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The distribution pattern of the Southeast Asian freshwater fishes can be divided into five zoogeographic regions. The first one is the Salween basin in Burma, with fishes mainly of the Indian subcontinent origin such as Amblypharyngodon atkinsoni, Bangana almorae and Brachydanio jayarami. The second zoogeographic area is the Mekong, Chao Phraya and Mae Khlong drainages which harbour fishes typical of the mainland of Southeast Asia such as Acanthorhodeus deignani, Barbichthys nitidus and Cirrhinus siamensis. The Malay Peninsula is the third region whose species composition is heavily influenced by the Siamese and Indonesian elements. The islands of Sumatra, Borneo and Java are the fourth zoogeographic area of fish distribution.

There is still disagreement about the taxonomy of the Pearl and Yangtze River populations. Adults begin spawning at about age 14 years and 21 years , and adults spend over 15 months in the river for reproduction. Sinensis were found every year since 1982 below the Gezhouba Dam, but it seems that insufficient suitable ground is available for spawning. Since 1983, commercial fishing has been prohibited but more measures need to be taken such as establishing protected areas and characterizing critical spawning, summering and wintering habitats. In a similar study on seasonal variations in benthic invertebrates, Dudgeon found hydrologic regime and geomorphology in stream environments, as the key determinants of their distribution and abundance.

Any changes within this system are most likely to influence the transmission potential. Species specific parasites in endangered animals will die out with these hosts. Parasites whose host population benefits from environmental changes, such as generalist and synanthropic species, will have transmission advantages due to the increased frequency with which they encounter suitable hosts.

Stream community metabolism switched between autotrophy and heterotrophy, depending on the degree of cloud cover. Successive cloudy days may be sufficient to result in anoxia in the stream, particularly in the benthos, where limited oxygen penetration into the sediments was recorded. Stable-isotope data indicated that little of the primary production from sugar cane or other C4 plants was transferred into the aquatic food web. The only significant contribution of C4 carbon was to the diets of some larger predatory fish, which must be directly dependent on terrestrial prey. In the absence of significant riparian inputs of C3 carbon, it appears that in-stream primary production supported the aquatic invertebrate community.

Each has a distinctive fish association with a relatively small number of frequent species. Blackwater associations can be divided into subassociations corresponding to different types of blackwaters. Tree country and riceland associations show some regional differentiation.

Kaluga Huso dauricus and Amur sturgeon Acipenser schrenckii are endemic to the Amur River. Both species still support fisheries, but stocks are declining due men’s metro health to overfishing. Acipenseriformes of the Yangtze River are primarily threatened by hydroelectric dams that block free passage to spawning and feeding areas.