Daphnia magna with eggs
Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style (although fleas are insects and thus only very distantly related). They live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.
The lifespan of a Daphnia does not exceed one year and is largely temperature dependent. For example, individual organisms can live up to 108 days at 3°C while some organisms live for only 29 days at 28°C. A clear exception to this trend is during the winter time in which harsh conditions limit the population in which females have been recorded to live for over six months. These females generally grow at slower rate but in the end are larger than ones under normal conditions.
Sometimes Daphnia may be used in certain environments to test the effects of toxins on an ecosystem. This makes Daphnia an indicator species, particularly useful in that area because of its short lifespan and reproductive capabilities. Because they are nearly transparent, their internal organs are easy to study in live specimens (an example might be to study the effect of temperature on the heart rate of these ectothermic organisms). They are often fed to tadpoles or small species of amphibians such as the African Dwarf Frog, Hymenochirus biettgeri. Daphnia are also a popular live food in tropical and marine fish keeping.
Several water flea species are considered threatened. The following are listed as vulnerable by IUCN: Daphnia nivalis, Daphnia coronata, Daphnia occidentalis, and Daphnia jollyi. Some species are halophiles, and can be found in hypersaline lake environments.
Chlorine is a toxic gas that irritates the respiratory system. Because it is heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Chlorine gas is a strong oxidizer, which may react with flammable materials.
Chlorine is detectable in concentrations of as low as 1 ppm. Coughing and vomiting may occur at 30 ppm and lung damage at 60 ppm. About 1000 ppm can be fatal after a few deep breaths of the gas. Breathing lower concentrations can aggravate the respiratory system, and exposure to the gas can irritate the eyes.
Chlorine's toxicity comes from its oxidizing power. When chlorine is inhaled at concentrations above 30ppm it begins to react with water and cells which change it into hydrochloric acid (HCl) and hypochlorous acid (HClO).
When used at specified levels for water disinfection, although chlorine reaction with water itself usually doesn't represent a major concern for human health, other materials present in the water can generate disinfection by-products that can damage human health.
Chlorine gas has also been used by insurgents against the local population and coalition forces in the Iraq War in the form of Chlorine bombs. On March 17, 2007, for example, three chlorine filled trucks were detonated in the Anbar province killing two and sickening over 350. Other chlorine bomb attacks resulted in higher death tolls, with more than 30 deaths on two separate occasions. Most of the deaths were caused by the force of the explosions rather than the effects of chlorine, since the toxic gas is readily dispersed and diluted in the atmosphere by the blast. The Iraqi authorities have tightened up security for chlorine, which is essential for providing safe drinking water for the population.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For more information (background, pictures, experiments and references): Daphnia, Chlorine