An inherent problem with both sewage and storm water runoff, is that they contain high levels of decomposing organic matter. When bacteria and other microorganisms break down these wastes, they use up a significant amount of oxygen found in seawater, oxygen that fish and other sealife need to exist.
The City of Los Angeles is exploring the feasibility of creating experimental filtration systems or "constructed wetlands" throughout the city to treat sewage and stormwater. These constructed wetlands allow decomposition to occur in a series of marsh-like detention areas.
The types of vegetation used in these areas, such as cattails, grow rather quickly and are very efficient at removing large amounts of phosphorus and nitrogen from untreated water and are also remarkably adept at soaking up heavy metal pollutants.