On the macro scale the biggest impact we're having on the ocean is through climate change warming oceans and acidification, so you could say that's land-based; it's human activity. Aside from the pollution element of what we do to our oceans the single largest affect on our oceans is the removal of biomass from the system. We take hundreds of millions of tonnes of biomass out of the ocean on a regular basis. As we know, we've done that to great extremes. That's a massive impact that we have to temper if we're hoping to maintain functioning ecosystems.
