Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 88

of energy required to split NN triple bonds, cannot use the gaseous N 2 form. A few
genera of bacteria, blue-green algae (cyanobacteria) that possess heterocytes (special-
ized cells present in most filamentous blue-green algae) and some unicellular forms
of blue-green algae without heterocytes are capable of nitrogen fixation. Nitrogen
fixation may be crucial to the acceleration of eutrophication in aquatic environments,
since fixation can occur when other sources of nitrogen are not available or are
insufficient for biological growth. 8,13,23 However, in almost all rivers, lakes, and coastal
marine ecosystems, loss of nitrogen via denitrification exceeds the inputs of nitrogen
via N 2 fixation. 18 For example, in Narragansett Bay, U.S.A., a small amount of nitrogen
fixation occurs in the sediment, but it is insignificant compared with nitrogen losses
through denitrification (less than 0.0007 mol N m −2 year −1 fixed in sediment compared
with 0.52 mol N m −2 year −1 denitrified). 34
Nitrogen fixation is less effective in making up nitrogen in marine systems than
in freshwater systems as the rates of nitrogen fixation in marine waters are generally
lower than those in freshwaters. 35 Many hypotheses have been proposed to explain
the difference in the rates of N 2 fixation in fresh and marine waters. 7,10 The most
likely explanation is the lower availability of two trace metals (iron and molybde-
num) in seawater that are required for nitrogen fixation, compared with their
availability in lakes. 34 Molybdenum is one of the active sites of the enzymes
involved in nitrogen fixation. The abundant sulfate in seawater interferes with the
uptake of molybdenum by fixers because of its steric similarity to molybdate;
therefore, seawater sulfate could reduce the activity of N 2 fixers. 7 In addition,
relatively large amounts of iron are required for the growth of cyanobacteria using
N 2 rather than NH 4 + or NO 3 − as the nitrogen source. Thus, the rate of N 2 fixation
might also be restricted by the low abundance of iron in seawater compared with
freshwater. 7
N 2 fixation by benthic cyanobacteria can be significant due to the direct supply
of iron and molybdenum from the sediment. However, low light penetration can
limit the growth of benthic N 2 fixers. 34 In unvegetated shallow coastal lagoons and
intertidal sediment where light is not limiting, dense populations of benthic nitro-
gen-fixing cyanobacteria can locally develop and contribute to nitrogen fixation.
However, even though the nitrogen fixed by cyanobacteria is locally important to
the mat communities themselves, the contribution to the total nitrogen budget is
minor in most shallow marine ecosystems due to the restricted distributions of
mat communities. 8 Since the cyanobacteria population varies greatly in time and
space, detailed measurements are required to estimate the total annual nitrogen
fixation rate. 13,23
Other physico-chemical parameters influencing nitrogen fixation activity in
benthic sediment include carbon availability, temperature, pH, dissolved oxygen,
inorganic nitrogen, and salinity. 8

4.1.2 P HOSPHORUS C YCLE

Phosphorus is one of the limiting nutrients for the growth of microorganisms
although the quantities of phosphorus needed are much smaller than those of C, Si,
or N. 23