Four of six key coral regions suffer higher domestic wastewater pollution inside MPAs than in unprotected waters.
Excess nitrogen from sewage weakens coral reefs, seagrass and mangroves and amplifies bleaching and disease.
It also undermines biodiversity goals, prompting calls to integrate wastewater control into 30x30 conservation plans.
Marine Protected Areas (MPA) designed to save tropical coral reefs are currently exposed to higher levels of untreated or poorly treated domestic wastewater pollution than unprotected waters, according to a new report. This increases their vulnerability to bleaching events, diseases and slow recovery after disturbances.
The study published in the journal Ocean and Coastal Management said that apart from the coral reefs, the growing and poorly managed threats are making the coastal ecosystems such as seagrass meadows and mangrove forests.
Across 1,855 MPAs within 50 km from the coast, the study is the first global assessment of total nitrogen (TN) loads that serves as a proxy for sewage from wastewater pollution within tropical ecosystems.
Globally, it is estimated that 55 per cent of coral reefs and 88 per cent of seagrass ecosystems are exposed to wastewater pollution. Despite its well-known extent, wastewater pollution is often overlooked in marine conservation initiatives, resulting in these ecosystems facing an unaddressed critical challenge, the study said.
“Nutrient enrichment from wastewater reduces coral reproductive processes, growth rates, and survival of early life stages, while increasing the prevalence of coral diseases and bioerosion processes,” it added.
These pollution pressures have aggravated drop in coral reefs species abundance and diversity worldwide. The wastewater pollution has other impacts such as prohibiting light penetration and reducing photosynthetic activity in seagrass meadows and introducing pathogens besides encourage growth of competing macro algae and epiphytes.
The wastewater also acts as a "threat multiplier" for climate change. Chronic nutrient loading makes corals significantly more vulnerable to bleaching events and hinders their ability to recover after a disturbance.
Additionally, marine heatwaves aggravate eutrophication and hypoxic events caused by wastewater pollution, threatening the perseverance of the rich fish biodiversity associated with tropical coastal ecosystems.
Mangrove forests become more vulnerable to erosion and less effective at storing carbon when exposed to wastewater pollution, it noted.
The study found that in four out of the six tropical regions, “protected” pixels actually showed higher median pollution levels than their non-protected counterparts.
The authors said that the East Africa and Middle East & North Africa regions presented similar proportions of MPAs above the 50th percentile — 53 per cent and 57 per cent respectively, while also showing the highest mean, median, and maximum TN loads as well as the highest standard deviations, indicating that several MPAs experience extremely elevated TN pollution.
MPAs in the Caribbean & Bahamas and the Coral Triangle showed similar metrics, with 57 per cent of MPAs below the 50th percentile for both regions.
The findings highlighted that many of the MPAs are currently located in areas exposed to high anthropogenic pressure, thus compromising on its ecological benefits.
The widespread, disproportionate exposure across MPAs underscores the pressing concern that, despite their protected status, many MPAs are vulnerable to wastewater pollution, and failing to deliver their ability to provide ecological outcomes, the study said.
“MPAs are a cornerstone of global biodiversity conservation and a key mechanism for achieving the Kunming-Montreal Global Biodiversity Frame-work Target 3, commonly referred to as “30x30,” which aims to protect 30 per cent of the ocean by 2030,” it noted.
Our results underscored the urgent need for targeted domestic wastewater management and policy interventions as a necessary complement to MPA design and management, they added.
The authors suggested aligning wastewater infrastructure and sanitation projects with coral conservation plans. Alongside, they said to achieve the 30x30 target, protection must extend beyond the geographical extent of the MPAs to address land-based pollution.
“A more holistic array of parameters to ensure MPA success must be established, where land-based pollution, including wastewater pollution, is integrated into metrics of MPA effectiveness. This requires integrating wastewater management into coastal conservation interventions, including thorough spatially explicit tools, policy reform, and targeted financing mechanisms,” it observed.
“Global push to achieve 30x30 will fail to deliver on its promise unless the effectiveness of protection from pollution is prioritised alongside extent. For MPAs to safeguard biodiversity, pollution sources that undermine their ecological integrity, such as wastewater, must be addressed,” the authors of the report concluded.