Bermuda does not have a sewage treatment plant. Rather it relies either on sewage pits or on pumping the raw sewage offshore in pipes. Some of these pipes end quite close to the shoreline and that raised concerns over public health as well as visual impact!
The Corporation of Hamilton (the capital of Bermuda) responded to these concerns in 1993-94 by extending the Seabright outfall pipe, delivering the effluent about 700 m from shore where it discharges over a 60 ' deep, broad sandy area. This was done to reduce nearshore impacts but has brought the effluent closer to deep offshore reefs. The Corporation of Hamilton asked BBSR's Benthic Ecology Research Programme team to set up a monitoring study of the offshore reefs and the sediments around the outfall. This study is part of a long-term planning exercise by the Corporation to determine the environmental impact of the sewage discharge and assess the capacity of the area to withstand anticipated future increases in sewage discharge.
The team chose two impact reef sites about 75 m from the outfall and two control reefs off John Smith's Bay. They set up permanent video transects and also permanent photographic quadrats on each reef. (The divers use the white frame to set the camera at a fixed height above the reef and to accurately line up the boundaries so that each photo taken over time is of exactly the same spot and surface area on the reef. The coral image is the result of one of the permanent photo quadrats). You can compare photos from each time to see if there are any differences. In the upper left is a healthy brain coral and the lower one has a disease called black band disease). The analyses of the tapes and photos showed that coral coverage was reduced slightly at the impact sites compared to the control reefs. Reef algae was also more abundant on the impact reefs. Future monitoring will help us assess if these are natural patterns unrelated to the outfall.
The overall impressi on of the impact reefs is that they appear healthy with high coral coverage . Diluted effluent has only been seen passing over the reef during infrequent periods of north winds. The movement of the effluent plume is generally controlled by the strong east-west tidal flow that keeps the effluent away from the outer reefs and may minimize its impact on these reefs.
The second component of this study was an assessment of the extent of the impact of effluent on the soft sediment animal communities surrounding the outfall. There is typically an abundant and diverse community of worms, clams, crustaceans and other invertebrates living within the sediments, called the infauna. These organisms are counted in sediment samples at 10, 50 and 400 m distance from the outfall with a grab sampler deployed from the Bermuda Biological Station's vessel the R/V Henry Stommel. The sediment samples are preserved and then later the organisms greater than 0.5 mm in size are sorted from the sand grains. The abundance and diversity of animals were reduced significantly at the 10 m sites compared to the 50 and 400 m sites with the sediments closest to the outfall only supporting a few worm and one clam species. Thus, it appears as though the deposition of the sewage from the outfall does have an effect on the sediment infauna but it is limited to less than 50 m from the outfall.
