CHAPTER 3 . Photochemical Processes in the Euphotic Zone of Sea Water
95
ters of the oceans. The levels of dissolved and particulate concentrations of all the
organic compounds are much lower in deep waters than in surface waters.
Dissolved marine organic matter is such an extremely complex and dilute mixture
of compound that only 10 to 20% can be full characterized. Measurements of dissolved
organic carbon, nitrogen and phosphorus compounds are standard measurements
used to obtain an understanding of the major types of organic compounds present in
the oceans. The dissolved organic matter in sea water consists largely of humic substances and more labile compounds from the major biochemically important compound classes such as carbohydrates, steroids, alcohol, amino acids, hydrocarbons and
fatty acids.
Particulate organic matter consists of a mixture of living and dead phytoplankton
and zooplankton, bacteria, their degradation products and macroscopic aggregates.
The distribution and nature of particulate organic matter has been found to be quite
variable both geographically, vertically, diurnally, and seasonally, and it is influenced
by a complex set of equilibria between sources, sinks and circulation patterns. In the
euphotic zone, the major portion of paM is due to phytoplankton, the chemical composition of which will vary with species as well as with environmental conditions,
hence, leading to significant variations in the nature of the paM. The metabolic products of phytoplankton can vary with changes in temperature, light intensity and nutrient availability. In the euphotic zone, the majority (90%) of poe is due to living
matter, while at depths of 2 400 m and more, less than 1% of the poe is due to living
organisms.
3.4
Role of Iron and Chlorine
Iron is an essential trace metal. In fact, as redox reactions and electron transport are
two major functions of iron, it affects the enzyme systems. Iron, along with molybdenum, is also essential for nitrogen fixation and nitrate reduction and thus for the assimilation of the major nutrient nitrogen.
It is hypothesized that iron availability limits specific rates of phytoplankton growth
(Martin et al. 1990; Brand 1991). Most marine photosynthetic organisms can only take
up iron in the dissolved form. Under sea water conditions, the solubility of iron is extremely low. Careful analytical studies in the oceans have shown that the concentrations of total dissolved iron range from 0.02 to 10 nM.
The role of chloride in the marine environment is also crucial. Several classes of
mechanisms for dechlorination of sea salt aerosol have been hypothesized. More than
one kind of mechanism is probably operating in the marine boundary layer, and the
relative importance of different mechanisms probably varies over space and time. An
improved understanding of inorganic CI chemistry in the marine boundary layer is
needed because of its potential importance for global chemistry (Keene 1995).
3.4.1
Inorganic (I Formation in the Marine Environment
The possible mechanisms for formation of atomic chlorine in the marine environment
have been widely investigated, and several possible sources have been identified.
95
ters of the oceans. The levels of dissolved and particulate concentrations of all the
organic compounds are much lower in deep waters than in surface waters.
Dissolved marine organic matter is such an extremely complex and dilute mixture
of compound that only 10 to 20% can be full characterized. Measurements of dissolved
organic carbon, nitrogen and phosphorus compounds are standard measurements
used to obtain an understanding of the major types of organic compounds present in
the oceans. The dissolved organic matter in sea water consists largely of humic substances and more labile compounds from the major biochemically important compound classes such as carbohydrates, steroids, alcohol, amino acids, hydrocarbons and
fatty acids.
Particulate organic matter consists of a mixture of living and dead phytoplankton
and zooplankton, bacteria, their degradation products and macroscopic aggregates.
The distribution and nature of particulate organic matter has been found to be quite
variable both geographically, vertically, diurnally, and seasonally, and it is influenced
by a complex set of equilibria between sources, sinks and circulation patterns. In the
euphotic zone, the major portion of paM is due to phytoplankton, the chemical composition of which will vary with species as well as with environmental conditions,
hence, leading to significant variations in the nature of the paM. The metabolic products of phytoplankton can vary with changes in temperature, light intensity and nutrient availability. In the euphotic zone, the majority (90%) of poe is due to living
matter, while at depths of 2 400 m and more, less than 1% of the poe is due to living
organisms.
3.4
Role of Iron and Chlorine
Iron is an essential trace metal. In fact, as redox reactions and electron transport are
two major functions of iron, it affects the enzyme systems. Iron, along with molybdenum, is also essential for nitrogen fixation and nitrate reduction and thus for the assimilation of the major nutrient nitrogen.
It is hypothesized that iron availability limits specific rates of phytoplankton growth
(Martin et al. 1990; Brand 1991). Most marine photosynthetic organisms can only take
up iron in the dissolved form. Under sea water conditions, the solubility of iron is extremely low. Careful analytical studies in the oceans have shown that the concentrations of total dissolved iron range from 0.02 to 10 nM.
The role of chloride in the marine environment is also crucial. Several classes of
mechanisms for dechlorination of sea salt aerosol have been hypothesized. More than
one kind of mechanism is probably operating in the marine boundary layer, and the
relative importance of different mechanisms probably varies over space and time. An
improved understanding of inorganic CI chemistry in the marine boundary layer is
needed because of its potential importance for global chemistry (Keene 1995).
3.4.1
Inorganic (I Formation in the Marine Environment
The possible mechanisms for formation of atomic chlorine in the marine environment
have been widely investigated, and several possible sources have been identified.