The impact of sewage discharge on nearby reefs is difficult to assess, but it still receives a fair amount of attention, in terms of regulation, research, the media, etc. Sewage can be introduced to Guam’s reef environment through the offshore sewage outfalls at Tanguisson, Hagatna, and Tipalao, overflow into Togcha and Toguon bays, and possibly through aquifer discharge contaminated by leaky sewage pipes and septic systems.
Personal observations, underwater video, and a limited amount of data obtained at the reef areas adjacent to these outfalls and other sewage discharge locations do not suggest that the outfalls have are having a major impact on these reefs. However, the effects of sewage may be less conspicuous than other threats, such as sedimentation, and may impact reefs over an extended period, such as by facilitating a “phase shift” after a large amount of coral is killed off by crown of thorns sea stars, coral bleaching, or some other acute disturbance. A phase shift occurs when a reef community changes from a coral-dominated to fleshy algae-dominated state (or some other state), or vice versa. Although it’s not clear how large an area would be impacted over the long-term by sewage, the rapid uptake of nutrients by plankton, algae, and other organisms would likely limit the extent of the impacts of sewage discharge.
Even with secondary treatment, nutrients – such as nitrates and phosphates – are present in the sewage discharge. Most coral reefs thrive in nutrient poor – or oligotrophic – water, and receive much of their energy through their symbiotic partnership with the microscopic algae (zooxanthellae) that live in their tissue. When nutrients are introduced into a system where they are a limiting factor in the growth of algae and cyanobacteria, the algae are given an edge over the coral and can sometimes encroach upon living coral tissue. But more often are cases where the coral is killed by something else and the algae quickly take over the freed-up space; the nutrients may accelerate this growth, and allow a greater amount of growth, thus enabling the algae to cover large areas with thick algal growth. Because the larvae of many coral species require fairly “clean” substrate, the presence of a thick canopy of algae can prevent them from settling on the reef. Even if they find a place to settle, they can be quickly covered over by algae. This may kill the young corals, or the reduced availability of light and the potential for damage by abrasion can hinder the coral’s growth.
The effect of nutrient enrichment is made worse in situations where the populations of herbivorous – or algae-eating – fishes are overfished. In fact, herbivorous reef fishes are so important to healthy reefs, that even in the absence of nutrient enrichment, algae can take over a reef if there are not enough algae-eating fishes. Nutrients are also known to make other problems worse. This type of an effect between two or more stressors is known as a “synergistic effect.” For example, research indicates that coral diseases can be made more severe in the presence of nutrients such as nitrates and phosphates. Sewage discharge may also be an actual source of bacteria and other pathogens that may harm or kill corals and other marine organisms. Click here for more about coral diseases.
Some welcome good news is that Guam Waterworks Authority is currently extending the sewage outfalls at Tanguisson (near the Northern Power Plant) and in Hagatna (near the Boat Basin). Prior to extension, the outfalls discharged sewage (sometimes untreated) into approximately 60-feet of water, a depth where coral and other reef organisms occur at a relatively high abundance. After the extension project is completed, the outfalls will occur at a much greater depth (> 200 feet). Much of the seafloor at these depths is covered with sand, with very sparse coral growth. While the fate of the sewage released from the greater depths is not yet certain, models predict that it will remain in the bottom water layer and will not reach the surface or impact nearby coral reef communities. However, monitoring of water quality and coral reef health should be conducted at these sites to ensure that there are no human and coral reef health impacts.