In the offshore areas of the Black Sea , most new organic matter is produced by phytoplankton. Below some 20-50 metres there is usually insufficient light for significant phytoplankton growth. Phytoplankton includes a large number of species with different requirements for light and nutrients. Diatoms for example, have external skeletons of silica resembling little glass boxes. Dinoflagellates on the other hand are often capable of limited movement and have no requirement for silica. As environmental conditions change during the year, individual phytoplankton species tend to ‘bloom' and fade in a continuous ‘boom and bust' sequence. The larger phytoplankton cells are often ‘grazed' by small zooplankton such as copepods. There are smaller zooplankton however - ciliates and even smaller heterotrophic nanoflagellates - typically less than 10µm (there are 1 million µm in a metre). These graze on smaller nanophytoplankton or even heterotrophic bacteria. The smallest zooplankton are consumed by larger ones (such as copepods) and these, in turn, may become the food of small pelagic fish such as anchovy or sprats. Larger predator fish like mackerel or bluefish consume small pelagic species. At the top of the food chain are the ‘apex predators' of which dolphins and humans are the most notable in the Black Sea . Hundreds of thousands of dolphins were killed by humans from most Black Sea countries in the 1950s – 1970s for food or though the perception that they threatened fish stocks.

The pelagic food web is illustrated in Figure 1. It would be naive to show it as a linear chain because of heterotrophs and omnivores that can span more than one trophic level. In a stable ecosystem, the populations of organisms at each trophic level would be balanced by the supply of food from lower trophic levels and the intensity of predation from higher ones. Humans can affect this balance by increasing the supply of nutrients (eutrophication) or overfishing predator species. Eutrophication results in increased phytoplankton populations. Removal of predators results in increased small pelagic fish stocks and decreased zooplankton numbers. This may also increase phytoplankton numbers. The consequences are described in the chemistry fact sheet. The Black Sea has suffered from serious eutrophication in the past as a consequence of large nutrient loads from rivers. These loads result from agricultural fertilizers, animal and human sewage and certain industrial and domestic effluents.

In the Black Sea , large benthic organisms can only live in water that does not contain hydrogen sulphide. The north-western shelf of the Black Sea and the Sea of Azov are the largest areas suitable for benthic communities (some 20% of the overall area of the Black Sea ) but there is a narrower band of shallow water around the entire Black Sea where these communities can develop.

Benthic food webs are more complex than pelagic ones. Where light levels are sufficient and wave or current energy is low, benthic macrophytes may develop. These provide another source of organic matter and also act as a habitat for a large number of animals. The other special feature of the benthic food web is the use of organic matter that collects on the sea floor, some of it from decaying macrophytes and plankton, some from terrestrial systems and rivers that discharge to the sea, and some from the metabolic products and faeces of animals in the water column and sea floor. This organic matter and the heterotrophs that live on it, provides food for a great variety of animals such as shrimps, mullet and carp, as well as filter feeders such as mussels and oysters (these also feed on plankton). Some of these in turn, provide food for larger animals notably fish such as sturgeon or turbot or some crustaceans such as the swimming crab (another introduced species).

In the next page, there is an illustration of an overall food web for the Black Sea (Figure 2). This is a schematic representation - the reality is even more complex – but it helps to understand the interactions between the various components of the ecosystem and humans as one of the apex predators (along with dolphins and, to some extent, some seabirds). A similar diagram drawn thirty years ago would have included monk seals as an apex predator. Human pressure (including disturbance of their breeding sites) has probably led to their extinction in the Black Sea ; there have been no reliable reports on their presence in the last decade.

Fig 2. Illustration of some of the pathways in the Black Sea shelf ecosystem . The diagram is not a complete picture of the system and is designed to show typical interactions; there are many more!

The pelagic system (see also Figure 1) includes phytoplankton (1), zooplankton (2), small fish such as anchovies (3) and larger predator fish (4) and also larger predators such as the spiny dogfish (5). The pelagic system is coupled with the benthic system in a number of ways; examples shown here are through sea anemones (6) and mussels (7) feeding on small fish and plankton respectively (as well as heterotrophic bacteria in the case of some bivalves). Mussels have a few predators such as the sea snail Rapana (8) but contribute to the benthic food chain mostly through the production of detritus supplying bacteria and worms such as polychaetes (11). Many invertebrate benthic predators such as Rapana, crabs (bottom living and swimming crabs are illustrated, 9, as well as hermit crabs, 13) can feed off one another at different stages of growth and compete fiercely for space (some are also omnivores). Bottom living fish such as the flatfish (10) graze small benthic animals. Benthic macroalgae (12) and sea grasses (14) act as a habitat for a wide variety of animals (gobies and shrimp are illustrated, 15) and also provide a source of detritus for supplying the benthos with organic matter. One of the pelagic pathways, the pathway from zooplankton to jellyfish (17), leads to a ‘dead end' since they have no obvious predators in the Black Sea . Jellyfish are a normal part of the Black Sea fauna but the ctenophore species Mnemiopsis leidyi (left hand of 16) was introduced from the ballast water of ships in the mid 1980s. It now has a predator , Beroe ovata (not shown) another ctenophore introduced in the same way as its prey. Humans, dolphins and some seabirds share a place as apex predators (17) of the system. Not all of the pathways to humans are illustrated; they also remove benthic fish, crabs, sea snails, bivalves and small pelagic fish. Humans are also responsible for many of the sources of organic matter and nutrients that flow from the land to the sea, as well as many contaminants (18). This diagram is based on the work of Dr. Alexander Vershinin (in Life of the Black Sea , p145, Moscow , 2003, in Russian). Tools for graphics (also Fig. 1) were kindly supplied courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science.

