Nitrogen Pollution with Heat stress increases Coral Bleaching

Climate change is increasing the frequency and magnitude of temperature anomalies, which cause coral bleaching, which leads to a substantial loss of stony coral that is fundamentally changing the structure and function of coral reefs. A recent study has shown that excess nitrogen can increase the chances of coral bleaching. However, researchers still don't know about the extent of damage the excess nitrogen can do to corals. After several surveys of coral bleaching and nitrogen content of that area, researchers concluded that nitrogen surely affects the corals and enhance bleaching patterns. Researchers have developed a theory that nitrogen interacts with heated water and alter the frequency of bleaching. Nitrogen almost doubles the frequency of bleaching in many areas. 

Global warming and the increasing temperature of ocean water are known to all. But there are many other silent factors (like acidification, calcification etc) that affect the marine ecosystem. Corals are one the most affected species due to these factors. The frequency of bleaching events has increased in the past few decades. Widespread coral bleaching and heat stress have increased coral mortality. Today the population of corals is decreasing at an alarming rate. It has now become crucial to understand and study the reasons behind this decrement.  

Nutrient pollution has increased to a great extent in the past century. Today amount of nutrients added through pollution is more than the nutrients produced naturally in the oceans. These extra nutrients will damage a lot of species especially corals. Nitrogen (N) and Phosphorus (P) will have a great impact on coral growth, reproduction and survival. These two elements are the reason for many severe coral diseases. A small scale increase in nitrogen also reduces the chances of recovery after bleaching, eventually increasing the mortality rate. To understand the effect of N on corals, researchers artificially added nitrogen in corals nursery. They concluded many physiological damages on the corals.

However, it is very difficult to understand the complete effects of nitrogen on corals in a human-created environment. Several studies in the Caribbean and The Great Coral Reefs showed no effect or very minor effect of nitrogen on corals. The effects of N differs from species to species. To understand the actual damage, in 2016, researchers studied a wide area of corals (covering about 50 sq km) in French Polynesia. They also studied more than 10,000 coral colonies across 167 locations. These surveys helped researchers to understand the relation of nitrogen with heat stress and bleaching. 

Pocillopora and Acropora are the two species of corals that showed the greater impact. These two are the centre of studies for the researchers. In those 167 locations, researchers studied 1500 Acropora colonies and around 8800 Pocillopora colonies. the final study concluded that around 38% of total Acropora and 30% of  Pocillopora colonies showed bleaching effects. Also, both species showed a high mortality rate after the bleaching event. Around 55% of the bleached corals of these species were dead. Researchers also found that larger colonies have less severity than the smaller colonies of the same species. The bleaching in Pocillopora was also related to the depth of the colonies of these species. The deepwater colonies were more bleached.   

Further surveys showed that the colonies where mortality of Acropora was high had an excess of nitrogen in the water. An increase in the content of N generally leads to more severity. Nitrogen also played a role in bleaching these two coral species. Excess nitrogen often damages the photosystems of the corals. This leads to a decrease in carbon content in the corals and eventually starvation. High nitrogen content means a high growth rate of algae. This leads to competition between algae and corals for sunlight and nutrients. Researchers finally concluded that nitrogen does more damage to corals than expected.  

However, heat stress and N were negatively related. Meaning, the colonies of Acropora showed high mortality in the areas of low heat stress and high N content. In the areas of low heat stress, high nitrogen content doubles the severity in the corals. This lead to another theory that heat stress and N mediate the impact of each other. For both the coral species, researchers found no evidence that N and heat stress together interact and influence the bleaching activity. As the heat stress goes on increasing the influence of nitrogen on both the species goes on decreasing. 

Another interesting result was the way both these species react to nitrogen. In Pocillopora, excess N increases both, the probability of bleaching and the mortality of bleached corals. However, in Acropora excess N decreases the probability of bleaching but increased the mortality in bleached corals. All these surveys lead to one outcome that the excess of Nitrogen in low stressed areas might lead to mass bleaching and mortality of corals. 

 Apart from French Polynesia, the corals of the Great Barrier Reef and Caribbean islands also showed the negative impacts of increased nitrogen. Many types of research and studies are still going on today to completely understand the influence of nitrogen on corals.