Hidden sponges determine coral reef’s nutrient cycle

Marine organisms hidden in caves, such as sponges, play an extremely important role in the nutrient cycle of coral reefs. Indeed they probably play the most important role of all, says Dutch biologist Sander Scheffers. And that is valuable information for nature conservationists who want to preserve the coral reefs.

In order to protect coral reefs it is important to understand how both the reefs and their environment function. Researchers often concentrate on subjects such as physical damage to reefs, the bleaching of coral and coral diseases. Sander Scheffers investigated a lesser-studied subject: the nutrient cycle on the coral reef and the role that organisms living in cavities, such as sponges, play in this.

Underwater camera

To determine the nature and size of this role, Scheffers first of all examined the precise appearance and quantity of these virtually inaccessible caves and their living communities. He did that on the Caribbean island of Curaçao using a special underwater camera. The films shot revealed that sponges were the most important inhabitants, followed by animals such as tube worms, tunicates and bivalves. Together they fill more than 60 percent of the cavities. Further the cavities were found to have a surface area eight times greater than that of the coral reef, as seen from above by divers.

Filterers

And according to Scheffers a larger living surface also means a larger filtering surface. Sponges filter the water. They take up planktonic particles such as bacteria and excrete inorganic nutrients. In turn, these nutrients can facilitate the growth of marine plants and other organisms.

Sponges filter at a phenomenal rate: if the seawater were to remain stationary, the sponges would have completely pumped it away within five minutes, i.e. they would have removed all of the small plankton from it. This is of course not the case, as there is a continuous supply of fresh water into the sea. According to Scheffers, these hidden organisms play a key role in the marine nutrient cycle due to their incredible capacity to convert enormous quantities of organic plankton into inorganic material.

The results from Scheffers’ research have been made available to the personnel from the Marine under water park of Curaçao and have been presented to the local government.

Sander Scheffers’ research was funded by NWO-WOTRO.