A consequence of anaerobic denitrification
is that inorganic nitrogen is generally
the limiting nutrient in wetlands. Ironically, the very presence of well-adapted
wetland species can contribute to denitrification: some of the oxygen transported
into their roots will diffuse out into the surrounding substrate and create an aerobic
environment in which nitrifying bacteria can convert ammonium to nitrate; this nitrate
can then be broken down into gaseous nitrogen by denitrifying bacteria. It is the
presence of aerobic and anaerobic conditions in very close proximity over a large
surface area around the roots which enhances this process.
To overcome the problems of nitrate availability,
wetland communities typically
support plant species with alternative sources of nitrates, including, in Europe, alder
(Alnus glutinosa) and bog myrtle (Myrica gale) which maintain a symbiotic
relationship with nitrogen-fixing bacteria in their root nodules. Ombrotrophic mires
support carnivorous plants such as sundew (Drosera spp.), which obtain nitrogen
compounds from insects which they trap and digest.
In saltmarsh soils, too, anaerobic conditions
promote denitrification and biologically
available nitrogen can be a limiting factor to growth.
Plants inhabiting coastal wetlands have
to withstand the further physiological stress
of high salinity, a situation complicated by the high requirement for nitrogen of
species such as Spartina alterniflora, possibly because nitrogen compounds are
required to maintain osmotic balance under saline conditions. Plant productivity in
estuarine saltmarshes is normally higher in the low marsh than in the high marsh,
even when the same species are compared. This can be explained by the frequent
influx of nutrient-rich water in low marshes, but also by the lower salinity of interstitial
water around the roots, as a consequence of flushing by diluted estuarine water.
The interaction between these two factors, and their relative importance in
determining productivity, are unclear.
Nutrient availability
Wetland macrophytes, even if submerged,
gain their nutrients only from the
sediment. They will, however, support a diverse epiphytic microflora of algae which
gain their nutrients mainly from the water. There is little transfer of nutrients from
algae to macrophyte, although some from macrophpyte to algae but, when they die,
macrophyte stems, along with the microflora they support, collapse into the
sediment. The nutrients absorbed from the water column by epiphytic algae are, in
this way, transferred into the sediment and therefore available for macrophytes. In
this way, epiphytic microflora, despite shading macrophytes and reducing their
photosynthetic efficiency, can be indirectly beneficial to their hosts.
Vertebrates and nutrient availability
Mobile vertebrates are important in their
ability to redistribute nutrients within
wetland systems. Wading birds in the Florida Everglades disperse widely to feed
but roost and nest aggregated into colonies in trees, creating areas where
excretion and mortality are concentrated. It has been has calculated that, during the
late-19th century, there were an estimated 2.5 million wading birds in the Florida
Everglades which, assuming 50% of excretion and mortality occurred at roosting
sites, would have transferred 400 t yr"1 of ash (the inorganic nutrient component of
excretory products and corpses) from the wider wetland to relatively small areas
around roosting sites. As the number of birds has been reduced by habitat loss and
degradation and, until early in the 20th century, hunting for plumes, so the extent of
this redistribution of nutrients will have declined significantly.
