Partial loss of habitat

Sedimentation is continuously stated as a major threat to marine ecosystems. Forest clearing, land development, land reclamation, agricultural run off, channelisation of waterways, mining, and dredging are all activities that add to sedimentation rates. Ecosystems that rely on clear water are the most susceptible to the increases in water turbidity caused by sedimentation. Seagrasses and coral reefs are the primary victims as they both rely on photosynthesis which is impossible without high levels of light. Sedimentation can smoother and kill both seagrass beds and coral reefs.

Examples include:

• Tracts of reef in Guam have been killed by sedimentation as a result of road construction.
• Nickel mining and bush fires have lead to intensive sedimentation around the New Caledonia barrier reef systems, a problem exacerbated by the destruction of sediment-retaining mangroves. Some reefs have been buried under more than 8 metres of silt.

Stress and depletion of these ecosystems causes a cascade of loss effecting all species that rely on them. Seagrasses and coral reefs are some of the most biodiverse and productive ecosystems in the world and losses within them could be catastrophic for the many user groups and communities that rely on them.

Eutrophication, caused by nutrient loading in coastal waters from land runoff, can instigate physical, chemical, and biological changes to occur in fauna and flora communities.

Nuisance algae growth due to high levels of plant nutrients. Courtesy of NOAA.

Fish kills due to low oxygen levels cause by eutrophication. Courtesy of NOAA.

Nutrients accumulate that the ocean does not have the capabilities to decompose. Marine phytoplankton is often limited by the amount of nitrogen (as nitrate or ammonia) rather than phosphate so fluxes in nitrogen, usually from rivers, cause phytoplankton blooms. Phytoplankton is short lived and eventually falls to the seabed to decompose. Higher biomasses will use up more oxygen creating zones of hypoxia (very low oxygen levels) where fish and other marine organisms cannot survive and fish kills have been known to occur. This decreases fish populations that communities rely on for food and commercial fishing. And the excessive decomposing algae can cause foul recreational areas and cause odour problems as it decays.

Visible differences in the transparency of coastal waters can be another consequence of eutrophication. Between 1930 and 1980 the transparency of the sea water in the Northern Baltic decreased by as much as 3-4 metres.