanded low oxygen zones beneath. Already there are signs that plankton productivity has fallen, probably as a consequence of warmer seas, and the volume of oxygen-starved waters has grown.8 Nutrient starvation of surface waters may also be due to the collapse in fish stocks from overfishing, causing nutrients to disappear into the depths rather than being recycled in shallow waters. Great whales may once have played a role as a nutrient pump from deep to shallow water.9 Many feed at depth and poop at the surface. Before whaling, when the oceans thronged with more than a million whales, that could have made a big difference to productivity.

  Tropical heating along western coasts will force faster redistribution of heat to the poles, strengthening winds that power upwelling of deep water. Upwelling could benefit fish production by raising nutrients from the deep, but intensified upwelling could push some regions into hyperproduction to create noxious eruptions of hydrogen sulfide like those that plague Namibia’s coast today. Upwellings might also pull low-oxygen water of higher acidity onto continental shelves, causing the mass mortality of bottom life like episodes seen recently along the Oregon coast.

  The effects of acidification are hard to predict. At the very least life is likely to get much more difficult for species with carbonate shells, which includes some of the most important primary producers in the sea, the phytoplankton that sustain food webs and release life-giving oxygen. We don’t yet know whether the photosynthesis boosting effect of extra carbon dioxide might outweigh or compensate its corrosive qualities in the balance between benefit and cost. If not, there are noncalcifying phytoplankton that might step up production. The worst case scenario would be a large-scale depression of primary production, which is the quantity of carbon fixed into plant material, which also means less oxygen produced.

  Any fall in the rate of plankton production would reduce the snow of organic debris that sinks from sunlit surface layers to the deep sea. Deep sea communities survive on meager handouts from above, and failure in supply would thin their numbers. As the sea is the ultimate sink for all of the carbon dioxide we are releasing into the atmosphere, it would also reduce the rate at which carbon gets locked away in bottom sediments. This is the “biological pump” that takes carbon from where it can be a nuisance to where it does no harm. If phytoplankton secrete less carbonate because of acidity, thei
r shells will be light and they won’t sink as fast. The pump will run more slowly, so the oceans will absorb less of the carbon dioxide we add to the atmosphere, and global warming will accelerate. Limited evidence from coccolithophores in deep sea sediments suggest their mass has actually increased by 40 percent since the Industrial Revolution. For them, the growth-boosting effect of extra carbon dioxide has so far outweighed the acidity cost.10 Some lab experiments suggest the reverse is true,11 and in the wild the skeletal weight of foraminifera in the Southern Ocean, another key plankton group, has declined by 30 percent to 35 percent since preindustrial times.12 The leading edge of science is always fraught with uncertainty. We still have much to learn.

  The Roman emperor Marcus Aurelius said, “Look back over the past, with its changing empires that rose and fell, and you can foresee the future, too.” There are more than enough portents to be able to see where the oceans are headed. Scotland’s Firth of Clyde is a window to a world without fish. Italy’s volcanic Ischia lets us dive in an acid sea. China’s Bohai Gulf shows how nutrient pollution and overfishing can carpet a sea in weed and spread plagues of jellyfish and toxic plankton blooms. The Gulf of Mexico, Oregon, the Baltic, and hundreds of other places across the world give us a taste of the oblivion spread by anoxic water. Europe’s North Sea reveals life on the move as the world warms. The Gulf of Maine and San Francisco Bay are filling with alien species. Sea level rise is eating away at the Mississippi and Nile deltas and threatens densely populated coasts like those of Holland, Germany, and Thailand. And the rotting bodies of Pacific albatross chicks, bloated and starved by a diet of plastics, show us how life struggles as the oceans fill with human refuse.

  Each of these places illustrates nature under stress from one or two of the impacts I have described in previous chapters. But these primary stressors are not the only ones they face. The Firth of Clyde, for example, has been polluted by decades of sewage-sludge dumping, the waste from a major city, and heavy industry. The Gulf of Mexico’s seabed is raked over year after year by shrimp trawlers. Multiple stresses have more serious effects because they can act in synergistic and often unpredictable ways.

  Stresses exert their insidious influences in several ways. Species are sensitive to different stressors to different degrees. If each stress was applied separately, a snail might be noticeably affected by slight acidification, made uncomfortable with a half degree rise in water temperature, and be indifferent to heavy metal pollution. Add two or more together and the effects could be severe. In one dramatic example, brittle stars are able to survive slight acidification, but add trace amounts of the antibacterial triclosan, an ingredient of hand washes and face creams, and it collapses into fragments.13 Stresses don’t usually combine to such spectacular effect, but their collective impact can erode the fitness of individuals and populations. Their effects are best understood in the currencies of life and death. Populations endure when the birthrate equals or betters the death rate. Stresses often drive down births and increase the likelihood of death, so they upset the balance of life’s equation.

  The extra physiological cost of coping in low-oxygen water, for example, could be met by increasing food intake or reducing calorie demand. Extra food may not be available, so animals could compensate by laying fewer eggs or skipping a breeding season. Or they might grow more slowly, which would also mean fewer offspring, since small animals usually produce fewer eggs. Low oxygen in itself is a powerful constraint on body size, favoring small animals over large because they can breathe more easily, and thereby also cutting egg production as big animals produce more eggs. High body burdens of toxic contaminants, many of them hormone disruptors, might further compromise reproduction through reduced fertility or miscarriage. When the tipping point is passed at which deaths exceed births, a population declines.

