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From delta to Black Sea, the color of Danube waters seen from space

In large river estuaries where hydrological regimes are complex, satellite observation of water turbidity helps studying delta dynamics and sediment transport towards the coastal zone.


Rivers bring to the sea huge amounts of mineral particles and of organic matter. In spite of its role regarding phytoplankton productivity, nutrient dynamics and the transport of pollutants, the time and space dynamics of waters flowing from the mouths of rivers remains most often poorly understood. There are various techniques to measure the transparence of water or its load in solid particles, but their practical use on the field may be limited in some environments. In this work, researchers used satellite remote sensing techniques to study the spatial and temporal distribution of turbidity in the Danube delta and the adjacent coastal zone.

The Danube delta seen from the sky


The Danube river extends over ten European countries and brings 59 % of the freshwater and 48 % of the solid materials reaching the Black Sea. Its delta covers more than 4,000 km² and is a complex hydrological system associating three main branches, a network of natural or artificial channels and about 300 lakes.


The studied area: Danube delta plain and coastal zone

The study covers four years: 2006 and 2007 (where uncommon events were observed, respectively inundations and low water levels), 2008 and 2009 (typical hydrological years). At sea, the MERIS sensor of satellite Envisat provided 52 images of medium ground resolution (300 m) but covering fifteen spectral bands. For the delta, the study used 32 images from various sensors of higher ground resolution (10 to 30 m) but with a lesser coverage of the spectrum portions appropriate to the study of water surface. For the coastal zone, the main two constituents of turbidity (suspended mater and plankton) were estimated separately by computations taking into account the various spectral bands. For the delta, a turbidity index was computed for each pixel by comparison with two reference sites chosen for the constancy of their optical properties.

The seasonal variations for a "typical" year (2008) were described:
- in winter, the Danube has a low discharge but very turbid waters bringing important amounts of suspended to the lakes. In the coastal zone, highly turbid plumes are rapidly oriented southwards by winds and currents: thence they keep very close to the shore and seldom extend farther than 15 km offshore.


Typical winter situation (February 2008) in the delta (left) and at sea (right)

- in spring, river water level peaks in April-May whereas lake turbidity starts to decrease, and most of them have very clear waters when the maximum level is reached. In the coastal zone, winds and currents still keep the plumes close to the shore in March and April, but short periods of considerable plume offshore expansion (up to 40 - 70 km) can be observed, with a maximal extension in late spring or early summer.

Typical early summer situation (early July 2008) in the delta (left) and at sea (right)

- in late summer, turbidity is usually low in both areas; in the coastal zone, plumes seldom extend beyond 10 km offshore. After September, conditions become less stable again, mainly at sea.

Typical late summer situation (late August 2008) in the delta (left) and at sea (right)

Turbidity is not controlled only by the river and the materials its carries: phytoplankton, aquatic plants and wind also play a role.
- high phytoplankton concentrations can strongly reduce water transparency in the lakes; this was observed in summer and autumn 2007, and generally from June to October but never in spring. The contribution of phytoplankton to global turbidity is probably minor in the coastal zone.
- aquatic plants growing in the lakes begin to develop in April and act as "filters" of suspended matter coming from the river distributaries; this could explain the low values of turbidity observed in most lakes in late spring and early summer. These plant communities are generally stable until the end of September, before their decomposition reduce their role of filter to almost nothing after November.

- turbidity can also be caused by the wind. Sediment resuspension by waves occurs mainly in the shallowest areas (< 20 m) of the coastal zone when wind speed exceeds 15 knots. In the lakes, the presence of floating or submerged aquatic plants efficiently prevents the effect of the wind on bottom sediments. When it blows in the same direction for more than half a day, the wind strongly influences the orientation of turbid plumes.

In spite of these influences, turbidity remains controlled by hydrological regime, as can be seen from the comparison of the two exceptional years. In 2006 the spring flood lasted until mid-summer; river turbidity was exceptionally high but the lakes remained very clear; the inner plume, the most turbid, extended up to 10-20 km offshore. In 2007 the spring flood pulse was almost inexistent; river water was very lightly loaded with sediments whereas lakes were very turbid because of phytoplankton; in the coastal zone, the inner plume barely reached distances of 5 km from the coast.

Opposite evolutions of turbidity in the delta and at sea in July 2006 (left) and July 2007 (right)

From the methodology standpoint, the use of satellite images and their processing with methods adapted to the spatial and spectral characteristics of the sensors provided reliable and inter-comparable information. The integration of these data over four contrasting years and several spatial scales made it possible to better understand the seasonal patterns of turbidity and to distinguish the relative importance of controlling factors all over the study area. Such a complex and variable system cannot be studied with punctual field data alone; even if realized repeatedly, these data should be complemented by satellite derived products.


The paper

Güttler F.N., Niculescu S., Gohin F., 2013. Turbidity retrieval and monitoring of Danube Delta waters using multi-sensor optical remote  sensing data: An integrated view from the delta plain lakes to the western–northwestern Black Sea coastal zone. Remote Sensing of Environment 132 : 86–101.

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The authors

This work was done in collaboration by researchers of the Géomer-LETG team (Littoral - Environnement - Télédétection – Géomatique) and of the laboratory Dyneco of Ifremer.


The journal

Remote Sensing of Environment is an interdisciplinary review published by Elsevier; it scope covers theory, applications and technology of remote sensing of resources and environment. The paper deal, at a local or global scale, with agriculture and forests, geography and spatial information, ecology, Earth sciences, hydrology and water resources, atmosphere and meteorology, oceanography and coastal zones, etc.



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