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Data Collection Scope and Methods


Physical and chemical oceanography(Ref. 8)

Physical and chemical oceanographers study the movement of ocean water (currents) and the physical and chemical characteristics of that water (e.g., temperature, salinity, density, dissolved gasses, nutrients, geochemistry, fluxes, light penetration and distribution, and other primary and derived physical and chemical parameters). Data are collected by taking direct measurements on site, by instrumented moored buoys that telemeter their data back by satellite or are retrieved at a later time, and by aerial surveys and satellite imagery.

Conductivity, temperature and depth, or "CTD" measurements

The CTD is a device that can reach 1,640 ft (500 meters) in depth, taking up to five water samples at different depths, and making other measurements on a continuous basis on descent and ascent. Temperature and pressure are measured directly. Salinity is measured indirectly by measuring the conductivity of water to electricity.

Scientists preparing
                      to lower a shipboard CTD unit
Scientists preparing to lower a shipboard CTD unit (Photo: NOAA/PMEL)

Chlorophyll, a green photosynthetic pigment, is measured indirectly by a fluorometer that emits purple light and measures fluorescence in response to that light. These measurements are made continuously, providing a profile of temperature, salinity, and chlorophyll as a function of depth.

Shipboard CTD measurements are typically taken at three locations around a given island or atoll: the windward and leeward sides, and at a standard oceanographic "station" assigned to each island or atoll that is being surveyed over a long period of time. There is one such station per each major island or atoll in the NWHI: Nihoa, Necker, French Frigate Shoals, Gardner, Maro Reef, Laysan Island, Lisanski Island, Pearl and Hermes Atoll, MidwayAtoll, and Kure Atoll (Ref. 6).

Shallow, handheld water CTD measurements are taken to understand the local reef ecosystem. These measurements are taken from a small jet boat every mile or two around the island/atoll between the 80 and 120 foot (24 to 37 m) isobath (a contour of equal depth in a body of water), and in a few places inside the atolls. Data are needed at more locations than just three, and at depths shallower than a ship can operate. A handheld CTD includes a temperature sensor, a depth sensor, and a conductivity sensor. It also has a transmissometer that measures the level of particulate matter in the water (a proxy for turbidity). These measurements are also made continuously as the device descends and ascends. Unlike the larger version, the small boat CTD does not take water samples and does not have a fluorometer. Separate devices are used for these purposes.

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A handheld CTD
A handheld CTD (Photo: NOWRAMP)

Water samples

Water samples are taken by a handheld water bottle consisting of a tube with spring loaded caps at both ends. The caps are set in the open position so water can flow through the tube as it descends. A weight is then slid down the supporting rope to hit a trigger and close the caps. Water samples are typically taken at three depths, 5, 30, and 60 feet (1.5, 9, and 18 meters). The sampler is then taken to the surface, where the water is used to first rinse out a sample bottle (to avoid contamination from other water), and then to fill the sample bottle. The bottle is opaque to prevent further modification of the chlorophyll content by light. (Water from the large CTD is also stored in these same bottles).

Solar radiation

A radiometer on the jet boat replaces the fluorometer, and also gives important information about available solar radiation. Two radiometers are coupled together to take readings above and below the surface. An instrument on the boat reads the amount of light arriving at the ocean surface. Another instrument is put in the water to read the light reflected back from the water. This is compared to the former surface measurement for reference. These measurements are made at light frequencies relevant to photosynthesis. The concentration of chlorophyll can be measured by reading reflected light at certain wavelengths.

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Sea Surface Temperature (SST) buoys
Sea Surface Temperature (SST) buoys (Photo: NOWRAMP)

Time series measurements

In order to understand variability of an ecosystem, it needs to be monitored over many years. The instruments described below record a series of measurements of water characteristics over time. They include NOAA Coral Reef Early Warning System (CREWS) buoys, Sea Surface Temperature (SST) buoys, and SST "pipe bombs".

CREWS buoys are large buoys that are anchored at a specific location and can telemeter data back daily to scientists via satellite. They provide data that may warn scientists as soon as possible of an unusual change taking place in the coral reef environment. These buoys have sensors both below and above the water surface that measure water and air temperature, salinity, wind speed, and barometric pressure. A few of them also have radiometers to measure solar radiation, but these can only be located where staff can get to them every few weeks to clean the sensors. There is one at French Frigate Shoals, serviced by U.S. Geological Survey staff on Tern Island.

A CREWS Buoy installed in the lagoon at Rose Atoll, American Samoa.
A CREWS Buoy installed in the lagoon at Rose Atoll, American Samoa. (Photo: NOAA)

CREWS buoys are large and expensive, so other instruments are also used that measure fewer parameters but can be deployed in more locations. Sea Surface Temperature (SST) buoys are round floating buoys that are anchored in a specific location. They measure water temperature and telemeter these data back to a data receiving station at regular intervals via satellite. SST pipe bombs are strapped to the reef at different depths and locations around an atoll. These are set to measure temperature every half an hour, and record it on a data chip. Divers must retrieve these devices in order to obtain the data.

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A Towed Optical Assessment device (TOAD) being readied
A Towed Optical Assessment device (TOAD) being readied (photo: NOWRAMP)

Night observations

In order to maximize ship use, after dark operations are conducted during which various sensors are deployed as the ship follows designated tracks in deeper water locations. Night operations include shipboard CTD deployment, and Towed Optical Assessment Device (TOAD) surveys. TOAD consists of a video camera and lights on a frame designed to be towed just above the bottom. TOAD video is used to certify or ground truth acoustic habitat sensing. Specifically, the TOAD videos show the composition of the bottom in a few locations, such as sand, rubble, sea grass, coral, etc. TOAD allows interpretation of broad area acoustic data by comparing it to the video information.

Water movement

Several instruments are used to measure water movement: Wave and Tide Recorders (WTDs) measure the tide 48 times a day, and record wave height eight times a day in the process. They are deployed at 50 to 100 feet (15.2 to 30.5 meters); The Acoustic Doppler Current Profiler (ADCP) generates a 3-dimensional current profile by analyzing the Doppler-shift of fixed-frequency acoustic echos. The ADCP is typically mounted to the bottom of a ship, or tied to a mooring and uses sound waves to detect the motion of particles in the water. Sound waves are emitted into the water column at a frequency of around 150 KHz and the echo of the sound bouncing off small particles in the water can be used to determine the motion of the particles; Drifting buoys follow water at 49.2 feet (15 m) depth, measuring Global Positioning System (GPS) position and water temperature over time.

Benthic habitat mapping

The Pacific Islands Benthic Habitat Mapping Center (PIBHMC), located on the University of Hawaii Manoa campus, is tasked with the delineation of the benthic habitat of coral reef ecosystems throughout the U.S. Pacific Islands, including the Hawaiian and Mariana archipelagos, American Samoa and remote, U.S.- affiliated islands such as Johnston and Palmyra Atolls. PIBHMC was established as a result of a long-standing relationship between the University of Hawaii's School of Ocean and Earth Science and Technology (SOEST) and NOAA.

Using acoustic and optical techniques, PIBHMC extends shallow-water maps into deeper waters where satellite and diver-based techniques are not feasible. Products such as gridded bathymetric maps of the NWHI (http://www.soest.hawaii.edu/pibhmc/pibhmc_nwhi.htm) were created using data gathered from multibeam soundings. The data are also being used for benthic habitat mapping, for locating Essential Fish Habitat, and for studying geologic features of the area. These products provide resource managers with comprehensive habitat maps on which to base decisions about Pacific coral reef ecosystems.

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