Boat sewage dumped into our waters may affect aquatic plants, fish, and other animals. The nutrients, microorganisms, and chemicals contained in human waste from boats have a negative impact on coastal and inland waters, particularly in sheltered or shallow areas not naturally flushed by tide or current.
- Sewage contains nutrients that fertilize algae in the water. This can make algae grow out of control, reducing the amount of light which reaches underwater grasses. Algae can also settle on the grass, smothering it. These grasses provide nursery areas for young fish and help prevent shoreline erosion.
- Flow-through treatment devices (Type I or II MSDs) reduce bacteria in the sewage, but do not reduce the nutrients or organic matter going into the water.
- Organic matter in sewage is decomposed in the water by bacteria. During this process, the bacteria use oxygen. As a result, the bacteria in the water may deplete the water's dissolved oxygen level, stressing fish and other aquatic animals that need oxygen in the water to survive.
- Microorganisms from the digestive system are found in human waste. Once they are in the water, they can pass diseases like hepatitis to people in contact with the water. They also can contaminate shellfish beds, passing disease on to people who eat raw oysters or clams.
- Chemical products used in onboard treatment devices, such as chlorine and formaldehyde, can be toxic to marine and estuarine life and could pose a problem in areas where boats congregate and where there is little tidal flushing action.