The Effect of Groundwater Discharge on Coastal Ecosystems
by Misha Tran
The next time you reach for that cup of coffee as you try to gain a few more hours for that problem set, consider this: you may be contributing to the presence of caffeine in coral reefs and other coastal ecosystems. Dr. Adina Paytan, Assistant Professor of Geological and Environmental Sciences at Stanford, and her team of researchers including Dr. Boehm in Civil and Environmental Engineering, have found that groundwater discharge is a source of pollution for coastal waters.
The State of Coral Reefs Today
Coral reefs, found in the clear, warm waters of tropical oceans, are an important source of productivity and diversity in the ocean environment. The corals maintain a symbiotic relationship with algae, and together they provide food and shelter for many different species of fish and marine life. Additionally, coral reefs protect coasts from erosion by slowing down and thus minimizing the impact of strong waves.
In recent years, however, coral reefs have been declining in most regions of the world due to climate change and disease, as well as human activities such as sediment overloading, over-fishing and nutrient pollution (eutrophication). Increasing nutrient pollution in some locations has led to algal blooms and a decline in the coral reefs. The coral is better adapted to water where the nutrient concentration is low, whereas macro algae flourish where there are more nutrients, particularly phosphorous and nitrogen. In the Bahamas and Jamaica, “It’s been documented that nutrient loading has caused a phase shift from coral to algae,” says Dr. Paytan.
Focusing on Groundwater
Dr. Paytan and her team explored groundwater as a potential source of this nutrient pollution. Groundwater is fresh water found in aquifers, underground formations of permeable rock, and it flows at a slow rate underneath the surface and ultimately into the ocean at land-sea interfaces. Percolation of surface water, such as rainfall, replenishes the supply of groundwater but also introduces contaminants from land use.
Scientists have long suspected groundwater, but according to Dr. Paytan, no extensive study was previously conducted to “directly quantify the nutrient and pollutant flux associated with this submarine groundwater discharge and its effect on the reef.” Although the percolation of groundwater into the ocean is typically not a visible process, groundwater seeps into the ocean wherever waves wash the coast. The impact of pollution on surrounding coastal communities, including corals, has been felt, but a specific source of pollution was not always identified. Dr. Paytan decided to investigate groundwater as this “hidden” source.
Quantifying Pollution
Over the course of several years, Dr. Paytan's team sampled ocean water and groundwater in Mexico, Hawaii, California, Florida and the Red Sea. To determine the extent of groundwater mixing with ocean water, the level of radium isotopes was monitored. These geochemical tracers produced in rocks and soil are found at a much higher concentration in groundwater than in sea water. The researchers discovered that radium isotopes existed at higher than normal concentrations in open sea water at all sites of the land-ocean interface, including where coral live.
They then measured the type and amount of pollutants in the groundwater, and used this data to determine how much pollution from land sources was present in the sea water. Dr. Paytan and her collaborators found the presence of bacteria, estrogen, caffeine, pharmaceutical drugs, nitrate and other pollutants in sea water.
Since pinpointing groundwater as a source of coastal pollution, the researchers have returned several times to Hawaii and documented high radium and nutrient levels at different sites. In this next phase of her research, Paytan hopes to elucidate the relationship between the health of coral reefs and the amount of nutrients and pollutants in sea water, as well as the effect of land use on these two factors.
Looking into the Future
Ultimately, Paytan wants her research to raise public awareness about the impact that humans have on coral reefs and other coastal ecosystems. “I want the public to realize that there is another conduit of pollution that’s not easily seen." Paytan says that groundwater "is like an invisible invader.” Paytan believes that the protection of coral reefs is vital to the maintenance of biodiversity in the oceans and to the survival of industries, such as fishery and tourism, that depend on them.
Therefore, she recommends that the health of coral reefs be taken into account in coastal developmental projects. For example, instead of building golf courses and other facilities adjacent to the shoreline, we should build these farther away to minimize the impact of groundwater discharge on coral reefs. The challenge for us in the future is balancing economic development and expansion with the maintenance of the fragile natural resources that are invaluable to coastal ecosystems.