Marine biodiversity describes the diversity of life in the seas and oceans. The term ‘Biodiversity’ (or Biological diversity) describes the variety of living organisms and is considered at the following scales:

Genetic biodiversity

This refers to the total number of genetic characteristics in the genetic makeup of a species. The greater the genetic diversity of a species, the more easily it can adapt to changes of the environment and therefore has a greater chance of survival.

Species biodiversity

This describes the number of different species that live in an area and is the parameter that is commonly used for estimating biodiversity.

Ecosystem diversity

This describes the variety of ecosystems in a specific area.

Why is marine biodiversity important?

Oceans and seas cover 71% of the earth’s surface and play a key role in influencing both local and global climates. Marine ecosystems provide a variety of goods and services which are used by human beings including food and energy. Therefore it is important to protect marine biodiversity and use it in a sustainable way.

Mediterranean Marine biodiversity

Mediterranean ecosystems are among the richest on earth in terms of species numbers. Although the Mediterranean Sea covers only 0.8% of the earth’s surface and accounts for 0.32% of water in the world’s oceans and seas, it hosts over 12,000 species of plants and animals. 25% of these species are found exclusively in the Mediterranean Sea.

Black Sea Marine biodiversity

The Black Sea supports approximately 3,700 species. The north-west is well known for its Phyllophora fields (a type of seaweed), whilst rocky biotopes and biogenic reefs are present in southern Crimea, the Caucasus coast, Anatolia and some capes in the south-western Black Sea. Coastal wetlands play an important role in the Black Sea ecosystem and cover approximately 10,000 km2. These areas provide reproduction and feeding grounds for rare and commercially valuable fish species as well as for wintering, migrating and nesting bird species.

Threats of marine biodiversity

Marine biodiversity is suffering a significant decline and increasing damage is caused to marine ecosystems. Much of this damage is caused by fishing, shipping, coastal development and pollution. These issues and the risks they pose are discussed in detail below:

Habitat loss or alteration

Alteration, partial loss or total loss of a habitat not only impacts the species that depend on the habitat but also the wider ecosystem. When habitat fragmentation occurs, populations may become isolated. These isolated populations may be too small to survive and risk becoming locally extinct. Habitat fragmentation also impacts ecosystems and degrades the wider marine environment.

Overexploitation of Resources (e.g. overfishing)

The overexploitation of resources is a common threat to marine ecosystems. Over fishing regularly occurs which negatively effects individual populations and the ecosystems they are part of. When large numbers of fish are removed before they have finished their breeding cycle, the populations decline dramatically. This can cause population explosions in species further down the food chain and equally effect species further up the food chain as their sources of food declines. These types of alterations can cause drastic changes in marine ecosystems as energy flows and processes are disrupted and changed.

Food Chain in the Sea
Pollution

The main types of pollution impacting marine biodiversity are:

  • Toxic discharges: These are metals, organic chemicals and suspended material, found in industrial waste and urban sewage that are directly driven into the sea. Toxic discharges can impact basic biological processes (feeding, reproduction, etc) by weakening or poisoning living organisms.
  • Bacterial infection: for example, faecal coliforms in urban waste water discharge.
  • Eutrophication (excessive concentration of nutrients): This is mainly caused by phosphates and nitrates that are discharged from fertilizers used in agriculture. These naturally occur in sea waters at very low concentrations, but when concentrations are increased by human activities a rapid growth of algae and aquatic plants is often observed. This limits the amount of oxygen and light available to other organisms in the ecosystem, causing an overall reduction in biodiversity and drastic changes in marine habitats.
Climate Change

Climate change is a real threat to marine biodiversity. Though marine species are adapted to different climatic conditions, the speed at which change is occurring is a serious threat, particularly to slowly evolving communities. Changes in sea temperatures and salinity levels can impact species and habitats by altering population dynamics. Plants and animals are especially sensitive to fluctuations of sea water temperature at the early stages of their development. Research on fossils as well as paleobiology studies have shown that time periods of rapid climate change are associated with mass extinction events.

