Phosphorus
Phosphorus is available in estuaries as dissolved inorganic
phosphorus (DIP), dissolved organic phosphorus (DOP),
and particulate organic phosphorus (POP). DIP is rapidly
assimilated by algae during the photosynthetic process.
It is essential for autotrophic growth and development in
estuaries.
Silicon
Silicon (Si) is a chemical element with atomic number
14 and atomic weight 28.0855. Silicon dioxide (SiO 2 )
occurs as silicate minerals in the earth’s crust. It is found
in natural waters as dissolved reactive silicon and
orthosilicic acid (Si(OH) 4 ) or H 4 SiO 4 ). Diatoms, radiolarians, and siliceous sponges use biogenic silica as structural
material to construct skeletons. Silicon is assimilated by
diatoms at rates similar to that of nitrogen, and it enters
estuaries via river flow carrying soil-leached products.
The crystalline form of silicon in skeletal material is
“biogenic silica.” The Si:N ratio influences the composition of phytoplankton. The growth of diatoms
depends on the presence of silicate, while the growth
of non-diatomaceous forms normally does not
(Day et al., 2012).
Sulfur
Sulfur (S) is a chemical element with atomic number
16 and atomic weight 32.065. It is an essential element
important for aquatic biochemical processes, metal
cycling, and ecosystem energetics (Day et al., 2012). It
occurs naturally as elemental sulfur, sulfide, and sulfate
minerals. In metabolic reactions, sulfur compounds serve
as both fuels (electron donors) and respiratory (oxygen
alternative) materials (electron acceptors). In anaerobic
sediments and pore water of estuaries, sulfate reduction
is an important process that passes energy from autotrophs
through the food web. Coal, biomass burning, and volcano
emissions inject SO 4 into the atmosphere, which can then
be further oxidized in the atmosphere and removed as
SO 4
2À in precipitation (Bianchi, 2007). Sulfur is widely
used in agriculture as a fertilizer which can be an important anthropogenic source of the element to receiving
waters.
Estuarine behavior
Nutrients are affected by estuarine mixing and circulation
processes. They are typically present at the air-water,
river-sea, land-sea (salt marshes), and sediment-water
interfaces. Nutrient concentrations in an estuary are
influenced by their hydrodynamic characteristics. Nutrient distributions in an estuary are determined in part by
the basin morphology, circulation, and tidal regime;
biological processes and regeneration; organic detritus
accumulation; and transport. Gradients in estuarine
processes are primarily driven by changes in river inflow,
tidal currents, waves, and meteorological forcing
(Bianchi, 2007; Day et al., 2012). Anthropogenic activities can alter the balance of estuarine nutrients.
Micronutrients
Micronutrients are trace elements essential for autotrophic
organisms in small amounts. They are necessary for photosynthesis, being key constituents of chlorophyllous
pigments and enzymes (Falkowski and Raven, 2007).
The trace elements include iron, cobalt, chromium, copper
[2], iodine, manganese, selenium, zinc, and molybdenum.
Iron has been reported by many researchers as the most
important bioactive trace element (Day et al., 2012).
Sources
As noted above, nutrient sources may be autochthonous, if
they result from organic matter remineralization in the
estuary, or allochthonous, if they enter the estuary from
external sources such as river input or atmospheric deposition. Free nitrogen (N 2 ) fixation by planktonic organisms
in estuaries is typically low, even when the N:P ratio is
low. The trend toward lower fixation may be due to low
availability of one or more trace elements required for
N fixation, including molybdenum and iron. Estuaries
often become highly productive areas due to enrichment
through soil weathering and nutrients supplied by rivers
which can support dense phytoplankton populations.
Limitation
Nutrient limitation leads to a reduction in autotrophic
growth and photosynthetic rates. According to Liebig’s
law of the minimum, the concentration of a nutrient at
the smallest relative amount required for organism
growth may be the limiting factor. The concept of
a limiting nutrient is essential to understanding biological
processes. The nutrient in short supply relative to the
others will be exhausted first and will thus limit cellular
growth (Falkowski and Raven, 2007). Physical processes
(e.g., transport, sedimentation/resuspension, and
sorption/desorption) together with biological processes
(e.g., regeneration and bioturbation) can significantly alter
nutrient concentrations in estuaries, whereby they exhibit
non-conservative behavior. The nutritional requirements
of phytoplankton have a relative ratio of nutrients, which
may be related to the relative proportion of the elements
available in the estuarine environment. Nitrogen is the
limiting nutrient in most estuarine waters because it is in
higher demand and is depleted more rapidly compared
with other nutrients. It also has a more complex biogeochemical cycle than phosphorus (Day et al., 2012).
Eutrophication
Nutrients are essential to the trophic web and estuarine
biodiversity, although excessive nutrient enrichment
results in eutrophication and environmental imbalance.
