Which of the following environmental issues represents the least immediate threat to ocean ecosystems?

A: entanglement of marine mammals in trawl nets and overfishing

B:melting of the ice caps and increased carbon dioxide levels in the atmosphere

C:nutrient runoff and industrial waste discharge

D:sewage and algal blooms

I think the correct answer from the choices listed above is option B. It would be melting of the ice caps and increased carbon dioxide levels in the atmosphere that would represent the least immediate threat to ocean ecosystems. The effects are seen as the years goes by than seeing the effect immediately. Hope this answers the question.