Threats To Marine Biodiversity

Source: Food and Agriculture Organization of the United Nations, 2013, Caroline Hattam, Plymouth Marine Laboratory,   "The Youth Guide to Biodiversity" 1st Edition (Chapter 8) Youth and United Nations Global Alliance. Reproduced with permission.

Chapter 8. Verbatim.

The number of marine dead zones has increased

over the decades.

Source: GBO-3, 2010

Marine biodiversity faces a number of threats that are causing changes in the mixture of species, where they are found, and in some cases, extinction. The IUCN lists 27 percent of corals, 25 percent of marine mammals and over 27 percent of seabirds as threatened. Particular threats include over-fishing (see box: “Fishing Down the Food Chain”), pollution (see box: “Pollution and Dead Zones”), climate change and ocean acidification and invasive alien species.

Climate change is leading to changing ocean temperatures, which in turn, cause species to migrate. In the northern hemisphere, some cool water species are moving north, while in the southern hemisphere cool water species are moving south. Warm water species are expanding their distribution into areas where cool water species once lived. 

Climate change is also affecting the pH (or level of acidity) of the sea. As more carbon dioxide (CO2) enters the air, more CO2 is absorbed by sea water through a natural chemical reaction. This is causing the sea to become more acidic. The full effects of this are not known, but scientists think that it will affect the building of shells or calcium-rich structures (such as corals) and reproduction in many species.

The combined effects of over-fishing, pollution and climate change are making it much easier for non-native species to establish themselves in new areas. Many of these alien species are not problematic, but some are; they are said to be invasive. Invasive alien species can be very difficult to eliminate of and may out-compete native species causing the whole ecosystem to change. Ships are the main culprits in the spread of invasive alien species, transporting them unintentionally on their hulls or in their ballast water. 

Fishing Down The Food Chain

Most of the fish we prefer to eat are large, slow-growing species (e.g. cod, tuna and snapper). As their numbers are falling, fishers are changing the species that they catch. They are increasingly catching smaller fish (e.g. mackerel and sardines) that are closer to the bottom of the food chain.These smaller fish may be the prey of the larger fish, so removing these smaller fish threatens the recovery of the larger fish. An example is cod fishing in Norway.  

As cod numbers decreased, fishers started targeting pout. Pout feeds on krill and copeopods. Krill also feed on copeopods, as do young cod. As the pout were caught, krill increased causing copeopods to decrease. The young cod then found it difficult to find food, making the recovery of the cod population even more difficult.

Pollution  And Dead Zones

Pollution enters the marine environment through a number of routes. It may come from ships as they move around the oceans, from the land (e.g. from industrial outlets, sewage outfalls and runoff from roads) and from rivers. It includes rubbish, sewage and many different chemicals, such as fertilisers, oil and medicines.

Pollution carried in rivers is becoming particularly problematic and in some cases, where river water containing high levels of fertilisers reachesthe coast, it is leading to “dead zones”. Dead zones are becoming much more common in coastal waters around the world. About 200 have been identified so far. Some come and go with the seasons, but others are permanent. The most well-known is found in the north of the Gulf of Mexico where the Mississippi River meets the sea. At its largest it covered 22 000 km2.

Dead zones appear when fresh water carrying lots of nutrients meets the sea. The fresh water floats on top of the salty water and prevents oxygen moving down. In the spring and summer, phytoplankton grow and multiply rapidly because of the high level of nutrients. Some of the phytoplankton are difficult to digest and may produce poisonous substances.

This means less food is available for other marine life.As the phytoplankton die, they fall to the seafloor and are broken down by bacteria. This process needs oxygen. As no oxygen can get through to the seabed, the oxygen already there gets used up quickly and the seabed is said to become hypoxic (almost no dissolved oxygen is present). Marine animals and plants, like those on land, need oxygen to survive. Those that can move leave the area, but those that cannot are left to die.