Paleoecology—Looking to the Past to Inform the Future

Fueled by heavy loads of phosphorus and nitrogen running off of farm land, harmful algal blooms have become an annual event in Lake Erie's western basin. Photograph: National Oceanic and Atmospheric Administration. The concept of the Anthropocene rose out of the muck at the bottom of polluted lakes. Eugene Stoermer, who coined the term, spent decades sifting through ancient lake sediments, reading the impacts of human ­pollution in the remains left behind by long-dead diatoms, a class of algae that form durable shells of silica. In the 1970s, Stoermer's experiments with living diatoms helped to show that phosphorus pollution from sewage was driving the degradation of Lakes Erie, Ontario, and Michigan—three of the Laurentian Great Lakes, which had long been thought immune to human impacts by virtue of their sheer size. Stoermer began using the word Anthropocene in the 1980s to describe the current geological epoch, in which human actions have reshaped the planet's chemical balance in profound ways. The idea was later popularized by the atmospheric chemist Paul J. Crutzen, who focused on greenhouse gases. But Stoermer saw the transformations of the Anthropocene from the perspective of an algal cell afloat in a lake overloaded with nutrients. Stoermer died in 2012, but the scientific techniques he developed remain critical as scientists work to understand and restore the ecosystems of nutrient-enriched lakes and estuaries worldwide. Paleoecologists are now expanding on that body of work, using an array of new techniques to understand the ecology of eutrophic lakes and estuaries. Recent studies show that the toxins produced by cyanobacteria are preserved in lake sediments and can be used to track their global increase and to infer the causes. Ancient insect remains are being used to understand the impacts of fossil-fuel combustion on some of the planet's most remote waters. Beneath the roots of threatened seagrass meadows, researchers have uncovered a valuable archive of data that reveals human impacts over centuries of time, on land and at sea. The pattern of eutrophication that Stoermer studied continues in Lake Erie, in Lake Okeechobee, in the Chesapeake Bay, and in the northern Gulf of Mexico, as well as in waters as far flung as China's Lake Taihu, the Baltic Sea, and Oyster Harbor in Australia. In all of these places, heavy loads of nitrogen and phosphorus trigger blooms of algae and cyanobacteria. Floating mats of surface growth shade out underwater plants, which wither away. As phytoplankton cells die off, they drift to the bottom, where bacteria break down their remains, using up oxygen in the process. The result is a dead zone, an expanse of oxygen-depleted water where no fish or invertebrates can survive. Diatoms, a class of algae that form durable shells of silica, are widely used as paleoindicators of the trophic state of lakes and estuaries in the past. Image: Berezovska. Today, 45 years after the passage of the Clean Water Act mandated improved treatment of sewage and industrial discharges, the major source of nutrient pollution in waters of the United States, as in other developed countries, is runoff from city streets and fertilized farm fields, nonpoint sources that remain difficult to control. In the 1970s, as water pollution reached a crisis point, scientists searched for a way to reach into the ecological past of affected lakes in the United States. Few aquatic ecosystems had been studied prior to being seriously polluted. Stoermer's long collaboration with limnologist Claire Schelske revealed the complex ways in which Great Lakes ecosystems had been changed by human activities since early Euro-American settlement. The key to their work was Stoermer's encyclopedic knowledge of diatoms. Each species of diatom builds a minute glass house with a distinctive shape. Under the microscope, these silica shells, called frustules, may resemble a truck tire, a star, or a pasta shell. Some diatoms thrive in clear waters, whereas others are adapted to the murk of nutrient-rich habitats. Thus, the traces left behind by diatom communities can map changes in nutrient loads over centuries. When species adapted to live in the sediments of clear waters fade and those that thrive by floating at the surface increase, they tell the tale of eutrophication from long ago. The amount of silica held in preserved diatom frustules can also be used as an index of primary productivity at the time different layers of sediment were deposited. William Kenney, a faculty scientist at the Land Use and Environmental Change Institute at the University of Florida, worked with Stoermer and Schelske. He continues to delve into the deep past of lakes using tiny fossils preserved in the muddy lake bottoms. His recent studies, for example, demonstrate that Florida's Lake Harris turned eutrophic in 1900, soon after its watershed was ditched to drain nearby wetlands for farming. Lake Harris had been a stretch of clear water for thousands of years, since its formation at the end of the Ice Age. “To truly understand the trajectory of an ecosystem, you need a long-term record,” says Kenney. He sees diatom remains as the “ultimate tool” in paleolimnology, because diatom communities