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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.
to lower a shipboard CTD unit (Photo: NOAA/PMEL)
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.
A handheld CTD (Photo: NOWRAMP)
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).
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.
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. (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.
A Towed Optical Assessment device (TOAD) being readied (photo: NOWRAMP)
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
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.