Distribution of Cystoseira s. l. species in the Sea of Azov

The information on distribution of the Cystoseira s. l. species in the Sea of Azov (SA) is presented based on the results of our own research and literature data. It is shown that the first record was made 100 years ago at Cape Khroni and at the Kazantyp Peninsula (Crimean coast). Currently, 22 reports from 18 sites have been registered along the Southern coast of the SA (17 along the Crimean coast and one at the Taman Peninsula). Treptacantha barbata (Stackh.) Orellana et Sansón (= Cystoseira barbata (Stackh.) C.Agardh) is the most widespread of the two species (17 sites). It forms dense communities with a biomass of 1.5–3.5 kg/m2. Carpodesmia crinita (Duby) Orellana et Sansón (= Cystoseira crinita Duby) was found in a small number only at two locations. It was found that the modern boundaries of the Azov fragment of the Cystoseira s. l. distribution range coincide with the boundaries of the Pre-Strait area of the SA, where salinity significantly higher, than in other areas. The distribution range has a linear configuration: it covers the Crimean coast from Cape Krasny Kut in the west to Cape Khroni in the east; is interrupted by the open water area of the Kerch Strait, and continues further on the Taman coast from Cape Achilleion to Cape Pekly. Outside the specified area, Cystoseira s. l. do not occur even if hard substrates are present. This shows, that salinity is limiting factor for the distribution of T. barbata and C. crinita in SA. In the past the system of marginal seas of the Mediterranean basin (to which SA also belongs) went through a series of transgressive and regressive phases. As a result, the boundaries, ecological conditions, and composition of biota have been changed several times in the entire basin and in its individual parts. The assumption was made that in the past Cystoseira s. l. species several times settled in the modern boundaries of the SA. The last invasion into the Pre-Strait area of SA (and inhabiting it up to the present time) should have happened in the Late Holocene 3.4–3.1 thousand years ago. Further alternation of regressions and transgressions, certainly, was accompanied by fluctuations of the distribution range boundaries, which either receded from the SA to the Kerch Strait, or again came back to its southern shores. At present, they can also fluctuate to a limited extent, following the salinity fluctuations. It is mostly relevant to the Crimean coast, where hard substrates are widespread. Considering the continuing salinization of the SA due to the climatically caused decrease in river runoff, the spread of Cystoseira s. l. species is possible on the Akmonay coast up to the top of the Arabat Bay. All this does not allow us to classify them as alien species in SA. However, in SA under modern conditions, the sustainable vegetation of the Cystoseira s. l. species with the formation of dense communities at a considerable distance from the Pre-Strait area is impossible. This must be taken into account when artificial reefs are installed and conservation strategies are developed.