organisms greatly increased the diffus- nematodes. Larger organisms (e.g., anne- 



ibility of oxygen into muddy sediment and lids) that also live in the anoxic zone 



extended the redox layer further below the tend to build tubes or burrows to the sur- 



surface. Despite the lack of oxygen, face that bring oxygenated water to the 



black reducing sediments contain a variety organism, 

 of small organisms such as bacteria and 



