Is Lightning a driver of the Sargassum blooms in the Atlantic?

It has been known for many decades that nitrogen oxide compounds (NO_x) are formed by lightning flashes due to the high temperatures in the lightning channel, which allows the otherwise tightly bounded N₂ and O₂ to react with each other. Lightning NO_x is then oxidized in cloud and rain drops to form nitric acid and deposited at the surface as nitrate (NO^-₃) in precipitation. This nitrate is a form of fixed nitrogen that can be taken up by ecosystems, especially where biological N fixation is limited.

Since 2011, researchers have repeatedly observed the so-called Great Atlantic Sargassum Belt, a gigantic carpet of seaweed that drifts from the equator towards the Caribbean when easterly winds prevail. Until now, the sources of nutrients fueling their rapid growth are unclear. It was hypothesized that nutrient runoff from overfertilization and rainforest deforestation might be responsible or upwelling of phosphorus-rich deep waters. However, these processes cannot completely explain the increase in Sargassum biomass observed during the past years. Nitrogen is a key element governing the dynamics and function of many ecosystems as many of them are limited in biologically available nitrogen supply. The lack of N is an important inhibitor on primary production in the tropics. Owing to this limitation, an increase in available N from lightning could increase the primary production and biomass accumulation.

Our analysis of the spatial distribution of lightning and Sargassum blooms over the tropical Atlantic show remarkable agreement during specific months of the year, as well as the annual cycle of the blooms that peak in the northern hemisphere summer.  While global lightning activity is expected to increase with rising global temperature, it is not clear that there has been a significant increase in lightning over the Atlantic in recent decades.  Nevertheless, lightning has not yet been considered as a possible source of nitrogen impacting the Sargassum blooms.