46
waters, however, excess nutrients can cause unchecked algal growth leading to
eventual collapse of the populations and ensuing anoxia due to bacterial respiration,
a condition known as eutrophication. Fertilizer in agricultural runoff, and human,
domestic animal, and food industry waste are some of the major anthropogenic
sources of nutrients to the aquatic environment. Habitat degradation with loss of
aesthetic quality, massive fish kills, and harmful algal blooms are some of the consequence of anthropogenic eutrophication of coastal waters. Economic consequences can be severe. Sustained observations of these nutrient concentrations in
nearshore waters provide forewarning of such events and assists in the identification
and remediation of hotspots.
Inorganic dissolved phosphorus exists in solution in seawater for the most part as
the bi-protonated phosphate ion (H 2 PO 4
−
) or its mono-protonated form (HPO 4
2−
).
These forms are collectively referred to as orthophosphate. Inorganic nitrogen on
the other hand can exist in a number of chemical species ranging from the highly
reduced ammonia/ammonium equilibrium couple (NH 3 /NH 4
+
) with a redox number
of -III, through molecular nitrogen (N 2 ) with redox zero through several other forms
including the laughing gas nitrous oxide (N 2 O) redox +I, to the highly oxidized
nitrite (redox +III) and nitrate (redox +V) ions. Of these, molecular nitrogen is the
most abundant with around 400 μmol.kg
−1
in solution in surface waters. Molecular
nitrogen is however largely unavailable to most taxonomic groups for biosynthesis
due to the strength of the triple bond between atoms. This bond can only be attacked
biologically by those organisms equipped with the nitrogenase enzyme, a few genera of bacteria and cyanobacteria. Thus, ammonium, nitrate and nitrate are the inorganic nitrogenous species most rapidly cycled in coastal marine waters. One more
element, silicon (Si), although not a true metabolite, is traditionally classed among
the essential nutrients due to its role in forming the skeletal material of diatoms as
amorphous silica (SiO 2 ). In the ocean it is found mostly as dissolved silicic acid
(H 4 SiO 4 ) and is referred to in the literature as silicate. Iron, while limiting in some
open ocean waters, is usually abundant in nearshore waters.
Traditional analytical procedures for characterization of these dissolved nutrients are performed through wet chemistry protocols during which reagents (added
in excess) react to form colored products. The color intensity of the resulting solution is dependent on the analyte concentration which constitutes the reagent limiting
to the reaction. The optical density or absorption (A) of the colored solution is measured by means of a spectrophotometer (an instrument equipped with a diffraction
grating monochromator) or a simpler colorimeter (equipped with colored optical
filters) against a set of standards of known concentration prepared in the laboratory
including a zero standard known as the reagent blank. Laboratory protocols for wet
chemistry analysis of reactive phosphorus and reactive silicate through the formation of colored molybdate complexes and of nitrite and nitrate through diazotization
(with prior reduction of nitrate to nitrite) have changed little since their original
descriptions. Strickland and Parsons (1972) provide authoritative accounts of the
development and practice of these procedures. Limits of detection using the standard 10 cm cell are: 0.3 μmol.l
−1
for phosphate (reactive phosphorus), 0.1 μmol.l
−1
2 Electronic Sensors and Instruments for Coastal Ocean Observing
waters, however, excess nutrients can cause unchecked algal growth leading to
eventual collapse of the populations and ensuing anoxia due to bacterial respiration,
a condition known as eutrophication. Fertilizer in agricultural runoff, and human,
domestic animal, and food industry waste are some of the major anthropogenic
sources of nutrients to the aquatic environment. Habitat degradation with loss of
aesthetic quality, massive fish kills, and harmful algal blooms are some of the consequence of anthropogenic eutrophication of coastal waters. Economic consequences can be severe. Sustained observations of these nutrient concentrations in
nearshore waters provide forewarning of such events and assists in the identification
and remediation of hotspots.
Inorganic dissolved phosphorus exists in solution in seawater for the most part as
the bi-protonated phosphate ion (H 2 PO 4
−
) or its mono-protonated form (HPO 4
2−
).
These forms are collectively referred to as orthophosphate. Inorganic nitrogen on
the other hand can exist in a number of chemical species ranging from the highly
reduced ammonia/ammonium equilibrium couple (NH 3 /NH 4
+
) with a redox number
of -III, through molecular nitrogen (N 2 ) with redox zero through several other forms
including the laughing gas nitrous oxide (N 2 O) redox +I, to the highly oxidized
nitrite (redox +III) and nitrate (redox +V) ions. Of these, molecular nitrogen is the
most abundant with around 400 μmol.kg
−1
in solution in surface waters. Molecular
nitrogen is however largely unavailable to most taxonomic groups for biosynthesis
due to the strength of the triple bond between atoms. This bond can only be attacked
biologically by those organisms equipped with the nitrogenase enzyme, a few genera of bacteria and cyanobacteria. Thus, ammonium, nitrate and nitrate are the inorganic nitrogenous species most rapidly cycled in coastal marine waters. One more
element, silicon (Si), although not a true metabolite, is traditionally classed among
the essential nutrients due to its role in forming the skeletal material of diatoms as
amorphous silica (SiO 2 ). In the ocean it is found mostly as dissolved silicic acid
(H 4 SiO 4 ) and is referred to in the literature as silicate. Iron, while limiting in some
open ocean waters, is usually abundant in nearshore waters.
Traditional analytical procedures for characterization of these dissolved nutrients are performed through wet chemistry protocols during which reagents (added
in excess) react to form colored products. The color intensity of the resulting solution is dependent on the analyte concentration which constitutes the reagent limiting
to the reaction. The optical density or absorption (A) of the colored solution is measured by means of a spectrophotometer (an instrument equipped with a diffraction
grating monochromator) or a simpler colorimeter (equipped with colored optical
filters) against a set of standards of known concentration prepared in the laboratory
including a zero standard known as the reagent blank. Laboratory protocols for wet
chemistry analysis of reactive phosphorus and reactive silicate through the formation of colored molybdate complexes and of nitrite and nitrate through diazotization
(with prior reduction of nitrate to nitrite) have changed little since their original
descriptions. Strickland and Parsons (1972) provide authoritative accounts of the
development and practice of these procedures. Limits of detection using the standard 10 cm cell are: 0.3 μmol.l
−1
for phosphate (reactive phosphorus), 0.1 μmol.l
−1
2 Electronic Sensors and Instruments for Coastal Ocean Observing