are exquisitely sensitive to changes in water quality. In the 1970s, the specific nutrient triggers of eutrophication were a matter of intense debate. Schelske and Stoermer created experimental microcosms to study the question in Lake Michigan. They submerged giant plastic bags; pumped them full of lake water; added different combinations of phosphorus, nitrogen, and carbon; and tracked the phytoplankton communities within each bag. They demonstrated a clear impact of phosphorus pollution—a controversial finding, given that phosphate-based detergents then contributed 50 percent of the phosphorus load in the Great Lakes. (Phosphate detergents would eventually be banned, after a long political battle with the detergent industry.) Freshwater phytoplankton, mainly diatoms and dinoflagellates found in Lake Chuzenji, Nikko, Japan. Photograph: NEON_ja. High concentrations of phosphorus meant that populations of diatoms, normally the dominant type of algae, boomed. Following a major diatom bloom, levels of dissolved silica in the water plummet, because the element is locked up in diatom frustules that sink to the bottom as the cells die off. The researchers warned that silica depletion would cause the decline of Great Lakes diatoms and favor green algae and cyanobacteria, which form more noxious blooms. That has indeed come to pass, especially in Lake Erie, the most intensely polluted of the Great Lakes. Lake Erie's harmful algal blooms of the last few decades have been dominated by a shifting array of cyanobacteria, which can produce nerve and liver toxins. Most recently, algal blooms in western Lake Erie, as well as in other eutrophic lakes as far flung as China and Japan, have been dominated by the cyanobacterium Microcystis, which relies on nitrogen dissolved in the water to produce a deadly liver toxin. Depending on the nature of its watershed, an undisturbed lake may range from oligotrophic (a body of clear water, low in nutrients, with little algal growth) to eutrophic or hypereutrophic (nutrient-rich murk thick with algae). Stoermer showed that dramatic changes in the diatom communities of the Great Lakes occurred as early as the 1800s, when European settlers were cutting down forests and draining wetlands. The loss of these habitats meant that increased loads of both nitrogen and phosphorus reached the lakes. The lower Great Lakes, Erie and Ontario, experienced their peak production of diatoms during the settlement era. Thereafter, silica depletion made other kinds of phytoplankton dominant. Although the dawn of the Anthropocene traces back to the eighteenth-century onset of the Industrial Revolution, the midtwentieth century was the moment of the “Great Acceleration.” The use of synthetic nitrogen fertilizers and phosphate detergents increased exponentially at that point, as did the use of fossil fuels. Lake sediments deposited after 1945 show rapid and profound changes in the biomass and composition of algal communities. That pattern holds in developed nations worldwide. In the shallower and more intensely polluted waters of Lakes Erie and Ontario, diatom abundance had dropped as early as the 1850s. The same pattern was repeated in Lake Michigan's deeper waters, but not until the intense pollution of the late 1940s and 1950s. Grace Brush, a paleoecologist at Johns Hopkins University, has tracked a similar pattern using a variety of data from sediment cores taken in different parts of the Chesapeake Bay. Her work on the bay's ecological past started in the 1970s, when submerged aquatic plants died off en masse, from the tributary waters of the Patuxent and Susquehanna Rivers to the Chesapeake's salty lower reaches. State officials ascribed the mass loss of habitat to some mysterious natural cycle. Using fossil seeds archived in the bay bottom, Brush was able to show that the sudden disappearance of underwater plants was unprecedented and correlated with a spike in sediment running off of land in the bay's vast and heavily populated watershed. “Grace's work gave us proof positive that the decline of seagrasses was not a ‘natural cycle,’ something that people didn’t need to worry about,” says Will Baker, director of the Chesapeake Bay Foundation. “That natural cycle argument has been used against those of us concerned about environmental impacts on all sorts of topics, from fisheries abundance to seagrasses.” The Chesapeake's mass plant die-off occurred after Hurricane Agnes struck in 1972, sending waters thick with sediment eroded from farm fields rushing into the bay. But long before the storm, the ecosystem had been badly weakened by nutrient pollution. Accumulation of detrital seagrass wrack (Posidonia australis) at the West Beach Boat Harbour, in South Australia, which occurs each winter. Using fossil evidence preserved on the bottom of Oyster Harbour in West Australia, Oscar Serrano has tracked a dramatic decline in P. australis meadows there beginning in the 1980s. Eutrophication, fueled by heavy phosphorus loads, triggered the mass loss of seagrass. Photograph: Bahudhara. Posidonia oceanica seagrass meadows in remote Portlligat Bay have proven to hold a rich archive of paleodata that reveal ancient human impacts on both land and sea. Photograph: Albert Kok. Nitrogen gas (N2) constitutes 78 