Mostly associated with climate change, coral reefs are one of the most threatened ecosystems of the planet, currently dying at record rates across the world. But while the rising temperatures are affecting them, they are just one part of the story.

The corals are threatened by rising temperatures — but there’s more to the story than that. Credit: USFWS (Flickr)

Researchers at Florida Atlantic University’s Harbor Branch Oceanographic Institute worked with 30 years of data from Looe Key Reef in the lower Florida Keys and discovered that coral bleaching isn’t happening just because of global warming but also from human pollution.

Improperly treated sewage and agricultural run-off have flowed for many years into the ocean waters of Florida, causing an increase in the nitrogen levels and lowering the reef’s temperature threshold for bleaching, according to a study published in the journal Marine Biology.

“Our results provide compelling evidence that nitrogen loading from the Florida Keys and greater Everglades ecosystem caused by humans, rather than warming temperatures, is the primary driver of coral reef degradation at Looe Key Sanctuary Preservation Area during our long-term study,” said Brian Lapointe, study senior author.

This has led to a steep decline in coral cover in the region, going from nearly 33 percent in 1984 to just six percent in 2008.

The researchers discovered that three bleaching events that occurred during that period only happened after heavy rainfall and increased land-based runoff. This means that if the amount of local pollution is reduced, the damage to coral reefs could be reduced.

An excess of nitrogen feeds blooms of algae that block out the light, also causing an imbalance of nutrients in the water that affects the coral’s life cycle. This leads to the corals being starved of phosphorus, making them unhealthy and susceptible to diseases, ultimately causing coral bleaching.

“Anthropogenic nutrient loading from local sources in the Florida Keys and regionally from the greater Everglades ecosystem is interacting with a changing climate to create conditions unfavorable for living coral,” the study reads.

Previous studies have also shown that between 1992 and 1996 – when Florida’s freshwater flows were directed south — there was a 404% increase in coral diseases throughout the Florida Keys National Marine Sanctuary (FKNMS), which also sits downstream from the Everglades.

The findings in Florida shows that actions taken by residents can have an impact on the health of coral reefs. The authors claimed better sewage and storm-water management can reduce nitrogen pollution, plus a better management of fertilizers used on lawns.

Coral reefs are considered the ocean’s most diverse and complex ecosystems, supporting 25% of all marine life, including 800 species of reef-building corals and more than one million animal and plant species.

A 2011 study by Burke said that 60% of coral reefs around the world are already seriously damaged by processes such as overfishing and coral bleaching. Rising ocean temperatures are also a massive issue for corals. It’s currently estimated that 75% of reefs are threatened.