By the very act of feeding, shellfish filter phytoplankton (nutrients including nitrogen and phosphorous) out of the water, cleaning and clarifying the water. In fact, a single clam can filter 50 gallons of water a day. Clearer water allows more sunlight to penetrate, which aids in the growth of crucial seagrasses, including eelgrass.
Without shellfish, algae can grow virtually unchecked, clouding the water and eventually falling to the bottom, where it decays, reducing the level of oxygen in the water. In addition, shellfish, like other carbon fixers such as corals, help reduce carbon dioxide levels by incorporating carbon into their shells.
Shellfish farming can only take place in the cleanest waters that have been certified under the National Shellfish Sanitation Program (NSSP), a stringent set of standards operated under the Food and Drug Administration. The NSSP standards led to the first estuarine/marine monitoring programs, and are the most stringent of all water quality classifications, far exceeding those required for swimming.
Regular monitoring is required to maintain certification of shellfish beds, and harvesting is banned if a problem is detected. These bans remain in effect until the problem is corrected and water quality monitoring indicates the area once again meets standards.
Click here to watch a time-lapse video of Manila Clams cleaning water.