From: Quarterdeck Volume 5, Number 1, Spring 1997

Tools of the trade

Mary Jo Richardson
Wilford D. Gardner

Conductivity-Temperature-Depth meter (CTD)

The primary work horse for water column sampling is the conductivity-temperature-depth meter (CTD), an electronic package that we deploy over the side of the ship at sampling stations.

The package measures conductivity by passing a current through the water. We convert the conductivity measurement to salinity by comparing the result to conductivities of water with known salinities. The CTD measures temperature with an electronic thermometer and depth with a pressure sensor.

Salinity, temperature, and pressure are used to calculate water density. These four parameters are the basic pieces of information we always need to evaluate other data from seawater.

The electronic information produced by most of the other underwater instruments discussed in this article can be linked through theCTD. The data travel to the ship through a conducting cable and we view them in real time on a computer screen.

We can also surround the CTD with a rack of water bottles that are open at both ends. As the bottles descend their ends can be closed at whatever depths we desire depending on the water characteristics revealed by the other instruments. On the surface we analyze the water and its particles for the property we are studying.


CTD [33K]


Bottles arranged in a rosette formation on a rack with a CTD can be closed at different depths to collect water for later analysis in the laboratory.

Transmissometer

Transmissometers measure the transmission of light of a given wavelength over a known distance in seawater. Different types of particles and dissolved organic matter in the water absorb and reflect light of different wavelengths. We choose wavelengths for our instruments based on which particles we are studying. The wavelength we use the most is about 660 nanometers(red).

Surprisingly and fortunately, the amount of dissolved salt in the water does not affect the transmission of light through it. Transmission through particle-free fresh water and salt water is the same.

Changes in the transmission of light through water are primarily related to changes in the abundance and type of particles present.Most variations in transmission come from particles less than 20 microns in diameter. Large particles and aggregates larger than 500 microns in diameter are not abundant in the ocean. Only a few exist in 1000 milliliters of water, so they rarely appear in the small sensing volume of the transmissometer (45 milliliters). When they do, they appear as singular large values which we remove from our data.

 


Transmissometer [18K]

Attenuation

The amount of attenuation of light (c) equals the sum of light scattering (b) and absorption (a). It's as simple as a+b=c.

Measuring these values is not so simple.

Most attenuation results from light scattering. Phytoplankton, however, contain little packets of chlorophyll in their cells that act as sponges for light, allowing them to absorb energy and use it in photosynthesis. Thus, living phytoplankton absorb more light than other particles.

With an instrument that can measure light absorption and attenuation (the a-c meter), we can start to distinguish living phytoplankton from dead plankton and other particles without taking water samples.

 

 

Optical backscatter meter (OBS or Light scattering sensor (LSS)

Light scattering is difficult to measure completely because light scatters in all directions and the angle of scattering depends on the particles' sizes. Furthermore, as soon as light bounces off of one particle it can bounce off of another. We cannot distinguish whether we are measuring primary, secondary, tertiary, or greater scattering.

Nevertheless, small scattering meters have been made that are useful-especially in turbid water-as they provide a crude measure of the particle abundance in the water. Scattering meters project a beam of light into the water while a detector next to the light source measures the amount of light scattered back into it.

The optical backscatter meter (OBS) built by Downing Associates and the light scattering sensor (LSS) built by SeaTech use an infrared light source and a detector for this purpose. To determine the amount of particulate matter in the water, these instruments (like transmissometers) have to be calibrated by filtering particles from a known volume of water and weighing them very precisely.

 
Optical backscatter meter (OBS) or Light scattering sensor (LSS) [11K]

Chlorophyll sensors

Most phytoplankton produce chlorophyll which they need for photosynthesis. The amount of chlorophyll in the water should depend on the abundance of phytoplankton.

This simple notion is actually more complicated because the amount of chlorophyll in an organism depends on its species and environmental factors.

Still, a measurement of chlorophyll is useful and can be accomplished with an a-c meter or a fluorometer. The presence of chlorophyll in seawater causes a dramatic change in attenuation of light with a wavelength of about 676 nanometers. We quantify that change with an a-c meter to estimate the amount of chlorophyll in the water.

Using a different method, a fluorometer acts as a false sun and emits a flash of light at one wavelength, which triggers a response in phytoplankton that causes them to fluoresce, or give off a tiny amount of light, at another wavelength. The fluorometer quantifies the light from plankton, and we convert that to a measurement of chlorophyll in the water. The fluorometer is calibrated with discrete measurements of known quantities of chlorophyll.


a-c meter [14K]


Fluorometer [12K]

Particle and Optics Profiling System (POPS)

The Particle and Optics Profiling System (POPS) is an assembly of instruments designed to count and measure particles and to determine optical and environmental properties of water with depth. The heart of POPS is the Large Aggregate Profiling System (LAPS), designed and operated by our colleague, Dr. Ian Walsh.

LAPS consists of a video camera synched with a strobe light which flashes at predetermined intervals as the system is slowly lowered through the water. The light is channeled so that it illuminates only a narrow slab of water. The video camera faces the slab of light and records images of large particles in the water.

Your eyes record the same phenomenon when sunlight passes into a darkened room through a narrow slit in the curtains. You see hundreds of dust particles wafting through the slab of light, but the particles on either side of the slab are invisible. If you put a ruler next to the illuminated particles, you could measure their size. If you take a photograph of an area of known size, you can count the number and calculate the size of dust particles in the air. This is what we do with LAPS in seawater.

The video images generally show white dots, marine snow, in a dark sea. They are analyzed with software that counts the number of particles and determines their maximum and minimum dimensions if they are not circular. We use these data to estimate a volume and mass of the imaged particles, which range from 250 microns to several millimeters in size.

In addition to LAPS, POPS can carry a CTD, transmissometer, fluorometer, LISST, LSS and a-c meters. All except the CTD are optical instruments that measure either visibility in seawater or the sizes of particles.


POPS [53K]

Laser In-Situ Scattering and Transmissometer(LISST)

A Laser In-Situ Scattering and Transmissometer from Sequoia Instruments measures the size distribution of particles five to 250 micronsin diameter. LISST is attached to POPS and generates diagrams of the size distribution of particles and small aggregates in the water column without disturbing them by collecting them in water bottles. Particles' sizes influence their optical properties and settling dynamics, but many of the aggregates fall apart in water bottles before they can be returned to the lab and measured.

To study smaller discrete particles we collect water samples and precisely measure the number and size of particles in the one- to thirty-micron size range with a Coulter counter. The Coulter counter pulls the water sample through a small hole in a tube through which an electrical current is also flowing. Particles in the sample decrease the current flowing through the hole by an amount proportional to their volume. Doctors use the same type of instrument to count your red blood cells.


Laser in-situ scattering and transmissometer (LISST) [16K]

Sediment Trap

Many particles in seawater are either neutrally buoyant or settle very slowly-only 10 centimeters to 10 meters each day. Most of the optical signal comes from these small, slowly sinking particles. Other particles, how-ever, settle quickly-as fast as 10 to 100 meters each day-and play a major role in biogeochemical processes in the ocean. To collect the rapidly settling particles we attach sediment traps to a wire held taut by an anchor at the bottom and glass floats at the top. The traps are simple cylinders that collect the settling particles that are carried into the traps by coastal currents (1-40 cm/sec). In the laboratory we determine the size and composition of the material the traps collect.


Sediment Trap [7K]


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