*3.6. Sea Level Rise*

Today, sea-level rise is not a serious problem for most of the shores of the Black Sea, but a combined effect of the sea-level rise and wave impact on the shores and beaches have already led to the destruction of beaches and shore tourism infrastructure in many places of the Russian coastal zone in the Black Sea and the Sea of Azov [61].

Ginzburg et al. [35] analyzed interannual changes in the level anomalies of the Black and Azov Seas according to the data of the along-track altimetry measurements of Topex/Poseidon and Jason-1, Jason-2, and Jason-3 satellites in the period of 1993–2020. Long-term variability of the Black Sea level is characterized by alternating periods of its

rise and fall. At different time intervals, the rate of the level rise varied from the minimum value of +2.64 cm/year from January 1993 to June 1999 to the maximum +25.74 cm/year from August 2012 to July 2013; the rate of the level drop varied from −1.23 cm/year from June 1999 to April 2003 to −8.59 cm/year from June 2004 to February 2008. The average for the 1993–2020 period linear trend of the Black Sea level is of +0.32 ± 0.16 cm/year, which was found to be approximately 2.5 times lower than in 1993–2012 (+0.82 ± 0.18 cm/year), although 1.5 times more than from the 1920s to the mid-1990s (0.17–0.18 cm/year). For the northeastern part of the Black Sea, Lebedev et al. [40] showed that for the time interval of 1993–2015, the Black Sea level off the coast of the Krasnodar Krai grew at an average rate of 0.29 ± 0.03 cm/year, with an increase in rates from the Kerch Strait (0.28 cm/year) to Adler (0.31 cm/year). An average increase in the Black Sea level of 3.2 mm/year is consistent with the average world ocean sea-level rise [17].

### *3.7. Algal Bloom and Introduced Species*

Algal bloom may be regarded as biological pollution of the coastal zone, which occurs as a result of an excess supply of nutrients (mainly nitrates and phosphates) in the coastal zone with river and land runoff, and high values of the SST. In the Black Sea, this problem occurs almost every year in the northwestern part of the Black Sea in the region of Odessa, and on shallow water sandy beaches of Anapa on the Russian coast [50,67]. The abundance of algae in the upper layers of the water makes the water green, not transparent, and entering into the water is unpleasant, and in some cases even dangerous (when swallowed), as it may contain toxic algae. In addition, the algae washed ashore starts to rot and the beach becomes unpleasant until cleaning (Figure 17).

**Figure 17.** (**a**) Typical summer green algal bloom on beaches of Anapa (https://turvopros.com/kogda-cvetet-more-v-anape, accessed on 16 July 2021); (**b**) concentration of brown algae in water and dead algae washed ashore with a storm near Dyurso on 11 October 2020.

Introduced species is ye<sup>t</sup> another problem for the Black Sea and for the development of coastal tourism in particular. Many alien species have become established in the Black Sea since the middle of the 20th century due to a discharge of ballast water from ships coming from adjacent seas and even oceans, as well as due to regional warming which facilitates northward expansion of species from the Mediterranean Sea. The most dramatic example of alien species introduction to the Black Sea was the invasion of a gelatinous predator, the polymorphic ctenophore *Mnemiopsis leidyi* (Figure 18a), and later on, the accidental invasion of ctenophore *Beroe ovata* (Figure 18b) [68]. On the international level, it is suggested that *Mnemiopsis* was brought to the Black Sea with ballast waters from the coastal regions of North America at the beginning of the 1980s. By 1988, *Mnemiposis* had spread over the entire Black Sea area and showed an enormous abundance outburst in the fall of 1989 [68–71]. This led to a decrease in the biomass, abundance, and species diversity of edible zooplankton, fish larvae, and eggs, which are the principal food objects of *Mnemiopsis*. As a result, by 1991, the commercial fish catches had decreased 10 times; this especially referred to the anchovy, Mediterranean horse mackerel, sprat, as well as higher trophic levels—predator fishes and dolphins, who feed mainly on anchovies and sprats [68]. From the Black Sea, *Mnemiopsis* spread to the seas of Azov and Marmara, and in 1999—to the Caspian Sea. Then, in 1997, a new invader—ctenophore *Beroe ovata*—was first found in the northwestern part of the Black Sea, and in August 1999, the first outburst in the *Beroea* development over the entire Black Sea was observed. This predator feeds, first of all, on *Mnemiopsis*, thus, *Beroe* radically reduced the *Mnemiopsis* population and this resulted in the restoration of the Black Sea ecosystem within several years [68].

Both jellies are placed in the palm of your hand and do not pose any threat to adults or children, however, their number in different years reached such values when there were several dozens of them in one cubic meter of water. Swimming in such water is extremely unpleasant.
