行星地球第三集：淡水資源 Fresh Water

[00:30.32]Only 3 percent of the water on our planet is fresh. [00:34.40] [00:36.84]Yet these precious waters are rich with surprise. [00:40.72] [00:53.40]All life on land is ultimately dependent upon fresh water. [00:58.68] [01:28.68]The mysterious tepuis of Venezuela - [01:32.64] [01:33.76]isolated mountain plateaus rising high above the jungle. [01:38.28] [01:47.68]This was the inspiration for Arthur Conan Doyle's 'Lost World,' [01:52.52] [01:52.60]an imagined prehistoric land. [01:55.64] [02:01.80]Here, strange towers of sandstone have been sculptured over the millennia [02:06.44] [02:06.56]by battering wind and torrential rain. [02:09.68] [02:22.48]Moisture rising as water vapour from the surface of the sea [02:26.44] [02:26.52]is blown inland by wind. [02:29.44] [02:41.20]On reaching mountains, the moisture is forced upwards [02:45.12] [02:45.24]and as it cools, it condenses into cloud and finally rain - [02:50.44] [02:50.56]the source of all fresh water. [02:53.64] [03:03.64]There is a tropical downpour here almost every day of the year. [03:08.52] [03:19.68]Fresh water's journey starts here, high in the mountains. [03:24.56] [03:40.72]Growing from humble streams to mighty rivers [03:43.84] [03:43.96]it will travel hundreds of miles to the sea. [03:47.52] [04:24.32]Angel Falls, the highest waterfall in the world. [04:28.68] [04:42.60]Its waters drop unbroken for almost a thousand metres. [04:47.60] [04:56.56]Such is the height of these falls [04:58.52] [04:58.64]that long before the water reaches the base in the Devil's Canyon [05:02.92] [05:03.00]it's blown away as a fine mist. [05:06.12] [05:37.20]In their upper reaches, [05:38.76] [05:38.88]mountain streams are full of energy. [05:42.20] [05:44.84]Streams join to form rivers, [05:47.72] [05:47.84]building in power, [05:49.24] [05:49.36]creating rapids. [05:51.16] [06:01.00]The water here is cold. [06:03.04] [06:03.16]Low in nutrients, but high in oxygen. [06:06.56] [06:10.24]The few creatures that live in the torrent [06:12.44] [06:12.56]have to hang on for dear life. [06:15.20] [06:17.28]Invertebrates dominate these upper reaches. [06:20.32] [06:20.44]The hellgrammite, its body flattened to reduce drag, [06:24.04] [06:24.20]has bushy gills to extract oxygen from the current. [06:28.40] [06:32.56]Black fly larvae anchor themselves with the ring of hooks, [06:37.00] [06:40.28]but if these become unstuck, [06:42.20] [06:42.32]they're still held by a silicon safety line. [06:45.84] [06:58.72]There are advantages to life in the fast stream - [07:02.32] [07:02.44]bamboo shrimps can just sit and sift out passing particles [07:06.28] [07:06.40]with their fan-like forearms. [07:08.60] [07:27.44]Usually, these mountain streams only provide enough food [07:31.32] [07:31.44]for small animals to survive. [07:34.12] [07:34.32]But with the spring melt here in Japan [07:37.44] [07:37.56]monsters stir in their dens. [07:40.88] [07:48.72]Giant salamanders, world's largest amphibian, [07:53.00] [07:53.12]almost two metres long. [07:55.76] [07:58.80]They're the only large predator in these icy waters. [08:03.08] [08:07.04]They begin their hunt at night. [08:09.80] [08:24.32]These salamanders have an exceptionally slow metabolism. [08:28.64] [08:28.76]Living up to 80 years they grow into giants. [08:32.80] [08:49.24]The fish they hunt are scarce [08:51.92] [08:52.04]and salamanders have poor eyesight. [08:55.08] [08:57.88]But sensory nodes on their head and body [09:00.64] [09:00.72]detect the slightest changes in water pressure. [09:04.36] [09:15.36]Free from competition, [09:17.20] [09:17.32]these giants can dine alone. [09:20.48] [09:33.52]Pickings