Assessment of the impact of the Seabright sewage outfall on the outer reefs of the South Shore, Bermuda
We chose two Impact reef sites about 75 meters from the outfall and two control
reefs off John Smith's Bay. We set up permanent video transects and also permanent photographic quadrats on each reef. The analyses of the tapes and photos showed that coral coverage was reduced slightly at the impact sites
compared to the control reefs, and 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 impression of the impact reefs is that they appear healthy with high coral coverage. We only ever observed diluted effluent 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 mitigate 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. We collected
sediment samples at 10, 50 and 400 meter distance from the outfall with a grab sampler deployed from BBSR's vessel the R/V Henry Stommel. We preserved the
sediment samples and then later sorted the organisms greater than 0.5 mm in size from the sand grains. We found that the abundance and diversity of animals was
reduced significantly at the 10-meter sites, compared to the 50- and 400- meter sites. The sediments closest to the outfall support only a few worm and one clam
species. Thus, it appears as though the deposition of organic material from the outfall has an effect on the sediment infauna but it is limited to less than 50 meters from the outfall.
The Corporation of Hamilton (the capital of Bermuda) extended the Seabright outfall in 1993-94, delivering the effluent about 700 meters from shore, where it discharges over
a 60-foot deep, broad sandy area. This was done to reduce nearshore impacts but the extended outfall pipe has brought the effluent closer to deep offshore reefs. The Corporation of Hamilton asked BBSR 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.
