Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 91

This can be a significant process in coastal waters. Across a variety of coastal
sediment, the regeneration of phosphate provides an average of 28% of the phy-
toplankton requirements whereas, in Narragansett Bay, U.S.A., it provides 50%. 7
Floating and submerged macrophytes also are important phosphorus sources
since they release phosphorus rapidly (within days) from the decaying leaves and
roots. 13
Phosphorus cycles rapidly through the aquatic food chain, and is seldom limiting
in the marine environment. 25 Howarth 34 states that during microbial decomposition,
phosphorus is released faster than nitrogen, presumably because the ester bonds of
phosphorus are more easily broken than are the covalent bonds of nitrogen.

4.1.2.3 Phosphorus Release from Sediment

Phosphorus retention and subsequent release from sediment to overlying waters may
be important in preventing/delaying the improvement of water quality. Therefore,
much study has been devoted to the phosphorus content of sediment and its move-
ment into the overlying water. 39
Exchanges across the sediment-water interface are regulated by mechanisms
associated with mineral-water equilibria, sorption processes (notably ion exchange),
oxygen-dependent redox reactions and microbial activities, as well as the environmen-
tal control of inorganic and organic compounds, i.e., enzymatic reactions. The release
of adsorbed phosphorus from sediment is controlled by physical-chemical factors such
as temperature, pH, and redox potential. 13 Lower redox potentials and high pH values
in the surface sediment cause phosphorus release during summer, while low temper-
atures, high redox potentials, and neutral pH help to retain phosphorus in sediment in
winter. 35 The rates of release of phosphate by a wide variety of marine sediment during
summer are reported by Nixon, 26 as

mol m −2 h −1 . Release rates are also
influenced by variable rates of turbulent diffusion and the burrowing activities of
benthic invertebrates. 13
The oxygen content of the sediment-water interface is one of the most important
features of the interface. As long as a few millimeters of the sediment is aerobic, the
phosphorus will be retained in the sediment efficiently. Phosphorus binding with ferric
oxides is particularly strong. At a neutral pH and redox potential greater than 200 mV,
Fe(OH) 3 is stable, 13 and sorption (chemisorption) of orthophosphate takes place. If
the pore water becomes anaerobic due to respiratory activity in the sediment, ferric
iron (Fe +3 ) is reduced to ferrous iron (Fe 2+ ) and the binding is weakened. 36 This redox
reaction causes the amorphous ferric oxyhydroxides to dissolve, making them unavail-
able to adsorb phosphates. The dissolved phosphate can leave the anaerobic sediment,
but some of the phosphate may precipitate as FePO 4 at the oxic-anaerobic interface,
and more is probably adsorbed onto the amorphous ferric oxyhydroxides, also at the
oxic-anaerobic interphase. In any case, the release of phosphate from anaerobic
sediment is faster than reoxidation and immobilization, resulting in a net phosphate
regeneration from anaerobic sediment into overlying water. 7,13
Caraco et al. 40 state that there is a correlation between sulfate abundance and
phosphorus release from anaerobic sediment that is a result of the interaction between
iron and sulfur. Enhanced sulfate reduction and the resulting formation of iron-sulfide

− 15-50 µ