The sea has a moderating effect on the climate, so temperatures do not fall much below in winter, and are not as high in the summer as in area further inland. The
Kaliakra cliffs in the north of Bulgaria are rich in flora, including many species in common with the neighboring
Steppic and
Mediterranean Regions. The western Black Sea Region is the
Via Pontica, Europe's second largest bird migration route. The migrating birds use the coastal lakes, marshes and lagoons behind the shoreline, and some spend the winter in these wetlands. The
Danube Delta is the best known of the wetlands. In Bulgaria and Romania the region is threatened by development of agriculture, industry, urbanization and tourism. The Black Sea itself, a very deep inland sea, is poor in oxygen and supports very little marine life in the deeper regions. However, it had a productive fishery until the 1960s, when stocks crashed in part because of over-fishing and in part from pollution and invasion by exotic species. ==Conservation==