are usually thin for the salamanders, [09:37.04] [09:37.20]but every year some of the world's high rivers [09:39.92] [09:40.04]are crowded by millions of visitors. [09:43.48] [09:50.52]The salmon have arrived. [09:53.04] [10:00.40]This is the world's largest fresh water fish migration. [10:04.96] [10:08.92]Across the northern hemisphere [10:10.56] [10:10.68]salmon, returning from the ocean to their spawning grounds, [10:14.24] [10:14.36]battle their way for hundreds of miles upstream. [10:17.88] [10:22.72]Up here, there are fewer predators to eat their eggs and fry. [10:27.44] [10:54.52]A grizzly bear. [10:57.00] [10:59.00]From famine to feast - [11:00.96] [11:01.16]he's spoilt for choice. [11:03.08] [11:09.92]This Canadian bear is very special - [11:12.48] [11:12.60]he's learnt to dive for his dinner. [11:15.72] [11:30.08]But catching salmon in deep water is not that easy [11:33.88] [11:34.00]and the cubs have lots to learn. [11:36.68] [12:13.00]The annual arrival of spawning salmon [12:15.52] [12:15.64]brings huge quantities of food into these high rivers [12:19.04] [12:19.16]that normally struggle to support much life. [12:22.12] [12:34.96]Although relatively lifeless, [12:36.96] [12:37.08]the power of the upland rivers to shape the landscape [12:40.20] [12:40.32]is greater than any other stage in a river's life. [12:44.16] [12:46.20]Driven by gravity, [12:47.64] [12:47.72]they're the most erosive forces on the planet. [12:51.24] [12:55.28]For the past 5 million years [12:58.00] [12:58.12]Arizona's Colorado river has eaten away at the desert's sandstone [13:02.80] [13:02.88]to create a gigantic canyon. [13:05.84] [13:16.32]It's over a mile deep [13:18.40] [13:18.48]and at its widest it's 17 miles across. [13:23.00] [13:36.00]The Grand Canyon. [13:37.88] [13:57.28]This river has cut the world's longest canyon system - [14:01.92] [14:02.00]a 1,000 mile scar clearly visible from space. [14:07.00] [14:32.48]As rivers leave the mountains behind, [14:35.08] [14:35.16]they gradually warm and begin to support more life. [14:40.00] [14:48.20]Indian rivers are home to the world's most social otter - [14:53.24] [14:54.04]smooth-coated otters form family groups up to 17 strong. [14:59.28] [15:10.08]Group rubbing not only refreshes their coats, [15:13.72] [15:13.80]but strengthens social bonds. [15:16.76] [15:26.04]When it comes to fishing [15:27.60] [15:27.68]there is real strength in numbers. [15:30.88] [15:43.20]Fishing practice begins when the cubs are four months old. [15:47.96] [16:06.04]Only the adults have the speed and agility needed to make a catch. [16:11.88] [16:48.84]Adults share their catches with their squabbling cubs. [16:53.24] [17:03.88]Most otters are solitary, [17:06.04] [17:06.20]but these rich warm waters can support large family groups [17:11.20] [17:11.64]and even bigger predators. [17:14.00] [17:32.80]Mugger crocodiles, four metres long, could easily take a single otter. [17:38.64] [18:03.52]But, confident in their gangs, [18:06.00] [18:06.08]the otters will actively harass these great reptiles. [18:10.36] [18:27.44]Team play wins the day. [18:30.16] [18:40.60]The Mara river, [18:42.12] [18:42.24]snaking across the plains of East Africa. [18:45.72] [18:47.64]As the land flattens out [18:49.28] [18:49.40]rivers slow down and lose their destructive power. [18:52.96] [18:53.76]Now they are carrying heavy loads of sediment [18:57.12] [18:57.20]that stains their waters brown. [18:59.68] [19:12.40]Lines of wildebeest are on their march. [19:15.96] [19:20.60]Each year nearly two million animals migrate across the Serengeti plains [19:25.84] [19:25.92]in search of fresh green