In situ monitoring of eutrophication
Contents
Introduction
In situ monitoring is the observation and / or measurement of events in its original place (Latin: situs). Oceanographic instruments containing different types of sensors are used to monitor eutrophication in coastal waters. Sensors detect and respond to electrical or optical signals and convert the physical, chemical or biological parameter into a signal which can be measured electrically.
Oceanographic instruments
CTD
The CTD[1][2] - Conductivity (salinity), Temperature and Depth (pressure) recorder - is the standard oceanographic tool for continuously measurement of physical properties of sea water. The CTD is mostly attached to a frame with water-collecting Niskin bottles (CTD rosette). From the deck the rosette is lowered on a cable down to the seafloor and once in the water data are transferred via a conducting cable connecting the CTD to a computer on a ship. The Niskin bottles are closed at predefined depths to target water samples for further analysis. Other sensors to measure chemical or biological parameters such as dissolved oxygen, chlorophyll fluorescence (phytoplankton concentrations) and water light transmission can be added to the cluster.
Sensors
In this section we focus only on the sensors that measure parameters that are important in the frame of the OSPAR Eutrophication Monitoring Programme: [3]
Temperature
The simplest mechanical way to measure temperature is by using a mercury-in-glass thermometer. They are commonly used to measure sea surface temperature by placing it in a bucket of sea water. Electrical temperature sensors such as the Resistance Temperature Detectors (RTDs) and the thermistors are more frequently used on a CTD. A thermistor is a type of resistor composed of a small piece of electrically semiconductor material (metallic oxides) such as a resistor which exhibits a large change in resistance proportional to a small change in temperature (negative temperature coefficient). RTDs are sensors used to measure temperature by correlating the resistance of the RTD element (pure metals usually platinum) with temperature (positive temperature coefficient).
More information: Temperature sensors
Salinity
Salinity is a measure of the quantity of salt in a volume of sea water and can be measured from a water bottle sample using a salinometer or in situ using a conductivity sensor on a CTD. The sensor measures the electrical conductivity (electrical current: ions in solution that flow through the sea water (dissolved salts conduct more electricity than water without salts)) at a known temperature and pressure and this conductivity is converted to salinity using a formula.
More information: Salinity sensors
Turbidity
Turbidity is a measure of water clarity or transparency caused by suspended particles such as soil particles, algae, plankton,... The more turbid the water the less light is transmitted. Many algae can keep out of the light causing an increase of turbidity. A common method for measuring ocean turbidity is the Secchi disk. The disc is being lowered into the water and the depth at which the disc is no longer visible, is a measure of the clarity of the water.
See also
References
- ↑ http://www.whoi.edu/instruments/viewInstrument.do?id=1003[1]
- ↑ http://noc.ac.uk/research-at-sea/nmfss/nmep/ctd[2]
- ↑ OSPAR Commission (2005), Agreement on the Eutrophication Monitoring Programme (Reference Number: 2005-4)[3]
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