Submarine groundwater as beneficial as rivers to the Mediterranean

A study led by the UAB and published in PNAS has for the first time estimated the magnitude of submarine groundwater discharge into the Mediterranean Sea, which can be similar or up to 15 time higher than what flows in from rivers, and the nutrient fluxes associated with this water.

17/03/2015

Submarine groundwater discharge (SGD) consists of a mixture of continental fresh water and salt water which permeates from the coastal aquifer. In addition to its importance in the water cycle, as a potentially exploitable resource and a source of water for brackish coastal waters such as marshes and lagoons, it also can serve as an important source of dissolved compounds such as nutrients, trace metals and toxic metals.

Now, a study led by researchers from the Institute of Environmental Science and Technology (ICTA) and the Department of Physics of the (UAB) has been the first to calculate the magnitude of SGD into the Mediterranean Sea, as well as the flux of dissolved nutrients. The study, published in Proceedings of the National Academy of Sciences of the United States of America (PNAS), also included the participation of researchers from the Bar-Ilan University of Israel (BIU; Ramat-Gan).

The study indicates that the annual volume of SGD for the whole Mediterranean basin ranges from 30,000 to 50,000 million cubic metres, which proves that this process is relevant at large scale and its discharge is similar or up to 15 times greater than that of river water inputs. The flux of nutrients associated with this discharge consists of an annual median of three million tonnes of nitrogen, twenty thousand of phosphorous, and three million more of silica, which represents a magnitude of inorganic nutrients comparable to that of external sources traditionally considered in marine studies, such as the atmospheric deposition and riverine runoff.

“The magnitude of SGD and fluxes of associated nutrients demonstrates their relevance in the biogeochemical cycles of the Mediterranean Sea, making it even more necessary to include this process in future marine studies, both at coastal and global level, given that until now this process has been ignored in these types of studies. Theses fluxes could have a large influence on the cycles seen in coastal environments, especially in areas with low availability of nutrients and in semiarid regions, such as the Mediterranean Sea,” explains Valentí Rodellas, UAB researcher in charge of directing the study as part of his PhD thesis.

“The entrance of inorganic nutrients associated to SGD which we quantified can be particularly relevant for the marine ecosystems of coastal areas located further away from river mouths, given that in these ecosystems they may be the only continual source of dissolved compounds reaching the sea”, explains Jordi Garcia-Orellana, UAB researcher and co-author of the study.

Pere Masqué, also co-author of the study, adds that, “in addition to nutrients, SGD can play a crucial role as a source of other dissolved compounds flowing into the Mediterranean Sea, such as iron, carbon and rare earth elements, if we take into account the magnitude of the calculated fluxes and the high concentration of these compounds in submarine groundwaters”.

To conduct the study, researchers used Ra-228, a natural radioactive tracer ideal in quantifying the submarine groundwater discharge into the sea at basin level. The application of this method required the participation of several oceanographic campaigns in order to distribute the Ra-228 throughout the Mediterranean Sea's column of water.

Original article:
Rodellas, V., Garcia-Orellana, J., Masqué, P., Feldman, M., Weinstein, Y. (2015). Submarine groundwater discharge as a major source of nutrients to the Mediterranean Sea. Proceedings of the National Academy of Sciences of the United States of America (PNAS).