pastures. [19:29.08] [19:29.20]For these thirsty herds [19:30.84] [19:30.96]the rivers are not only a vital source of drinking water, [19:34.32] [19:34.44]but also dangerous obstacles. [19:37.44] [19:56.36]This is one of the largest concentrations of Nile crocodiles in Africa, [20:02.24] [20:02.52]giants that grow over five metres long. [20:06.12] [20:21.92]From memory, the wildebeest are coming [20:25.44] [20:25.56]and gather in anticipation. [20:28.20] [21:30.48]The crocodile's jaws snap tight like a steel trap - [21:34.64] [21:34.72]once they have a hold, they never let go. [21:37.52] [21:50.00]It took over an hour to drown this full-grown bull. [21:54.48] [22:02.40]To surprise their prey [22:04.28] [22:04.40]crocodiles must strike with lightning speed. [22:08.40] [22:32.52]Here, only the narrowest line separates life from death. [22:39.28] [23:14.00]Most rivers drain into the sea, [23:16.60] [23:16.72]but some end their journey in vast lakes. [23:20.96] [23:23.76]Worldwide lakes hold twenty times more fresh water than all the rivers. [23:30.04] [23:32.04]The East African Rift Valley holds three of the world's largest: [23:36.92] [23:37.00]Malawi, Tanganyika, and Victoria. [23:41.24] [23:42.92]Lake Malawi, the smallest of the three, [23:45.60] [23:45.68]is still bigger than Wales. [23:48.08] [24:00.76]Its tropical waters teem with more fish species [24:04.48] [24:04.60]than any other lake. [24:06.32] [24:07.04]There are 850 different cichlids alone, [24:10.40] [24:10.48]all of which evolved from just one single ancestor [24:13.92] [24:14.00]isolated here thousands of years ago. [24:17.28] [24:31.36]These two-metre wide craters are fish-made. [24:36.08] [24:45.20]Fastidiously maintained by the males, [24:47.96] [24:48.04]these bowls are courtship arenas. [24:51.00] [25:03.68]Cichlids are caring parents. [25:07.32] [25:10.72]Brooding young in the mouth is a very effective way of protecting them. [25:15.56] [25:17.04]This lake can be a dangerous place. [25:20.12] [25:32.20]After dark, predatory dolphin fish emerge from their daytime lairs among the rocks. [25:39.20] [25:43.68]Like packs of sharks, they're on the prowl for sleeping cichlids. [25:48.88] [25:55.80]In the darkness these electric fish hunt [25:59.28] [25:59.36]by detecting distortions in the electric field they create around their bodies. [26:04.92] [26:22.80]Any cichlid that trenches out will be snapped up. [26:27.20] [26:47.88]The floor of Lake Malawi drops 700 metres into an abyss. [26:54.08] [27:01.28]Here, in this dead zone [27:03.12] [27:03.20]the larvae of lake fly midges hide out away from predators. [27:07.60] [27:11.12]In the rainy season they balloon up to the surface [27:13.88] [27:14.00]and undergo a magical transformation. [27:17.00] [27:26.56]At dawn the first adult midges start to break out. [27:31.12] [27:34.92]Soon, millions upon millions of newly hatched lake flies [27:39.08] [27:39.16]are taking to the wing. [27:40.92] [27:51.16]Early explorers told tales of lakes that smoked, as if on fire. [27:56.52] [27:58.04]But these spiralling columns hundreds if metres high [28:01.88] [28:02.00]are mating flies. [28:03.96] [28:17.76]Once the flies have mated, [28:19.48] [28:19.60]they will all drop to the water surface, [28:21.88] [28:22.00]release their eggs and die. [28:25.00] [28:34.48]Malawi may look like an inland sea, [28:37.52] [28:37.64]but it's dwarfed by the world's largest lake - [28:41.72] [28:42.80]Baikal in Eastern Siberia. [28:46.24] [28:56.24]400 miles long and over a mile deep, [28:59.28] [28:59.40]Baikal contains one fifth of all the fresh water [29:02.64] [29:02.76]found in our planet's lakes and