The distribution of benthic marine organisms is largely determined by physical and chemical factors including the substrate (rocky, sand, mud, etc.), the wave or current energy, the salinity (amount of salt in the water), the oxygen content, illumination by sunlight, temperature, the supply of organic matter and the amount of nutrients in the water column. These factors contribute to their rather patchy distribution. Assemblages of plants and animals tend to form in particular range combinations of the above conditions (biotopes). In some senses they can be regarded as small ecosystems within the overall Black Sea system.

Each assemblage is usually characterised by a few ‘keystone' species that are completely indispensable to its existence; remove them and the assemblage can no longer survive. Table 2 illustrates one interpretation of the benthic assemblages on the north-western shelf of the Black Sea (this is fairly representative of the entire Black Sea ). With one exception, the key species cited are either bivalves or macrophytes. Both types of organism are vulnerable to environmental condition; macrophytes are limited by the available light and bivalves cannot survive for prolonged periods without dissolved oxygen.

Table 2: Types of assemblage found on the north-western shelf of the Black Sea . The assemblages are ordered by depth but there are a number of other factors determining their presence, notably the substrate type and the oxygen availability. Adapted from Kiseleva M.I. (1979) Bottom biocoenoses and their biomass. In: Fundamentals of biological productivity of the Black Sea . Kiev : Naukova Dumka, pp 218-239 (in Russian).

It was this vulnerability that led to the massive loss of a major part of the biota on the north-western shelf in the early 1970s. One of the consequences of eutrophication is impaired light penetration to the bottom of the sea (eutrophication increases phytoplankton and debris concentrations in the water, blocking the passage of light). As eutrophication increased, macrophytes beds shrank in size. Zernov's Phyllophora field on the north-western shelf (the largest red algal community in the world) collapsed spectacularly during the 1970s (see Figure 3).

The loss of Phyllophora and increased eutrophication led to even greater loading of organic matter to the bottom of the sea and decreased oxygen (see Chemistry fact sheet). Soon, much of the north-western shelf became hypoxic and even anoxic during summer months of each year and this led to the decline of many of the bivalve assemblages in the region. Thousands of tons of dead animals were washed up onto the shores of Romania and Ukraine .

Figure 3. Spectacular reduction of Zernov's Phyllophora field in the 1970s (data from Zaitsev, 1992)

Eutrophication and hypoxia are not the only reason for the decline of many Black Sea benthic communities. Introduced species have caused major changes in their structure. The softshell clam Mya arenaria (the keystone species of an assemblage cited in Table 2) was introduced in the second half of the 20 th century, possibly from the North Sea . The voracious predatory sea snail Rapana thomasiana arrived in the middle of the 20 th century and led to the massive loss of many benthic communities. Another reason for decline has been human disturbance by dredging for shellfish, the extraction of Phyllophora for agar and by the indiscriminate use of heavy trawls for fishing. Dumping of waste from some countries also occurred in the Black Sea in the past.

Despite all of these serious environmental problems, the Black Sea appears to be in gradual recovery from the worst period of eutrophication during the period from 1973 to 1990 (see section 4 of this fact sheet). This is more a consequence of economic problems in some of the coastal countries than good management however.

Fig 4. Map of major Black Sea wetland areas (from BSEP, Black Sea GIS, 1996)

The survival of many species of wading bird depends on the use of Black Sea wetlands as a resting place during their migration between the Arctic and Africa . The Black Sea is one of the most important migratory flyways in the world and there are indications that the number of birds in transit has decreased by over 50% in the past few decades.

There are many reasons why the Black Sea ecosystem deteriorated so rapidly from the 1970s. Eutrophication was a major factor, but not the only one. Many scientists believe that the current intensity of fishing in the Black Sea will not allow the full recovery of its food webs and are also concerned at the pace of destruction of marginal ecotones, including wetlands. Black Sea countries have joined a number of conventions designed to give additional protection to its biodiversity. These conventions include the Ramsar convention on wetlands, the UN Biodiversity Convention and the Bucharest Convention. There are a number of important initiatives to create protected areas (e.g. by the designation of coastal Ramsar sites) in all Black Sea countries. Unfortunately, there are also alarming accounts of wetlands and wild coastal areas being developed in a thoughtless way that puts the short-term profit of some people before the longer-term (and ethically and economically wiser) objective of development in harmony with nature.

One of the key tools for protecting marine ecosystems is the creation of networks of marine protected areas (MPAs). These areas act as a refuge for marine life and enable representative biotopes to be maintained in the hope that these will enable the entire system to recover when other stresses are removed. As yet, there are very few of these MPAs in the Black Sea and those that exist are very small in size and lack sufficient staff and funding to be fully effective. Hopefully, programmes such as the Black Sea Environmental Programme (of which the Global Environment Facility's Black Sea Ecosystem Recovery Project is part) will help the governments in the region to take appropriate measures to implement the various agreements they have signed up to.

The main lessons from this very brief introduction to the ecology of the Black Sea are:

•  Marine ecosystems are constantly adapting to natural changes in geology and climate.

•  The Black Sea 's biota is a result of tens of thousands of years of adaptation but it has suffered impacts of unprecedented severity in the past 50 years due to human activities.

•  There is recent evidence that some of the biotopes are beginning to recover in the last decade, which demonstrates that something can really be done to protect the sea from further deterioration.

•  However, the components of the ecosystem are linked in a complex manner and action to protect it must take into account these linkages.