Abstract
The diversity of centric diatoms is documented for the transitional zone of the lower part of the Southern Bug River (Ukraine) just before entering the Dnipro-Bug Estuary and compared to earlier results from the upstream sampling sites of the same river system. Benthic samples of the following sites were investigated: north of Mykolaiv City (approximately 5 km), in Mykolaiv City (near Varvarivskyi Bridge), and 5 km south of the city. Twenty-four centric diatom taxa belonging to 11 genera were identified, analysed, and documented by scanning electron microscopy (SEM) and light microscopy (LM). Among them, Aulacoseira nivalis is the first report for Ukraine, A. islandica and is the first confirmed record for the studied area since the 1930s. The maximum number of centric diatom taxa found in one station was 21, the minimum 10. Melosira subglobosa was the most common (documented in 57–80% of sites with centric diatoms) and abundant species 7.3–15.7% in relative abundance to all diatom taxa. The discovered diversity of taxa and its comparison with previous results is discussed with regard to the relevance of estuary zones in the research of diatoms.
Highlights
Transitional waters are the continuum between freshwaters and coastal marine waters which according to the EU Water Framework Directive (The Directive 2000/60/EC) are defined as “bodies of surface water in the vicinity of river mouths which are partially saline in character as a result of their proximity to coastal waters but which are substantially influenced by freshwater flows”
This area could be termed a transitional zone because it represents a transition community consisting of freshwater and marine species being at the edge of their ecological range
Among the centric diatoms we found three alien or potentially neophytic species (Actinocyclus normanii, Skeletonema subsalsum, Thalassiosira incerta) that might be considered as an immigration of marine species to freshwaters (Kaštovský et al 2010; Korneva 2014; Vidaković et al 2016), their monitoring is important for biodiversity conservation
Summary
Transitional waters are the continuum between freshwaters and coastal marine waters which according to the EU Water Framework Directive (The Directive 2000/60/EC) are defined as “bodies of surface water in the vicinity of river mouths which are partially saline in character as a result of their proximity to coastal waters but which are substantially influenced by freshwater flows”. The salinity gradient in the Bug estuary itself varies in a wide range of 0.3–9.5 g/dm, and the mean salinity equals 3.6‰ (Mykolaiv regional state administration 2019) These waters move upstream to Mykolaiv City forming a buffer zone, which may be defined as having two overlapping gradients formed by major saline stressors: freshwater species from the river and marine species from the estuary. This area could be termed a transitional zone because it represents a transition community consisting of freshwater and marine species being at the edge of their ecological range
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