Danube Delta

• Category: Inland Aquatic Biomes.
• Geographic Location: Eurasia.
• Summary: This freshwater-dominated estuarine deltaic complex is threatened by land use, pollution from upstream areas, and climate change; conservation efforts are in place to address further decline.

Central Europe's Danube River flows through or acts as a border for ten countries, from the Schwarzwald (Black Forest) mountain range in Germany southward and eastward to the Black Sea. The modern Danube Delta, where the river meets the Black Sea, is a primarily freshwater-dominated estuarine ecosystem complex about 6,000 years old. The Danube Delta is home to more than 300 bird and 45 fish species, is the 2nd largest delta in Europe, and spans two countries and 1,603 square miles (4,152 square kilometers). The southern portion, in Romania, encompasses approximately 1,331 square miles (3,446 square kilometers), and the much less extensive northern section, in Ukraine, covers about 273 square miles (706 square kilometers).

Water systems in the delta include the Danube arms, the main distributaries that flow into the Black Sea, and secondary dead-end distributaries (creeks, backwaters, and dredged channels), which do not reach the Black Sea. There are three primary arms of the Danube Delta: the Chilia, the northernmost arm and the only arm that flows through Ukraine; the Sulina, the most direct arm; and the Sfântu Gheorghe arm, the river's farthest point south. Together with the nearby Razelm-Sinoie, an ecologically similar complex of lagoons physically separated from the delta by a major sandbar, the Danube Delta covers approximately 2,042 square miles (5,290 square kilometers).

Based on climate alone (without water inputs from flooding), the delta would be characterized by xeric (arid-evolved) scrub and herbaceous-dominated ecosystems. For this reason, elevation relative to both sea level and the river itself is an important factor in hydrology and ecology. Variation in elevation ultimately results in 30 distinct ecosystem types (23 natural, seven anthropogenic), with natural ecosystem types ranging from open-water lakes to upland forests. Although somewhat degraded by dams, land use, eutrophication (excess nutrient input), and overfishing, the Danube is considered the best-preserved large deltaic system in Europe.

Biodiversity

Areas near the Black Sea are affected by various amounts of tidal salt inflow and typically are dominated by halophytic (salt-loving) plants, such as glasswort and alkaligrass. Estuarine wetlands provide critical habitat for many fish and bird species that rely on tidally influenced ecosystems for foraging and nesting. The coastal lagoons of Razelm-Sinoie vary in salinity and provide habitats in a broad range of conditions, leading to greater biodiversity.

Running waters encompass a small but important part of the delta. These waters serve as a habitat for many plant species, including attached varieties such as mint, submerged aquatic ones like hornworts, and floating macrophytes including fern and duckweed. The underlying substrates are important habitat for invertebrates, such as worms and mollusks, which serve as important food sources for historically important commercial fish species such as the Danube mackerel and Danube sturgeon (fishing for the latter fish has been suspended due to overexploitation).

The ecological structures of lakes, ponds, and lagoons can be quite complex; submerged aquatic vegetation, such as Eurasian watermilfoil, may dominate shallow ponds. Vegetation in deeper lakes and lagoons is generally characterized by floating macro- and microphytic species such as the edible water chestnut, the carnivorous bladderworts, and phytoplankton. In these deeper systems, detritus and associated bacteria are particularly important components of trophic structure, making up a large part of the base of a food web that supports important fish species such as tench and perch.

A large expanse of the delta, as much as 70 percent, is covered by marshes, primarily dominated by reed, Carex sedge, and cattail. Although many of these reed swamps or plaur are attached to the underlying substrates, a large portion of this marshland is actually rooted in a thick peat mat that floats on top of the underlying water table. The plaur are generally located along lake margins. It is expected that over time, the plaur will move inland, over the lake edges, and the areas that are currently occupied by these floating mats margins will succeed to drier, more upland habitat.

These transitional marsh zones are an extremely productive and particularly important ecosystem type, serving as habitat for many native waterbirds (such as ducks, geese, cormorants, egrets, pelicans, and pheasants) and carnivorous mammals (such as otters, mink, foxes, and wolf species). Since human settlement in the region, and particularly over the past 50 years, exotic mammal species (tanuki, muskrat, nutria, feral cat, and feral hog) have established populations and could pose problems for native species.

Where the banks of the primary and secondary distributaries are shallow, riparian wetlands dominate. Areas of the floodplain closest to the river may be inundated frequently and are often dominated by marsh vegetation. These riparian marshes are similar in ecological function to the plaur at lake margins. Farther away from the river, woody shrubs and small tree species begin to dominate as the marshes grade into periodically flooded swamplands, with primarily early successional species, such as willow and white poplar.

