Examining the Chemistry of Seawater and Coral to Promote the Health of West Maui’s Coral Reefs
Coral reefs are among the most diverse and biologically complex ecosystems on Earth. They provide nursery grounds for fish and shellfish; they protect harbors, beaches, and coastal communities from storm-wave damage and erosion; and they serve millions of people as areas of natural beauty and recreation. These important ecosystems are facing multiple threats worldwide, including climate change, disease, destructive fishing practices, coral collecting, and pollution and sediment from adjacent lands.
During fieldwork in March 2016, USGS scientists focused on understanding the link between land-based pollutants and coral reef health along the coast of west Maui in the Hawaiian Islands. Many reefs in this area are being overgrown by invasive seaweeds, a concern since the late 1980s. The algal growth has been linked to pollutants from the land, including elevated levels of nutrients—such as nitrogen and phosphorus—in water from wastewater injection wells. This nutrient-rich water enters the groundwater system and then discharges into the ocean near the reefs. In 2014, the federal district court in Honolulu ruled that Maui County’s use of injection wells at the Lahaina Wastewater Reclamation Facility violates the federal Clean Water Act and imposed civil penalties for the county’s violations.
Problems with nutrients and contaminants are not unique to the reefs of west Maui. Around the world, heightened nutrient loads have been attributed to recent land-use changes—such as residential subdivisions, golf courses, agriculture, suburban and commercial developments that rely on septic systems—as well as the establishment of wastewater treatment facilities that discharge water with elevated nutrient levels.
Corals are already being threatened by the effects of climate change, including warmer water and ocean acidification. Excessively warm water causes coral bleaching: the loss of single-celled organisms that live in the tissues of many corals, giving them color and providing them with vital energy. Ocean acidification—the lowering of seawater pH that results from increasing amounts of carbon dioxide (CO2) entering the atmosphere and dissolving into the ocean—makes it difficult for marine organisms such as shellfish and corals to build their calcium carbonate shells and skeletons.
Land-based sources of pollution that increase nutrients and change pH levels in coastal waters can magnify the global stressors to corals. One result is accelerated bioerosion—the breakdown of coral by other organisms. Bioerosion is a natural process that removes old dead coral and makes room for new coral, but if it proceeds more quickly than calcification (the process by which new coral is built), the reef will decline. With global warming and ocean acidification projected to compromise calcification, managing the compounding effects from local stressors is a top priority in coral reef management.
For two weeks in March 2016, research oceanographers Nancy Prouty of the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California, and Kim Yates of the USGS St. Petersburg Coastal and Marine Science Center in St. Petersburg, Florida, joined forces in an intensive field sampling program along the reefs offshore of Kāʻanapali, west Maui. Their goal was to quantify the key variables controlling the delicate balance between reef erosion and calcification. Together with geologist Nathan Smiley from the St. Petersburg center and lab assistant Chris Gallagher from the Santa Cruz center, they sampled seawater for a suite of chemical parameters (pH, alkalinity, nutrients, and isotopes) every four hours for six days in order to quantify how the carbonate chemistry along the reef varied over time and from place to place. This effort builds on previous studies of seawater flow and sediment movement in the area (for example, see Winds, Waves, Tides, and the Resulting Flow Patterns and Fluxes of Water, Sediment, and Coral Larvae off West Maui, Hawaiʻi) and underwater discharge of groundwater into the ocean near the reef (Observations of nearshore groundwater discharge: Kahekili Beach Park submarine springs, Maui, Hawaiʻi).
By applying a rigorous, high-resolution sampling protocol that Yates and Smiley have successfully used at reef sites in the Atlantic, Pacific, Gulf of Mexico, and Caribbean (for example, see Diverse coral communities in mangrove habitats suggest a novel refuge from climate change), the team is producing an unprecedented data set to quantify the exposure of reefs to high nutrient/low pH waters. Preliminary results from Yate’s real-time pH measurements suggest that the pH of freshwater discharging from underwater vents in the study area can range from 7.3 to 8.0. (Average seawater has a pH of 8.2.)
Yates’ measurements will complement research by Prouty and her team, who are analyzing coral cores collected along the reef to quantify bioerosion rates and inputs of nutrients over the past several decades. CT scans (a series of X-ray images) of the coral cores reveal skeletal density and the history of bioerosion. Chemical analyses reveal the amount of nutrients and other seawater constituents that have been incorporated into the coral over time. The team’s results, to be presented at the 13th International Coral Reef Symposium in June 2016, suggest that nutrient input from human activity is driving a disproportionate escalation of bioerosion rates under ocean acidification conditions. Reducing such stressors as nutrients and pollutants from nearby land is imperative for future coral reef resiliency.
In addition to doing their fieldwork, the team met with local residents at Kahekili Beach Park for a “talk story,” or informal chat, about their work and preliminary findings. (Read an article in the Lahaina News announcing the gathering and the fieldwork: USGS studying impacts of groundwater quality upon coral reef health.) The group included members of the West Maui Kumuwai, a movement to protect the area’s ocean and way of life, and members of the State of Hawaiʻi Makai Watch Program, a collaborative, statewide program in which citizens and nongovernmental organizations assist Hawaiʻi’s Department of Land and Natural Resources in the management of marine resources through promoting compliance to rules, education, and monitoring.
The scientists expect that their results will help resource managers make informed decisions to improve the health of coral reef ecosystems and associated watersheds.
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