Final answer:
The Black Sea's recovery during the 1990s was aided by a reduction in eutrophication option (b), which resulted from decreased fishing and improved marine ecosystems, as well as the potential impact of bivalve filter feeders in reducing excess nutrients.
Step-by-step explanation:
The Black Sea's recovery during the 1990s can be attributed to the reduction in eutrophication, which is the decrease in excessive nutrients that can lead to dense plant and algal life, ultimately resulting in the depletion of oxygen within a body of water.
One significant factor that helped decrease eutrophication was decreased fishing. Overfishing had previously led to imbalances in marine ecosystems, but with decreased fishing pressure, fish populations started to rebound, promoting healthier marine environments.
Additionally, bivalve filter feeders like oysters have the potential to alleviate eutrophic conditions. These organisms can enhance growth rate, increase abundance, and maintain a clearer water column through their filtration process.
The introduction and cultivation of bivalve filter feeders thus contributed to the recovery as they can clear out excess nutrients and prevent the growth of extensive algal blooms. However, this option is not mentioned in the choices provided in the question.
Therefore, to answer the given question, the recovery of the Black Sea during the 1990s was helped by choice 'B. Reduction in eutrophication'.