Does sewage discharge into the sea harm marine life?

Answer 1: 

Pollution from oil sewage and industrial wastes is the biggest threat to ocean eco system.

Coral reefs teem with variety of wild life. The waters are not rich in nutrients but the reef inhabitants recycle them quickly so that nothing is wasted. Algae live in the body of the corals and they need sunlight to make food. They can live in only warm clean water less than 30 m (100 ft) deep so that sunlight reaches them.

Coral reefs are threatened by pollution. Human excreta has the worst effect on the aquatic environment. Some corals, which are part of the world's third largest barrier reef, have been found to be tainted with bacteria and viruses found in human faeces.

Researchers also report that drugs flushed into water ways can kill or maim the plankton that are the mainstay of freshwater eco systems. Pollution, over fishing and global warming have also been found to kill the corals' symbiotic algae. Sewage can also provide nutrients to fuel the growth of algae that attacks corals.

Further research done in the U.S. has revealed that high concentrations of chemicals from drugs to bug repellents and narcotics kill zooplankton — the tiny animals that are an integral part of the food chain in ponds and streams.

A combination of human sewage and shipyard discharge may be responsible for the development and spread of deadly black band disease in corals, say American researchers. "Black band disease is characterised by a ring-shaped bacterial mat that migrates across a coral colony, leaving dead tissue in its wake," say geologists. Diseased corals may be experiencing increased environmental stress due to pollution, which in turn decreases the coral's resistance to bacterial infection.

Healthy corals contain a natural population of bacteria within a mucous-rich biofilm that provides protection from light, exposure and sedimentation. "Environmental stresses cause corals to secrete more of this mucous to coat their outer tissues. This leads to elevated levels of natural microbial populations, as well as the introduction of new, potentially harmful bacterial populations." "Like a tropical rainforest, a coral reef system is a cradle of biodiversity. If we destroy the reefs, we destroy the ocean's ability to reproduce."

Answer 2: 

Sewage wastes are categorised under organic pollutants. Dissolved oxygen (DO) is essential for sustaining the plant and animal life in any aquatic system. When the sewage materials undergo degradation by bacterial activity in presence of dissolved oxygen deoxygenation process results which leads to depletion of DO.

Also small amounts of nitrates and phosphates occur in all aquatic systems which are very essential to maintain a balanced biological growth. In wastewater, these nutrients are present in abundance as nitrates, phosphates, ammonia or combined organic nitrogen. Thus, when large concentrations of nutrients are present in water bodies, an excess growth of algae, known as algal bloom appears. Algae bloom is a heavy growth of algae in and on a body of water as a result of high phosphate concentration from farm fertilizers and detergents. This produces an unsightly green slimy layer over the surface of water body. The slimy layer reduces light penetration and restricts atmospheric reoxygenation of the water. The dense algal growth eventually dies and the subsequent biodegradation produces an oxygen deficit which can result in foul-smelling anaerobic condition.

Source : The Hindu