Article: Earthworms are Oligochaetes that support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water. Although blood-letting is no longer in favor with doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength.

Now answer this question: How do burrowing marine annelids help ocean ecosystems?
enabling water and oxygen to penetrate the sea floor
The context states that marine polychaetes, in addition to encouraging the development of ecosystems by enabling water and oxygen to penetrate the sea floor.
Since this is a short passage with only one focus on annelids (segmented worms), it can be inferred easily that burrowing helps ocean ecosystem by allowing other organisms such as bacteria and plankton to survive near the shores..