How human activities compromise coral health and resilience

How human activities compromise coral health and resilience

Human activities are fundamentally altering the chemical makeup of coral reefs, leaving them more vulnerable to environmental stressors. A study led by the University of Hawai‘i at Mānoa and published in Nature Communications reveals that anthropogenic disturbances create a direct relationship between the accumulation of dangerous contaminants and declining coral health.

Researchers focused their study on the waters around Maui, Hawai‘i, where they analyzed 380 rice corals (Montipora capitata) and lobe corals (Porites lobata). The team sampled these specimens across 16 different sites off the south and west coasts of the island. By examining the metabolomes—the pool of chemicals within coral tissues—the scientists discovered that 25 contaminants from industrial, pharmaceutical, and agricultural sources had accumulated in the soft tissues of the corals.

The impact of these contaminants was consistent across different species.

"It was extremely surprising that the metabolomes of both coral species had almost identical trends,"

Zach Quinlan, research biologist at the Hawai‘i Institute of Marine Biology, via soest.hawaii.edu

Quinlan noted that because these two coral species employ very different life strategies, researchers would not typically expect them to respond to disturbances or accumulate contaminants in the same manner. He stated that this similarity demonstrates the strength of anthropogenic forcing.

The study identified two primary mechanisms by which human activities reduce coral resilience. First, corals accumulate anthropogenic molecules, including industrial byproducts and pharmaceuticals. Second, the pressure from these human activities forces corals to utilize their own nitrogen and energetic reserves.

This depletion of resources makes corals significantly less capable of surviving warmer or more acidic waters. According to the research team, in areas with higher ecosystem disturbance, nitrogen and energy reserves in the tissues decreased while stress chemicals were enriched. The team verified this by looking at historical coral cover trends at five of their sampling sites; the locations that experienced the most severe declines in coral cover following the 2016 bleaching event were the same sites with the most impacted metabolomes.

Beyond chemical shifts, reefs face a variety of physical and systemic threats. Land-based runoff from sewage treatment plants, coastal development, deforestation, and agricultural activities can introduce insecticides, oil, sediments, and chemicals into the water. Such runoff can increase nutrient levels, which promotes the rapid growth of organisms and algae that smother the corals.

Other human-led damages include:

• Destructive Fishing: The use of cyanide to stun fish kills coral polyps, while blast fishing using dynamite can crack corals and cause them to expel their zooxanthellae. Deep water trawling and muro-ami netting, which involves pounding reefs with weighted bags, also destroy habitats.
• Collection and Physical Damage: Corals are often destroyed when collected for the jewelry and aquarium trades. Additionally, anchors dropped from vessels and untrained divers can trample or break fragile colonies.
• Pollution: Plastic debris can entangle and smother coral. While petroleum spills may not always affect corals directly due to evaporation, oil can damage eggs and sperm during spawning periods, disrupting reproductive success.
• Climate Change: The burning of fossil fuels and deforestation increase greenhouse gases, leading to rising ocean temperatures, altered storm patterns, and sea level rise, which trigger more frequent bleaching events.

The broader implications of these findings suggest that anthropogenic contaminants are escaping into marine ecosystems at an alarming rate.

"We see increasing evidence that anthropogenic contaminants have broad cumulative impacts, undermining the resilience of coastal ecosystems,"

Megan Donahue, HIMB director and senior author, via soest.hawaii.edu

The research team proposes that monitoring the metabolomes of coastal seafloor species or corals can be used as a tool to track the hidden impacts of human disturbance on marine life. To address these threats, scientists are now looking for ways to increase the energy and nitrogen reserves in coral tissues through controlled experiments to see if such enrichment improves resilience.