Acidification of Sea Water

Increased concentrations of CO2, both in the atmosphere and in the sea, is a major threat to global biodiversity. Marine plants can absorb carbon from the atmosphere but this can have negative impacts on marine ecosystems. As marine ecosystems absorb more CO2 the pH level of the sea decreases and becomes more acidic. This is known as 'acidification' and it threatens the survival of a large number of marine species. The most threatened species are those who consist of Calcium Carbonate (CaCO3), such as corals, molluscs and phytoplankton (Coccolithofors). High acidity levels in the seas make it difficult for marine calcifying organisms to form, causing deformations and stunted growth whilst also causing mass coral bleaching events, leading to losses of living coral.

  • Phytoplankton (Coccolithofors). Right = with the current CO2 concentration and Left = with CO2 concentrations forecasted for 2100.
Species Invasion

Non-native or exotic species are organisms that move from their natural habitats via human induced activity or changes and establish in areas outside of their normal range. Some non-native species compete aggressively against native species and can eventually replace them. These species are known as ‘invasive species’.

Invasive species impact marine ecosystems as they alter the food-chains and increase the risk of introducing new diseases, which can destroy sensitive native species. They also bring sudden changes in biodiversity and in the relative abundance of species. This causes a reduction of the number of native species in a habitat while they are replaced with "opportunistic" species, causing the homogenisation of ecosystems.

Due to relatively recent colonisation, the Mediterranean Sea seems to be more susceptible to new colonisers than older evolutionary ecosystems, such as the Atlantic, where the available space for new species is limited and competition among them is high.

Non-native invasive species are entering the Mediterranean Sea through the Gibraltar Strait and Dardanelles Strait. Human activities also result in non-native species invasion in the Mediterranean Sea through aquaculture and transport of species on ship hulls and ballasts. In the eastern Mediterranean Sea, tropical species from the Indo-Pacific Ocean are becoming more common as they enter through the Suez Canal.

The Black Sea is also susceptible to invasive species colonisation. Forty-onetourismon-native species are known to exist in the region with this number continuing to increase. The main causes of these introductions are species imported for aquaculture and accidental introductions through ballast water and organisms on ships hulls. Comb jellyfish Mnemiopsis leidyi is the most well-known of the non-native species present in the Black Sea. The species has gained huge biomass and feeds on zooplankton and fish larvae. Decomposing individuals are also creating hypoxic (low oxygen conditions) in some areas, making the environment inhospitable for native species.

  • Nudibranch on the alien alga Caulerpa racemosa

The rise of mean sea temperature due to climate change plays an important role in species invasion as it facilitates the entry of new species into ecosystems. Species whose range was previously limited by sea temperatures are now able to colonise new areas.

The arrival of non-native species can have economic impacts since their existence increases the risk of extinction of commercially valuable native species. Tourism can also be impacted by the introduction of non-native species which are harmful to human health.

Marine Traffic

Deep-sea and shallow-water habitats are highly sensitive environments which are vulnerable to the impacts of marine traffic. Enclosed seas, such as the Mediterranean and Black Seas, are particularly vulnerable, due to large number of shipping routes and high volume of traffic.

During the last fifty years, shipping has been greatly expanded in the Mediterranean and Black Sea basins. From 1985 to 2001, a 77% increase of the volume of ship movements through Mediterranean ports was recorded. The estimated total of 200,000 commercial ships cross the Mediterranean Sea annually and approximately 30% of international sea-borne volume originates from or is directed towards the 300 ports of the Mediterranean Sea. These values are expected to multiply up to three or four times in the next 20 years.

As a result impacts on marine biodiversity are expected to increase in the future. The most common impacts expected are: pollution from ships and collisions, damage caused by grounding and anchoring and the transport and introduction of non-native species.

Noise pollution is also a problem in areas with heavy traffic and developed coastal zones. Noise pollution is caused by marine traffic, marine developments and exploration (oil rigs, wind turbines, etc). Intense underwater noise is generated by airguns, widely used for geophysical exploration for the oil and gas industry as well as for military or civilian purposes (high-power sonars).

The high noise levels may directly injure animals even far from the source of the sound, since sound travels five times faster in water than in air. Due to the density of seawater, acoustic energy transmits very efficiently over much greater distances than in air, the effects of underwater noise may be felt over very large distances.

Marine mammals are especially sensitive to noise pollution as they use sound to communicate; navigate; find prey; avoid obstacles, predators and other hazards. Noise pollution can produce temporary or permanent hearing loss, forcing them to abandon their habitat, change their behaviour, or mask their acoustic signals over large areas.