Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 92

mineral (e.g., FeS, FeS 2 ) were responsible for increased pore water phosphate accu-
mulation. However, in sulfate-free sediment, much of the phosphate released during
anaerobic microbial reduction of Fe +3 was captured by solid phase reduced iron
compounds (e.g., Fe 3 (PO 4 ) 2 ). In freshwater systems, sulfate concentrations in sedi-
ment are generally low compared to coastal marine systems. Therefore, freshwater
systems have a greater capacity of retaining phosphorus in the sediment. The phos-
phorus in freshwater sediment is bound more tightly and proportionally less is
released back into the water column. 35

4.1.2.4 Sorption of Phosphorus

Phosphates adsorb readily under aerobic conditions onto amorphous oxyhydroxides,
calcium carbonate, and clay mineral particles. Thus, phosphate seldom travels far
in sediment, except when transported by the movement of particles. Phosphate also
precipitates with cations such as Ca 2+ , Al 3+ , and Fe 3+ . 7 With a few exceptions, surface
waters receive most of their phosphorus load from surface flows rather than from
groundwater, since phosphates bind with most soils and sediment. 36
There is an adsorption-desorption interaction between phosphates and sus-
pended particulate matter in the water column. The subsequent settling of suspended
solids together with the adsorbed inorganic phosphorus can be a significant phos-
phorus loss mechanism in the water column and is a major source of phosphorus to
the sediment.
Although phosphorus exchange by adsorption-desorption within the sediment
and between sediment particles and interstitial water can be as rapid as a few minutes,
the rate of phosphorus exchange across the sediment-water interface depends on
the state of the microzone. 13
In Thau Lagoon, France, the release of phosphates adsorbed onto Fe(OOH)
and also onto CaCO 3 is maximum in summer due to the low redox potential and
low pH. 28

4.1.2.5 Significance of N/P Ratio

Net primary production in many marine ecosystems is probably limited by nitrogen,
but phosphorus also may limit production in some ecosystems. 6-8,34 There is a shift
from phosphorus to nitrogen limitation in moving from freshwater to coastal waters.
Some of the reasons for this are more efficient recycling of phosphorus, 36 the high
losses of nitrogen to the atmosphere due to denitrification in coastal waters, 34,36 the
role of sulfate in recycling phosphorus in coastal sediment, 36 and the low N:P ratio
in nutrient inputs to many coastal waters with limited planktonic nitrogen fixation. 34
Differences in nutrient limitation are the result of changes in the ratio of total
nitrogen to total phosphorus in nutrient inputs and the dynamics of internal bio-
geochemical processes. 34 Analyses of algal cells show that the mean ratio of carbon
to nitrogen to phosphorus is C:N:P 106:16:1. It seems reasonable, therefore, to

assume that the cells require these elements according to this ratio. Liebig's law of
minimum states that if the ratio of elements in the water deviates widely from this
ratio, elements present in excess cannot be utilized. 41 The observed N:P ratios that

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