rivers. [29:05.80] [29:08.80]For five months of the year it's sealed by an ice sheet over a metre thick. [29:14.72] [29:29.92]Baikal is the oldest lake in the world [29:32.68] [29:32.80]and, despite the harsh conditions, life flourishes here in isolation. [29:38.64] [29:39.88]80 percent of its species are found nowhere else on Earth, [29:43.32] [29:43.40]including the world's only fresh water seal. [29:47.60] [29:54.16]With this seal [29:55.36] [29:55.44]and its marine-like forests of sponges [29:58.12] [29:58.20]Baikal seems more like an ocean than a lake. [30:01.80] [30:20.44]There are shrimp-like crustaceans - giant amphipods - as large as mice. [30:26.60] [30:32.24]They are the key scavengers in this lake. [30:35.60] [30:35.68]The water here is just too cold for the bacteria that normally decompose the dead. [30:41.68] [30:55.04]Most rivers do not end in lakes [30:58.24] [30:58.36]but continue their journey to the sea. [31:01.04] [31:06.08]The planet's indisputable super-river is the Amazon. [31:11.24] [31:12.92]It carries as much water as the next top-ten biggest rivers combined. [31:19.12] [31:22.00]Rising in the Peruvian Andes, its main trunk flows eastwards across Brazil. [31:28.08] [31:28.24]On its way the system drains a third of South America. [31:32.92] [31:34.00]Eventually, over 4,000 miles from its source, [31:37.72] [31:37.80]it empties into the Atlantic Ocean. [31:41.04] [31:48.28]The Amazon transports a billion tonnes of sediment a year, [31:52.40] [31:52.52]sediment clearly visible at the mixing of the waters [31:56.44] [31:56.56]where one massive tributary, the Rio Negro, flows into the main river. [32:01.88] [32:07.88]Its waters are wonderfully rich. [32:10.40] [32:10.48]To date over 3,000 species of their fish have been described - [32:14.72] [32:14.80]more than in the whole of the Atlantic Ocean. [32:17.88] [32:45.36]The Amazon is so large and rich in fish [32:48.52] [32:48.60]that it can support fresh water dolphins. [32:51.76] [32:51.84]These botos are huge - two and a half metres long. [32:57.08] [32:58.92]In these murky waters they rely on sonar to navigate and hunt. [33:04.48] [33:21.52]They work together to drive shoals of fish into the shallows. [33:26.32] [34:27.36]Botos are highly social [34:29.28] [34:29.40]and in the breeding season there is stiff competition for mates. [34:34.04] [34:34.24]The males hold court in a unique way. [34:37.72] [34:48.72]They pick up rocks in their jaws [34:51.16] [34:51.28]and flaunt them to their attending females. [34:54.36] [34:58.88]Maybe each male is trying to show how strong and dexterous he is [35:03.00] [35:03.12]and that he therefore is the best father a female could have for her young. [35:08.24] [35:19.12]Successful displays lead to mating. [35:23.08] [35:37.32]Even for giant rivers like the Amazon [35:40.04] [35:40.16]the journey to the sea is not always smooth or uninterrupted. [35:45.48] [36:12.48]Iguassu Falls on the border of Brazil and Argentina [36:16.76] [36:16.88]is one of the widest waterfalls in the world - [36:19.80] [36:19.88]one and a half miles across. [36:22.80] [36:29.04]In flood 30 million litres of water spill over every second. [36:34.84] [37:20.44]All the world's great broad waterfalls: [37:23.76] [37:23.84]Victoria, Niagara and here, Iguassu, [37:27.56] [37:27.68]are only found in the lower courses of their rivers. [37:31.48] [37:39.28]In their final stages [37:40.92] [37:41.04]rivers broaden and flow wearily across their flat flood plains. [37:45.76] [37:46.68]Each wet season here, in Brazil, [37:48.80] [37:48.92]the Parana river overflows its banks [37:51.52] [37:51.60]and floods an area the size of England. [37:54.68] [37:58.24]The