Other countries are already contending with the effects of major pollution. Fertilizer, pesticide, and livestock-waste runoff from farms in the American Midwest, for example, have created a dead zone at the mouth of the Mississippi River in the Gulf of Mexico. There, coastal algae populations thrive on the influx of nutrients and the misfortune of their natural predators, which are often curtailed by the pesticides. From spring until late summer, immense algal blooms rob the Gulf’s water of oxygen. Such hypoxic conditions chase the swimming creatures away and kill clams and other sedentary species on the spot. The Gulf’s seasonal dead zone now encompasses more than 8,000 square miles, an area the size of New Jersey, every spring and summer. Much smaller dead zones occur on the Mekong, too. Worldwide, there are 146, every one increasing in size, intensity, and often duration.
Besides agricultural runoff, pollution from industry and municipalities is also a big problem for freshwater systems. In addition to contributing extra nutrients that promote algal overgrowth, municipal wastewater also carries thousands of chemicals from products used in daily life: cosmetics, soaps, pharmaceuticals, cleaning supplies, and more. Most of it winds up in aquatic systems.
The long-term consequences of dumping so many chemicals in the water are just coming to light. More than 200 species are thought to have adverse reactions to endocrine disruptors—such as estrogen and its chemical mimics—that get into the environment via human and veterinary pharmaceuticals in wastewater and farm runoff. Sightings of frogs with deformities, such as extra legs, mushroomed in the Midwest about a decade ago. Ecologists think chemicals or an interaction between chemicals and parasites could be causing the deformities. Indeed, chemicals in freshwater may be a factor in the alarmingly sharp worldwide decline of amphibians.
Biological introductions to waterways, like chemical introductions, are extremely problematic. In their own communities, most species are held in check by natural predators or other environmental constraints. But organisms from afar can crowd, devour, or outcompete native species in their new neighborhoods, and can even change entire ecosystems. Most biological introductions by people are accidental, but some, such as fishes stocked for anglers or plants brought in to stabilize soils, are intentional.
Mimosa pigra, a spiny shrub native to the Americas and planted abroad as an ornamental or to control erosion, is now one of the world’s worst aquatic invasive species. Once established, it quickly forms dense stands and outcompetes native plants. First spotted on the Mekong in 1979, it spreads in floodwaters and in truckloads of construction sand, and is now devastating parts of the watershed. The mimosa has taken over several irreplaceable wetlands, doubling its area almost every year in some places. Several endangered water birds that depend on native grasses for food and shelter are undergoing population declines as mimosa stands replace their habitat.
Controlling freshwater invaders and mitigating the damage they cause costs some 9 billion dollars each year in the U.S. alone. Yet the rate of invasions everywhere is on the rise as global commerce, trade, and travel increase.
So much for the organisms people add to freshwater systems. What about the ones—too many—that we take out? Overexploitation for food, medicine, and recreation poses a major threat to freshwater birds, crocodiles, fishes, frogs, and turtles, as well as some invertebrates. More than 40 million people rely on the waters of the Mekong River Basin for their protein and income, and they are overfishing numerous species—indeed entire fish assemblages in certain areas—as a result.
The Mekong giant catfish, Pangasianodon gigas, is just one of the region’s struggling, overfished residents. Reaching nine feet in length and more than 600 pounds, it is the world’s largest catfish. With such grand proportions, a jackpot of succulent flesh that once sold at a premium to urban restaurants, the giant catfish was a fisherman’s prize catch. In the mid-twentieth century, hundreds of giant catfish—a naturally rare species—were caught each year, but recently the annual catch has declined to fewer than ten. Overfishing is the main cause of the decline, but habitat fragmentation and alteration of spawning grounds by dams and navigation projects also contribute. Today, the giant catfish is critically endangered, its range is greatly restricted, and the average size of individuals is declining. In recent years, Cambodia, Laos, and Thailand have outlawed catching the giant catfish. But the species is migratory, so a regional agreement may be necessary to prevent its demise.