In some areas, banks are steep enough that the swamps eventually grade into forested stands that are rarely flooded. These forests may contain a network of jaspas—low-lying ephemeral pools that remain under standing water following river flooding. Sand dunes along river banks protect forested stands on the highest levees from flooding, allowing later-successional oak species to grow in the intervening low-lying spaces, called hasmace. Letea and Caraorman forests are notable for their extensive hasmace, which encompass oak and ash forests replete with a diverse array of understory species, such as blackthorn and wild hop. These areas also serve as habitat for the endangered Danubian meadow viper. The dunes are sparsely vegetated with stress-tolerant grasses such as fescue.

Effects of Human Activity

Most of the delta is not densely populated. The Romanian city of Tulcea, however, located at the western edge of the area, has a population of approximately 65,000 people, while the Ukrainian city of Izmail, on the northeastern rim, is home to some 85,000 people. There are several smaller towns, villages, and rural settlements in the region's less densely populated areas that are more integrated into the surrounding landscape and habitat matrix.

Even areas that are not currently settled may have been converted for human use. Many of these constructed ecosystems are the result of draining surrounding marshes and other wetlands. Drained wetlands are quite fertile and are used for farming and forestry. In some cases, such converted lands later become abandoned, resulting in ecosystems with novel biological communities. Elsewhere, overfishing of many lakes has caused declines in fish stocks. As a result, some wetlands are not drained but dammed, resulting in human-made ponds used for fish farming. These fish-farm ecosystems may be quite unstable, and nutrient-rich feeds may result in eutrophication.

Major alterations of the delta channels had occurred since at least the 19th century when the Sulina arm was channelized to ease navigation. For a large part of the 20th century, the delta was controlled by the Soviet Union. As part of plans to convert the entire delta into an agro-industrial complex, the Soviet Union oversaw extensive wetland drainage.

An additional problem is nutrient loading from agricultural runoff, which is far upriver. Nutrients in fertilizers from fields as far away as Germany may find their way to the Danube Delta, where they contribute to wetland eutrophication. This process may be particularly devastating to floating plaur, for which buoyancy depends on slow decomposition. Similar to the problem of agricultural runoff, other forms of pollution from the many cities upriver may be deposited in the delta. Also, erosion is slowly driving the loss of land as sediments become deposited in the Black Sea. The effects of erosion will become exacerbated over the next century by rising sea levels, which will also cause longer periods of inundation with saltwater.

Global warming and climate change also threaten the region. According to the International Community for the Protection of the Danube River, global warming will cause the air temperature to increase by 1 to 1.5 degrees Celsius, causing more frequent heat waves and cooler winters. Water temperature will rise by 2 degrees Celsius, which causes algae growth and reduced oxygen levels in the water, which will reduce fish stocks. Small rivers in the region will have 5 to 25 percent less water, especially in the summer. Consequences of climate change and increased temperatures could include flooding, longer periods of drought, deteriorating water quality, and declining fish populations.

Conservation Efforts

In anticipation of and response to these challenges, the United Nations has established the entirety of the Danube River Delta and the Razelm-Sinoie coastal lagoon complex as a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Biosphere Preserve site. Similarly, the Romanian government established the delta as a Natura 2000 site, barring actions that could significantly affect its species and habitats. Still, the system is not safe from alteration. Despite the delta's unique ecology and natural beauty, the Ukrainian government currently has long-term plans to further expand navigational channels.

Bibliography

Ganea, Iolanda-Veronica, and Alexandru Sabin Badarau. "How a Romanian Village Resurrected the Danube Delta after the Fall of the Iron Curtain." The Conversation, 21 Nov. 2021, theconversation.com/how-a-romanian-village-resurrected-the-danube-delta-after-the-fall-of-the-iron-curtain-169180. Accessed 26 Oct. 2024.

Panin, Nicolai, and Dan Jipa. "Danube River Sediment Input and its Interaction With the North-Western Black Sea." Estuarine, Coastal and Shelf Science, vol. 54, no. 3, 2002, pp. 551-62.

Sarbu, A. "Inventory of Aquatic Plants in the Danube Delta: A Pilot Study in Romania." Archiv für Hydrobiologie Supplement, vol. 147, no. 1-2, 2003, pp. 205-16.

Unluata, Umit. Environmental Degradation of the Black Sea: Challenges and Remedies. Kluwer Academic, 1998.