  The cocktail of human stresses is mixed differently from place to place, but whatever the combination, its collective impact causes worse harm than each one acting alone. In other words, the kill rate from multiple stresses acting in concert is greater than the summed kill rate from each of the same stresses alone. There is a parallel here between the environment and the experience of intensive-care doctors. They must jump in quickly to support patients when they have only one problem to deal with, like blood loss or stroke. When problems begin to cascade through the complex systems of the body, mortality shoots up.

  When a species succumbs, the effect multiplies through the ecosystem via the web of interactions that connect species. Some creatures are strongly connected, like salmon sharks and salmon, anemonefish and anemones, or dugongs and sea grass. Others form loose associations. A crab might be happy to live among the branches of half a dozen different types of coral, or a fish might feel equally at home amid the waving fronds of several different seaweeds. Still more are linked indirectly through intermediaries; the food your prey eats is ultimately your food (a truth we too should keep in mind in our relations with the rest of the biosphere).

  Stresses act on ecosystems at this higher level when they weaken or knock out links among species. When one species goes down, it takes others with it that, in turn, affect the animals and plants with whom they are connected. The more different stresses we add to the blend, the wider the spectrum of life that is directly affected, so the consequences penetrate further through the tangle of existence. As species dwindle or disappear, ecosystems adapt and re-form, as remaining inhabitants find other ways to make a living. Weak links can strengthen as animals find alternative food or living space.

  Knocking out one species, inserting something else, or adjusting the abundance of another lead to effects on species that at first seem completely unconnected with them. The nineteenth-century American national parks pioneer John Muir’s famous phrase is apt here: “When one tugs at a single thing in nature, he finds it attached to the rest of the world.”14 The scale of our ignorance of these interconnections is breathtaking. It would take a thousand lifetimes of research to figure out all of the ways in which we are affecting the species in an ecosystem of only moderate complexity. Even within a species, effects differ. While adult lobsters may happily cope with increased acidity, the same change halts development of one of their several juvenile stages.

  When species that play key structuring roles disappear, their loss has profound consequences. Longline fisheries in the Pacific Ocean have cut down populations of large ocean-going sharks like the thresher to a tenth of their former abundance.15 In the wake of their departure, smaller species, such as the pelagic stingray, have proliferated. Off America’s East Coast, a similar reorganization has played out. Megapredators, such as hammerhead, great white, and tiger sharks, have collapsed, leaving the field open to their prey.16 Five species of smaller shark and ray have grown in abundance, some spectacularly so. The cownose ray, blunt-faced stingrays with dark round eyes and long whiptails, travel in groups and raise thick billows of sand as they excavate buried clams. With no big sharks to prey on them, their groups have grown to hundreds strong. Added together there could be as many as forty million up and down the East Coast. Cownose rays are particularly partial to bay scallops, and with this many mouths to feed, there is scarcely a scallop left, leaving U.S. fishing communities empty-handed and bewildered. It isn’t an obvious connection, that fishing-out giant sharks would cause the collapse of scallop fisheries, but that is how impacts proliferate. It does illustrate the difficulty of predicting how stresses will reshape ecosystems, even when there is only a single stress involved.

  Canadian cod may have fallen victim to another kind of ecological reorganization, since their population crashed in the 1990s and a moratorium on cod fishing was imposed. Cod once reigned supreme in the chilly waters off Newfoundland. Their ferocity and indiscriminate tastes made them almost universally feared and avoided. Lobsters lurked and crabs cowered as throngs of cod advanced over the continental shelf to their spawning grounds. Like most fish, however, cod are vulnerable when young. Eggs and larvae are snapped up by capelin, h
erring, mackerel, jellyfish, and hosts of others. When cod were mighty they produced so many young that they overwhelmed their predators. It mattered not that tiny cod poured in endless streams into almost limitless mouths because plenty escaped. But when cod were humbled by overfishing, the survivors could only produce a tiny fraction of the former abundance of young, few of which survive the carnivorous gauntlet today. Here is another of those role reversals in which a predator falls victim to its prey.

  When stresses take out habitat-defining species like kelp, oysters, or sea grass they can set off chains of events that remodel the habitat so it becomes hostile to the original inhabitants, just as political revolutions sweep aside old regimes and those who thrived in their orbit.17 When they go, or are pushed out, space frees up for competitors. If strong or predatory interlopers establish, they can change conditions in ways that inhibit recovery of the original species. European coasts were developed at the rate of over half a mile per day between 1960 and 1995, causing widespread wetland loss. Below water, rocky reefs carpeted in lush seaweed forests were subjected to siltation, reduced light, anchoring and dredging, and both the direct effects of destructive fishing and indirect effects of removal of key fish species that kept grazing herbivores in check. When large, canopy-forming seaweeds disappeared from Mediterranean reefs they were replaced by shorter, turf-forming seaweeds. Beneath the old canopies, sediments were swept away by fronds that lashed back and forth under the influence of waves and currents. Turf seaweeds, by contrast, trapped sediment to create an unstable layer over the rock so that it cannot long sustain larger weeds that require firm anchors to hold them in place.