Geoscience Reference
In-Depth Information
environmental conditions. Orthophosphate is taken up by phytoplankton and incorpo-
rated into cells during growth. Some fraction of the phosphorus taken up is released
in forms readily available to other algal cells. Other phosphorus compounds released
have to be mineralized and/or hydrolyzed into inorganic form before their use in
growth. It is likely that phosphorus is also excreted directly by invertebrates, similar
to zooplankton. 13 Algal growth produces an increase in particulate organic phosphorus
(POP), and the death of algae releases dissolved organic phosphorus (DOP). Some of
the POP is transformed to dissolved inorganic phosphorus (DIP) as particles decay in
the water column, but some POP settles onto the bottom sediment. In the sediment,
further degradation of settled organic phosphorus to DIP can take place, and some of
this formed DIP is subsequently precipitated or adsorbed. 7 Bacteria excrete some
phosphate, and DIP is also generated by microbial hydrolysis of the esters of the DOP.
4.1.2.1
Uptake of Phosphorus
DIP (orthophosphate) is the only form of phosphorus that can be assimilated by
bacteria, algae, and plants. 13,36 The growth process is usually represented by the
Monod equation, and the half-saturation concentration of nutrients ( Ks ) in the Monod
equation varies depending on the organisms involved.
An organism with a lower Ks value, has the advantage over other organisms
when the nutrient in question (here, phosphorus) is in short supply. 27 For example,
algae have lower Ks concentrations (0.6 -1.7
µ
M) while bacteria have higher Ks
values (6.7-11.3
M). 7 The ability to store nutrients also makes algae a more
important reservoir than bacteria.
Because aquatic macrophytes have the ability to use sedimentary nutrient
sources 37 and the nutrient concentrations in sediment are much higher than those in
the water column, high Ks values for macrophytes do not result in severe nutrient
limitation. 27
µ
4.1.2.2
Phytoplankton Death and Mineralization
During respiration and death of phytoplankton, a fraction of the phosphorus released
is in inorganic form. The remaining fraction is in organic form, which must be
mineralized and/or hydrolyzed into inorganic form to be made available to other
organisms. This transformation usually occurs in the sediment in coastal ecosystems
because these systems are shallow and the settling process is more effective. After
the transformation, inorganic phosphorus is either released to the overlying water
and made available for growth or adsorbed or buried deeper into the sediment.
In fish farms located in lagoon systems, the fish are fed with rich food distributed
in large quantities in the water column. This input could constitute an additional
source of organic matter to the sediment that might increase benthic nutrient fluxes.
On the other hand, the greater recycling rates of organic matter could explain the
significant increase in benthic nutrient fluxes observed in the lagoon fish culture
systems. 38
The release of available phosphorus from sediment to overlying water stimulates
primary production in the water column and is known as regeneration of phosphorus.
 
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