Restore the plankton and you've restored the bottom of the food chain.
The plankton have died off by at least 40% over the past 60 years. John Martin at the Monterey Bay Aquarium Research Institute hypothesized in the early 90's that the die-off was due to diminishing iron in the ocean surface waters. He was quoted as saying "Give me a freighter full of iron fertilizer and I'll give you an ice age." meaning that spraying iron onto the ocean's surface would re-populate the plankton and they in turn would consume the excess CO2 that's currently acidifying the oceans.
In 2002, MBARI validated his hypothesis that spraying iron fertilizer would engender a plankton bloom. Subsequent studies have replicated MBARI's results.
Seems to me that if someone were to claim a 100 square mile chunk of ocean, they could fertilize it, seed it with anchovies and start a very profitable aqua farm. They would be harvesting a variety of predator fish such as bass and tuna once they discovered the anchovies feasting on the plankton. Since the farm wouldn't harvest all of the carbon the plankton consumed, it'd be a net carbon sink.