Human activities have increased the input of nitrogen
and phosphorus to estuarine and coastal waters, thereby
increasing the Si:N and Si:P nutrient ratios and causing
NUTRIENTS
465
Phosphorus is available in estuaries as dissolved inorganic
phosphorus (DIP), dissolved organic phosphorus (DOP),
and particulate organic phosphorus (POP). DIP is rapidly
assimilated by algae during the photosynthetic process.
It is essential for autotrophic growth and development in
estuaries.
Silicon
Silicon (Si) is a chemical element with atomic number
14 and atomic weight 28.0855. Silicon dioxide (SiO 2 )
occurs as silicate minerals in the earth’s crust. It is found
in natural waters as dissolved reactive silicon and
orthosilicic acid (Si(OH) 4 ) or H 4 SiO 4 ). Diatoms, radiolarians, and siliceous sponges use biogenic silica as structural
material to construct skeletons. Silicon is assimilated by
diatoms at rates similar to that of nitrogen, and it enters
estuaries via river flow carrying soil-leached products.
The crystalline form of silicon in skeletal material is
“biogenic silica.” The Si:N ratio influences the composition of phytoplankton. The growth of diatoms
depends on the presence of silicate, while the growth
of non-diatomaceous forms normally does not
(Day et al., 2012).
Sulfur
Sulfur (S) is a chemical element with atomic number
16 and atomic weight 32.065. It is an essential element
important for aquatic biochemical processes, metal
cycling, and ecosystem energetics (Day et al., 2012). It
occurs naturally as elemental sulfur, sulfide, and sulfate
minerals. In metabolic reactions, sulfur compounds serve
as both fuels (electron donors) and respiratory (oxygen
alternative) materials (electron acceptors). In anaerobic
sediments and pore water of estuaries, sulfate reduction
is an important process that passes energy from autotrophs
through the food web. Coal, biomass burning, and volcano
emissions inject SO 4 into the atmosphere, which can then
be further oxidized in the atmosphere and removed as
SO 4
2À in precipitation (Bianchi, 2007). Sulfur is widely
used in agriculture as a fertilizer which can be an important anthropogenic source of the element to receiving
waters.
Estuarine behavior
Nutrients are affected by estuarine mixing and circulation
processes. They are typically present at the air-water,
river-sea, land-sea (salt marshes), and sediment-water
interfaces. Nutrient concentrations in an estuary are
influenced by their hydrodynamic characteristics. Nutrient distributions in an estuary are determined in part by
the basin morphology, circulation, and tidal regime;
biological processes and regeneration; organic detritus
accumulation; and transport. Gradients in estuarine
processes are primarily driven by changes in river inflow,
tidal currents, waves, and meteorological forcing
(Bianchi, 2007; Day et al., 2012). Anthropogenic activities can alter the balance of estuarine nutrients.
Micronutrients
Micronutrients are trace elements essential for autotrophic
organisms in small amounts. They are necessary for photosynthesis, being key constituents of chlorophyllous
pigments and enzymes (Falkowski and Raven, 2007).
The trace elements include iron, cobalt, chromium, copper
[2], iodine, manganese, selenium, zinc, and molybdenum.
Iron has been reported by many researchers as the most
important bioactive trace element (Day et al., 2012).
Sources
As noted above, nutrient sources may be autochthonous, if
they result from organic matter remineralization in the
estuary, or allochthonous, if they enter the estuary from
external sources such as river input or atmospheric deposition. Free nitrogen (N 2 ) fixation by planktonic organisms
in estuaries is typically low, even when the N:P ratio is
low. The trend toward lower fixation may be due to low
availability of one or more trace elements required for
N fixation, including molybdenum and iron. Estuaries
often become highly productive areas due to enrichment
through soil weathering and nutrients supplied by rivers
which can support dense phytoplankton populations.
Limitation
Nutrient limitation leads to a reduction in autotrophic
growth and photosynthetic rates. According to Liebig’s
law of the minimum, the concentration of a nutrient at
the smallest relative amount required for organism
growth may be the limiting factor. The concept of
a limiting nutrient is essential to understanding biological
processes. The nutrient in short supply relative to the
others will be exhausted first and will thus limit cellular
growth (Falkowski and Raven, 2007). Physical processes
(e.g., transport, sedimentation/resuspension, and
sorption/desorption) together with biological processes
(e.g., regeneration and bioturbation) can significantly alter
nutrient concentrations in estuaries, whereby they exhibit
non-conservative behavior. The nutritional requirements
of phytoplankton have a relative ratio of nutrients, which
may be related to the relative proportion of the elements
available in the estuarine environment. Nitrogen is the
limiting nutrient in most estuarine waters because it is in
higher demand and is depleted more rapidly compared
with other nutrients. It also has a more complex biogeochemical cycle than phosphorus (Day et al., 2012).
Eutrophication
Nutrients are essential to the trophic web and estuarine
biodiversity, although excessive nutrient enrichment
results in eutrophication and environmental imbalance.
Human activities have increased the input of nitrogen
and phosphorus to estuarine and coastal waters, thereby
increasing the Si:N and Si:P nutrient ratios and causing
NUTRIENTS
465