3.3.3 Chlorophyll and Cyanobacteria Sensors
Chlorophyll and cyanobacteria sensors measure the amount of algal biomass in the
water, which is an indication of the primary production capability of the system,
and the overall health of the water body. Too little algae in the system can result in a
lack of food for larger zooplankton and a lack of dissolved oxygen. An
overabundance of algae in the system can be indicative of eutrophication and can
result in reduced light penetration and sags in dissolved oxygen during peak periods
of respiration or when the algae die off and are decomposed by the microbial loop.
Some algal blooms can also produce toxins which may be harmful to living
resources and/or human health. Algal biomass is measured by exciting the chlorophyll and phycobilin pigments in the algal cells with an LED of specific wavelengths and measuring the intensity of the emitted light, which is proportional to the
concentration of the algal biomass. The excitation and emission wavelengths of
these sensors vary for different manufacturers; as an example, for YSI EXO total
algae sensors, the emission is 685 Æ 20 nm, the chlorophyll excitation is
470 Æ 15 nm, and the phycocyanin excitation is 590 Æ 15 nm.
3.3.4 Optical Nitrate Sensors
Optical nitrate sensors operate on the principle that nitrate ions absorb ultraviolet
(UV) light (wavelengths less than 220 nm) which is measured by a photometer and
then converted to a nitrate concentration. Optical nitrate sensors are designed to
convert spectral absorption properties measured to a nitrate concentration by using
laboratory calibrations and integrated algorithms that account for interferences
from other absorbing ions and organic matter. These sensors allow for real-time
nitrate measurements without the need for chemical reagents and demonstrate good
in-stream accuracy, typically within 3–5 % of laboratory data [15].
Optical sensors offer numerous advantages over ion-selective electrodes (ISEs)
and wet chemistry analyzers including higher resolution, accuracy, precision,
measurement range, chemical-free operation, faster response time, and the benefit
of additional spectral information. However, these instruments are currently very
power hungry and require some form of infrastructure for deployment (e.g., DCP
station with solar power). Figure 14 illustrates the results of real-time nitrate
concentrations measured by optical nitrate sensors (reported as nitrogen) and
streamflow (discharge) in Smith Creek near New Market, Virginia (USGS Site
01632900).
Advances in Water Sensor Technologies and Real-Time Water Monitoring
191
Chlorophyll and cyanobacteria sensors measure the amount of algal biomass in the
water, which is an indication of the primary production capability of the system,
and the overall health of the water body. Too little algae in the system can result in a
lack of food for larger zooplankton and a lack of dissolved oxygen. An
overabundance of algae in the system can be indicative of eutrophication and can
result in reduced light penetration and sags in dissolved oxygen during peak periods
of respiration or when the algae die off and are decomposed by the microbial loop.
Some algal blooms can also produce toxins which may be harmful to living
resources and/or human health. Algal biomass is measured by exciting the chlorophyll and phycobilin pigments in the algal cells with an LED of specific wavelengths and measuring the intensity of the emitted light, which is proportional to the
concentration of the algal biomass. The excitation and emission wavelengths of
these sensors vary for different manufacturers; as an example, for YSI EXO total
algae sensors, the emission is 685 Æ 20 nm, the chlorophyll excitation is
470 Æ 15 nm, and the phycocyanin excitation is 590 Æ 15 nm.
3.3.4 Optical Nitrate Sensors
Optical nitrate sensors operate on the principle that nitrate ions absorb ultraviolet
(UV) light (wavelengths less than 220 nm) which is measured by a photometer and
then converted to a nitrate concentration. Optical nitrate sensors are designed to
convert spectral absorption properties measured to a nitrate concentration by using
laboratory calibrations and integrated algorithms that account for interferences
from other absorbing ions and organic matter. These sensors allow for real-time
nitrate measurements without the need for chemical reagents and demonstrate good
in-stream accuracy, typically within 3–5 % of laboratory data [15].
Optical sensors offer numerous advantages over ion-selective electrodes (ISEs)
and wet chemistry analyzers including higher resolution, accuracy, precision,
measurement range, chemical-free operation, faster response time, and the benefit
of additional spectral information. However, these instruments are currently very
power hungry and require some form of infrastructure for deployment (e.g., DCP
station with solar power). Figure 14 illustrates the results of real-time nitrate
concentrations measured by optical nitrate sensors (reported as nitrogen) and
streamflow (discharge) in Smith Creek near New Market, Virginia (USGS Site
01632900).
Advances in Water Sensor Technologies and Real-Time Water Monitoring
191