Elena Zhmud, Roman Sizo, AZBOS

 

The reed Phragmites australis Trin. ex Steud is a typical edificator of plavni (overflow areas in the delta) ecosystems in the south of Ukraine and has an important ecological and economical importance.

In the framework of enviroGRIDS project we made an attempt to estimate ongoing changes in the area of reedbeds in the Ukrainian Danube Delta. For this purpose we have selected two plots of different type and with different extent of man-made transformation: Stensivsko-Zherianski Plavni (SZP) and Kubanu Island.

 

  1. Stensivsko-Zherbrianski Plavni – the largest in the Ukrainian Danube Delta embanked plavni areas with regulated hydroregime. They belong to the zone of regulated protected regime of the Danube Biosphere Regime. This plavni ecosystem is divided into two parts by the Danube-Sasyk Canal. Over the last 25-30 years the water level in plavni has been artificially maintained and gradually increases. Increase of water level is artificially retained by water discharge during the Danube flood. When the water level starts decreasing the sluices are closed. But the reed forms perennial roots which annually, with rise of water level, come up several cm higher. All these factors have led to reduction of the water table and depletion of species composition of associations. The surface of open water area is gradually reducing and the system to a great extent loses its mosaic and attractiveness for faunistic elements.

  2. Kubanu Island is one of the most well-preserved in a natural state islands in the Danube Delta with minimal human impact

Methods

As a basis for analysis there were taken satellite images LandSat 4-7 for 1984 (1986 for Kubanu Island), 1993, 1999, 2002, 2006, 2009 and 2011. The period of satellite survey is September-November. To receive data on the state of territories in 1975 there were used maps with scale 1:100000.

Satellite images were ordered from glovis.gov.ua and received as archives, containing geotiff files for each channel of survey. Using the programme ERDAS IMAGINE the channels were combined in one file. As a result seven separate .img files corresponding to seven acquisition years were obtained.

Borders of territories were digitized in ERDAS IMAGINE using a tool ‘Area of Interest'. Then, the function of unsupervised classification was applied to each image with limitation of the area in correspondence with ‘Area of Interest' with the following parameters: maximum iterations — 20, convergence threshold — 0.96.

For visual perception and further analysis by a tool ‘Reclassify' of ArcGIS Spatial Analyst there were obtained three result classes:

  1. open water
  2. rarefied reedbeds
  3. dense reedbeds

Taking into account that percentage of dense and rarefied reedbeds is subject to significant changes due to different factors (watering, fires, etc.) for the dynamics of ratio it was used the surface area of open water and total area of reedbeds.

Results are represented lower in maps and graphs:

Stensivsko-Zherianski Plavni

1984

1993

 

 

1999

2006

 

 

2009

2011

Fig. 1. Distribution of reed in SZP in different years.

 

Kubanu Island

1986

1993

1999

 

 

 

2002

2006

2011

 

 

 

Fig. 2. Distribution of reed in SZP in Kubanu Island

 

Conclusions

  • Both on the territory of SZP and Kubanu Island it was observed the overgrowing of the territory with reed and reduction of open water surface for the studied period

  • The surface area of open water in SZP reduced from 21.29 km2 in 1975 to 12.01 km2 in 2011. A sharp reduction of the area in the period from 1975 to 1984 is apparently connected with building in that period the Danube-Sasyk Canal which divided SZP in two parts and broke the water exchange

  • The surface area of open water on Kubanu Island reduced from 3.16 km2 in 1975 to 2.44 km2 in 2011. The process of overgrowing of the area was much more temporally even, compared to SZP, and is connected with natural processes of the delta growth.