Pantanal - [37:59.52] [37:59.60]the world's largest wetland. [38:02.52] [38:09.36]In these slow-flowing waters aquatic plants flourish [38:13.72] [38:13.88]like the Victoria giant water lily with leaves two metres across. [38:18.84] [38:41.48]These underwater forests are nursery grounds for fish. [38:46.40] [38:47.92]Over 300 species breed here, including red-bellied piranha [38:53.00] [38:57.48]and other predators, like the spectacle caiman. [39:01.76] [39:27.60]Ripening fig trees overhanging the water's edge [39:31.08] [39:31.16]provide welcome food for shoals of hungry fish. [39:34.68] [39:41.32]The commotion attracts dorado, [39:44.00] [39:44.12]known locally as the river tiger. [39:47.40] [39:54.72]They patrol the feeding shoals, [39:57.08] [39:57.20]looking for a chance to strike. [40:00.00] [40:36.68]And waiting in the wings, [40:38.40] [40:38.52]ready to pick off any injured fish, [40:41.24] [40:41.36]are the piranhas. [40:43.24] [40:55.00]The feeding frenzy quickly develops. [40:58.20] [41:15.60]Piranha can strip a fish to the bone in minutes. [41:19.76] [41:26.20]Great numbers of fish sustain vast flocks of water birds. [41:32.28] [41:35.12]The rose-eared spoonbill is just one of the 650 bird species found in the Pantanal. [41:42.40] [41:49.20]They nest alongside wood stocks in colonies thousands strong. [41:54.68] [42:22.44]Spectacle caiman linger below, [42:25.04] [42:25.16]waiting for a meal to fall out of the sky. [42:28.08] [43:23.52]When rivers finally reach the sea [43:25.52] [43:25.64]they slow down, release their sediment and build deltas. [43:30.24] [43:32.08]In Bangladesh the Ganges and Brahmaputra rivers join [43:36.04] [43:36.12]to form the world's biggest. [43:38.16] [43:40.80]Every year almost 2 thousand million tonnes of sediment [43:45.24] [43:45.32]eroded from the Himalayas is delivered to the ocean. [43:49.12] [43:54.04]At the delta's mouth - the largest mangrove forest in the world, [43:58.36] [43:58.44]the Sundarbans. [44:00.20] [44:06.52]These extraordinary forests spring up throughout the tropics [44:10.64] [44:10.72]in these tidal zones where rivers meet the sea. [44:14.12] [44:24.28]Crab-eating macaques are mangrove specials. [44:27.72] [44:31.92]In Indonesia these monkeys have adopted a unique amphibious lifestyle - [44:38.80] [44:45.64]they fish out fallen food. [44:48.36] [45:11.32]The troop also uses the waters to cool off during the heat of the day. [45:17.04] [45:21.60]But the channels are also the playground for restless young macaques. [45:26.84] [45:32.32]Some of the young have even taken to underwater swimming. [45:36.60] [45:42.04]They can stay down for more than 30 seconds [45:44.92] [45:45.04]and appear to do this just for fun. [45:48.00] [46:00.08]Yet these swimming skills acquired during play [46:03.96] [46:04.04]will certainly be useful later in life in these flooded mangrove forests. [46:09.44] [46:17.64]In cooler climes, mud, laid down in estuaries, [46:21.36] [46:21.44]is colonised by salt marsh grasses [46:24.40] [46:24.48]and form one of the most productive habitats on the planet. [46:28.20] [46:57.24]400,000 greater snow geese flock to the estuaries along the Atlantic coast of the United States [47:04.40] [47:04.56]to rest and refuel on their long migratory journeys. [47:08.84] [47:44.72]This is the end of the river's journey. [47:48.00] [47:48.08]Collectively they've worn down mountains [47:50.72] [47:50.80]and carried them to the sea. [47:52.52] [47:52.60]And all along the way, [47:53.96] [47:54.08]their fresh water has brought life and abundance to planet Earth. [47:59.16]

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