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Yes, Black Sea. Wiki says:

The Black Sea underwent a catastrophic environmental collapse in the early 1990s caused by chronic nutrient pollution from farms, feed lots, and municipal sewers throughout its vast watershed. Massive algae blooms resulted in hypoxic conditions across the ecologically-critical northwest shelf, wiping out much of the flora and fauna there. Perhaps because of the attendant ecological disruptions, an alien species - Mnemiopsis leidyi -- was able to establish itself in the basin, exploding from a few individuals to an estimated biomass of one billion metric tons.[16]

Recently, scientists have noted signs of ecological recovery, in part due to the collapse of agriculture (and subsidized fertilizer usage) in much of the lower Danube basin and the construction of new sewage treatment plants in Slovakia, Hungary, Romania, and Bulgaria in connection with membership in the European Union. Mnemiopsis leidyi populations have been checked with the arrival of another alien species which feeds on them.[17]

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