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Over the past decade, there has been a notable change in seafloor-bottom type along west Maui, Hawai‘i. Nearshore areas, once dominated by corals, are now mostly overgrown by algae, suggesting a local nutrient imbalance that warrants further investigation. Previous studies by the U.S. Environmental Protection Agency (EPA), U.S. Geological Survey (USGS), State of Hawai‘i, and University of Hawai‘i have documented the magnitude of change along this section of the coast and have investigated the causes driving these changes.

In 2009, the Hawai‘i Department of Land and Natural Resources’ Division of Aquatic Resources (HI-DAR) established the Kahekili Herbivore Fisheries Management Area (KHFMA), designated to improve the health of the coral reef ecosystem by increasing the number of herbivores (plant eaters) to help reduce the turf algae and macroalgae that have overgrown the corals at the site.

In early 2011, the U.S. Coral Reef Task Force (USCRTF) Watershed Partnership Initiative selected the Kā‘anapali area of west Maui as the second of three national priority study areas on which to focus its research and restoration efforts. Driven by this national support, the Hawai‘i Coral Reef Strategy identified the coral reef ecosystem of west Maui as a priority management area. Likewise, the West Maui Ridge to Reef (R2R) Initiative published the Wahikuli-Honokāwai Watershed Management Plan, which provides a framework for reducing the generation and transport of land-based pollutants (generally sediment, but also nutrients, such as nitrogen from agricultural fertilizers) to improve water quality and the health of west Maui’s coral reef ecosystems.

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In cooperation with HI-DAR, the USGS Coastal and Marine Geology Program initiated a multidisciplinary effort in 2012 to provide scientific information in support of both USCRTF and HI-DAR priorities in west Maui. This effort includes the following:

• Constructing a high-resolution map of the underwater environment along the west coast of Maui.

• Recovering coral cores to provide historical records of sediment and nutrient inputs to the reefs along west Maui, in order to put recent instrument measurements into longer-term context.

• Making time-series measurements (measuring the same variable at regular time intervals) of nutrient and contaminant inputs to coastal waters from the submarine discharge of fresh groundwater (flowing seaward in underground aquifers and discharging from the seafloor either by diffuse flow from sediment pores or through discrete submarine vents).

• Deploying satellite-trackable drifters from key reefs to measure near-surface currents that transport coral larvae, in order to better understand the connectivity between different reefs and islands.

Susan Cochran and Ann Gibbs (USGS Pacific Coastal and Marine Science Center [PCMSC], Santa Cruz, California) worked with Darla White (HI-DAR) in winter 2012–2013 to begin constructing high-resolution maps of the underwater environment along the west coast of Maui (see map above). They used existing satellite imagery, bathymetric data from airborne lidar (light detection and ranging), sidescan-sonar data, and georeferenced underwater video. Maps of dominant structure (such as mud, sand, reef) and major biological cover (such as coral, macroalgae, coralline algae) were created using methodology and a classification scheme used by the National Oceanic and Atmospheric Administration (NOAA) but at much finer spatial resolution.

A minimum mapping unit (MMU) of 100 square meters was used (as opposed to the NOAA MMU of 1 acre [4,047 square meters]), and smaller features were mapped if they carried habitat significance (for example, an individual coral colony 2 meters in diameter located in an otherwise uncolonized area). During a multifaceted interagency effort in July 2013, the USGS Coral Reefs Project, along with HI-DAR personnel, collected seafloor photographs and video for the ground-truth survey.

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Also in July 2013, Nancy Prouty, Josh Logan, Tom Reiss, Pete Dal Ferro, and Curt Storlazzi (USGS-PCMSC) collected several cores along the west Maui coast. Given the length of the cores, it may be possible to document environmental conditions as far back as pre-European times. Prouty and Storlazzi instrumented the coral head from which one of these cores was collected with temperature, salinity, turbidity, and chlorophyll sensors in October 2012. Comparing the measurements from these sensors with the chemistry of recently formed layers of the coral skeleton will help scientists quantify relationships between coral chemistry and seawater chemistry. This calibration will enable them to use ancient coral layers to learn about the chemistry of the seawater in which those layers formed.

Processes related to the sustained discharge of coastal groundwater at a site off west Maui were studied by Peter Swarzenski, Pamela Campbell, Cordell Johnson, Randy Russell, and Storlazzi (USGS-PCMSC); Kevin Kroeger (USGS Woods Hole Coastal and Marine Science Center [WHCMSC], Woods Hole, Massachusetts); and Joe Fackrell and Meghan Dailer (University of Hawai‘i, Honolulu). Their diverse list of analyses included time-series measurements of two radioactive isotopes that occur naturally in groundwater and so serve as groundwater “tracers”: radon (222Rn, half-life = 3.8 days) and thoron (220Rn, half-life = 55.6 sec). These measurements were complemented with data from strategically placed thermistor arrays (for measuring temperature), wave- and pressure-sensors, and an acoustic Doppler current profiler (ADCP, for measuring current velocities at different heights above the seafloor). The scientists also deployed an acoustic Doppler velocimeter (ADV) capable of accurately measuring wave-induced pumping of focused coastal groundwater discharge.

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A suite of water samples were collected from both nearby and distant groundwater wells, surface and bottom water, and sites of submarine groundwater discharge to better understand the geochemical character and composition of the discharging groundwater. Additionally, a thermal-infrared (TIR) camera installed in a building near the beach collected time-series images to record variations in submarine groundwater discharge as revealed by temperature contrasts between the discharging groundwater plume and ambient surface water. Essential logistical support was most ably provided by White of HI-DAR and Sulinn Aipa and Ryan Nobriga of the Westin Ka’anapali Ocean Resort (in which the TIR camera was installed).

Lastly, Storlazzi, Logan, Reiss, Cochran, and Russell partnered with HI-DAR staff to deploy a series of satellite-tracked, near-surface current drifters during the nighttime spawning of the reef-building coral Montipora capitata. The drifters were released on two nights during the July 2013 spawning event, from one reef off south-central Moloka‘i and three reefs off west Maui, two of which are also State of Hawai‘i marine managed areas (Honolua-Mokule‘ia Bay Marine Life Conservation District and Āhihi Kīna‘u Natural Area Reserve). These drifters were tracked for six days and elucidated the interconnected nature of the coral reefs in the main Hawaiian Islands, as drifters from Maui reached the reefs of O‘ahu and Kaho‘olawe during the time frame when the developing larvae were still in their positively buoyant, planktonic stage.

In order to better communicate the recent findings and goals of our ongoing studies to Federal, State, and local partners, Storlazzi briefed the USCRTF, HI-DAR, and the Maui Nui Marine Resource Council one evening after field operations, and both he and Prouty gave invited lectures at the University of Hawai‘i Maui College at the end of field operations. All in all, a wide range of multidisciplinary data was acquired, and interaction with partners occurred in support of National, State, and local needs.

Learn more about the USGS Coral Reef Project.

Learn more about USGS coastal groundwater studies.

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