percent of Earth's atmosphere. In the gas, two nitrogen atoms are joined by chemical bonds so strong they can be broken only by a bolt of lightning or a few nitrogen-fixing bacteria. These microbes transform nitrogen into other chemical forms that can be absorbed by plants. Under anaerobic conditions, in the mucky bottoms of wetlands, lakes, and bays, a separate set of microbes reverses the process, freeing nitrogen from organic compounds and pumping N2 gas into the air. The twentieth-century invention of the Haber-Bosch process, which allows industrial fixation of atmospheric nitrogen, made production of cheap synthetic nitrogen fertilizers possible. Their use skyrocketed in the years following World War II, dramatically increasing the loads of the bioavailable nitrogen reaching waterways. Brush has built a vivid mental picture of the landscape walked by the first Chesapeake colonists—an ideal home for the microbes that process nitrogen. “The land was forested, and it was soggy wet,” she says. “There were a lot of beavers, and their dams were everywhere, creating marshy areas. Rain soaked into the ground and didn’t run off.” As people destroyed the forests and wetlands where nitrogen is cycled, they began to dump heavy loads of fertilizer on their fields. Intense doses of nutrients washed into the bay with every rainstorm. Shifts in the Chesapeake's diatom community reveal a transformation typical of eutrophic waters. Prior to European settlement, the bay's bounty of fish and crabs relied on bottom-dwelling organisms, including aquatic grasses and diatoms that grew on the sediment. As they transformed the sun's energy into green growth, the underwater plants pumped ­oxygen into the depths. Shad and striped bass dined on invertebrates that lived in the sediments: A favorite prey item was the Nereis worm. These little beasts ate detritus on the bottom and were wildly abundant. Although worms tend to decompose, leaving no fossil trace, the chitinous jaws of the Nereis worm remain preserved in bottom sediments for hundreds of years, allowing Brush and her colleague Angela Sowers to track their decline in a study published in Estuaries and Coasts in 2014. Helen Bennion at Loch Shiel, Scotland, with a sediment core. Bennion uses an array of paleoindicators, including fossil remains of diatoms, chironomids, cladocerans, and aquatic plants, to track eutrophication and recovery of European lakes. Photograph: Philip Henderson. By the late 1800s, forests and wetlands were gone from the watershed, translated into tidy farm fields or buried under city streets. The waters were clouded with blooms of floating algae. Aquatic grasses began to dwindle, and with them went the Nereis worms and benthic diatoms. The bay's bottom turned barren, depleted of oxygen. The whole ecosystem turned inside out. Once benthic and diverse, it became planktonic, able to support only a few kinds of plants and animals adapted to overfertilized waters. In a 2016 article published in Global Change Biology, Oscar Serrano, a marine ecologist at Edith Cowan University, documented an Australian parallel to Brush's findings in the Chesapeake. European settlers built the city of Albany, in Western Australia, in the 1820s. As they cleared the local forests for farming and ranching, they sent a cascade of fine sediments and nutrients flowing into nearby Oyster Harbour. Meadows of the native seagrass, Posidonia australis, were affected by the pollution but survived into the 1960s, when loads of phosphorus in Oyster Harbour climbed dramatically. The manufacture and widespread use of phosphate fertilizers in the watershed drove devastating changes in the estuary. By 1990, 80 percent of the harbor's original seagrass meadows had vanished, and the waters were dominated by floating algae and cyanobacteria. Twenty-nine percent of the planet's seagrass meadows were lost between 1880 and 1990; seven percent of the surviving meadows have vanished each year since. Seagrasses provide critical habitat for an array of fish and invertebrates. They buffer shorelines from the erosive impacts of waves. They are also among the world's most important carbon sinks, locking away great stores of organic matter in rich mats of marine peat. In Portlligat Bay, on the remote coast of Catalonia, Serrano and his colleagues have discovered that seagrass mats also act as archives of human ecological impacts—on land and in the sea. The bay holds extensive meadows of Mediterranean seagrass, Posidonia oceanica. Woven into the centuries-old layers of peat built by the meadows are markers of human impacts dating back to Roman and medieval times. When ancient farmers cut down forests and plowed the land for crops, they caused a reduction in the amount of glomalin-related soil protein (GRSP) in the watershed. GRSP is produced by mycorrhizal fungi that grow as symbionts on the roots of native trees. During times of intense cultivation, the amount of GRSP washed into Portlligat Bay and preserved in seagrass mats declined. This implies a significant decrease in soil-carbon storage onshore. Thus, living seagrass meadows act as archives of information on soil ecology in the watershed, as well as markers of change in the marine ecosystem. 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