Based on the materials of a 2019-2020 study, a comparative hydrobotanical characterisation of two nearby coastal water areas of the Black Sea (Southern Coast of Crimea - SCC) is presented. Both water areas are similar in terms of their complex of natural and climatic conditions; but contrast in terms of the anthropogenic transformation level and eutrophication. In total, 81 species of macrophytes were identified in the study area. In the natural water area, located within the boundaries of the "Cape Martyan" Nature Reserve, 61 species were registered (<i>Chlorophyta</i> - 14 (22.6%), <i>Ochrophyta</i> - 12 (19.7%), <i>Rhodophyta</i> - 34 (55.8%), <i>Tracheophyta</i> - 1 (1.6%). To the east of it, the marine water area adjasent to the Gurzuf village is located. This has been transformed and is subject to local, but constant and intense eutrophication from a shallow emergency sewer. In total 63 species were registered here (<i>Chlorophyta </i>- 15 (23.8%), <i>Ochrophyta</i> - 16 (25.4%), <i>Rhodophyta</i> - 32 (50.8%). The ratio of macrophytes ecological and floristic groups by number of species (NS) does not reveal fundamental differences between the two water areas; the tendency of change in the total biomass (BM) of communities with increasing depth along the profiles is similar, and the values at the same depths are comparable (up to 0.8 kg · m<sup>-2</sup> in the pseudolittoral zone and up to 6.2-7.0 kg · m<sup>-2</sup> in the sublittoral). Significant differences are found in the BM ratio of macrophytes ecological and floristic groups, which ultimately determines the structural features of the vegetation cover of each water area (although their general nature, due to the type of substrate, is similar). Near the Cape Martyan, both in terms of NS and BM, stenobiont (oligosaprobic marine) <i>Phaeophyceae</i> species dominate: in the pseudolittoral - seasonal summer ones; in the sublittoral - perennial ones (belt of <i>Cystoseira</i> s.l.), which is typical for the SCC in summer. In the water area off the Gurzuf village, the algal communities of the pseudolittoral and the shallowest sublittoral zones are formed by eurybiont (polysaprobic brakish-water) <i>Chlorophyta</i>. Such features of the composition and structure are an adaptive response of phytocenoses to intense eutrophication. With a decrease in the trophic content of waters, they could quickly return to a relative norm. However, the situation is complicated by the progressive invasion of the new transformer species <i>Bonnemaisonia hamifera</i> Hariot, which suppresses and displaces native species near the SCC. This changes the appearance, structure, and production indicators of the vegetation cover of the sublittoral zone, and in Gurzuf even leads to local degradation (at depths of ≥ 5 m) of the most productive and species-rich <i>Cystoseira</i> s.l. communities, which are of key importance for maintaining the ecological balance in the coastal marine waters of the region. Presumably, eutrophication is one of the factors that stimulate the spread of <i>B. hamifera</i>, which in the conditions of the significantly isolated Azov-Black Sea basin can lead to a regional ecological disaster. In this case, a decrease in the trophicity of coastal waters can slow down the process of invasion and the resulting transformation of the macrophytobenthos.
