What role does groundwater play in coastal seas?
A new study in Nature Reviews Earth & Environment highlights the major importance of nutrient inputs to coastal oceans from groundwater. Such nutrients can play a critical role in the function and vulnerability of coastal ecosystems worldwide.
Nutrient budgets in the ocean are subject to a variety of influences. Rivers contribute industrial residues and fertilizers from agriculture to the coastal ocean, wastewater from settlements or aquaculture facilities is discharged directly in many countries, and even wind blows significant amounts of nutrients into the ocean, especially in desert regions.
But nutrients also enter the oceans through groundwater. Marine research has only been devoting more intensive attention to this aspect in recent years, and it has hardly been taken into account in coastal management. This is because the sources are almost invisible, they usually lie below the water surface and are highly variable in their number and activity.
A new article in Nature Reviews Earth & Environment now evaluates the results of numerous studies. They were conducted at more than 200 sites in coastal areas worldwide, ranging from polar to tropical seas. The studies quantify nutrient fluxes from groundwater inputs and explain their effects on marine ecosystems. The international collective of authors includes hydrogeologist Prof. Dr. Nils Moosdorf of the Leibniz Centre for Tropical Marine Research (ZMT).
60% of the studies show that at the locations investigated, nutrients such as nitrogen, phosphorus and silicon enter the sea in greater quantities via groundwater than for example via rivers, the largest sources of nutrients in the oceans. This makes groundwater the most important nutrient supplier to coastal ecosystems at many coastal locations. Projections showed that groundwater worldwide releases around 140 million tons of silicon, 40 million tons of nitrogen and 9 million tons of phosphorus into the sea each year.
Fresh groundwater makes its way through layers of rock or sand into the sea, where it seeps out in coastal areas. But the studies also considered nutrient inputs from marine sediment. Organic material is deposited there and decomposed. It can lie buried for hundreds of years until the nutrients are washed out of the sand again.
Depending on the location, such substance inputs can be a blessing or a curse for coastal ecosystems. For example, groundwater sometimes promotes coral reef growth.
Local fisheries can also benefit. Plankton thrive particularly well due to the nutrients, so there is plenty of food for fish. The nutrient-rich groundwater sources of Australia’s „Wonky Holes,“ for example, are hotspots for fish such as giant bass and are therefore of importance for fishing and tourism.
However, there are also negative, sometimes very massive, impacts of groundwater flows on coastal ecosystems. In many places, excess nutrients lead to algal blooms. Toxic pollutants can enter the sea via groundwater. Also, fresh groundwater usually has a low pH and can contribute to coastal water acidification in some places.
Such negative effects of nutrients are seen off the coast of Hawaii, for example, where the U.S. Supreme Court recently ruled that groundwater must be better protected to guard the coastal ocean from harm,
„The studies show that groundwater is a very important source of nutrients for the oceans and is an essential component of coastal ecosystem function and vulnerability, both locally and globally,“ Moosdorf said.
Groundwater inputs should therefore be given greater consideration in research and coastal management, according to Moosdorf. Climate warming and increasingly intensive land use are expected to change the chemical composition and volume of inputs. A proper understanding of their role in the ecological and economic context is needed to develop effective coastal management strategies.
Prof. Dr. Nils Moosdorf
Head of the Submarine Groundwater Discharge Unit at the Leibniz Centre for Tropical Marine Research (ZMT)
Tel: 0421 / 23800-33
Isaac R. Santos, Xiaogang Chen, Alanna L. Lecher, Audrey H. Sawyer, Nils Moosdorf, Valentí Rodellas, Joseph Tamborski, Hyung- Mi Cho, Natasha Dimova, Ryo Sugimoto, Stefano Bonaglia, Hailong Li, Mithra- Christin Hajati and Ling Li: Submarine groundwater discharge impacts on coastal nutrient biogeochemistry. Nature Reviews Earth & Environment. Link: https://www.nature.com/articles/s43017-021-00152-0