Algae as Bioindicators - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 139

water within the surrounding marine environment.
This plume of water extends into the surrounding
ocean and is characterised by reduced salinity, high
levels of inorganic nutrients, high levels of suspended
material and a distinctive phytoplankton population.
The freshwater tends to lie on the surface of the sea-
water (forming a 'freshwater lens'), so that the loca-
tion of the freshwater plume can be considered both
in terms of horizontal and vertical distribution.
A good example of this is the Amazon River
plume,whichextendsintotheWesternTropicalNorth
Atlantic (WTNA) ocean (Foster et al ., 2007). The
Amazon River represents the world's largest fluvial
outflow, releasing an average 1.93 × 10 5 m 3 s −1
of water to the WTNA. The influence of the Ama-
zon River plume is geographically far-reaching, with
freshwater lenses containing enhanced nutrients and
distinct phytoplankton populations reported as far
away as

Studies by Foster et al . (2007) on the occurrence of
nitrogen-fixing algae in the WTNA targeted the verti-
cal and horizontal distribution of seven diazotrophic
populations, including Trichodesmium (typical of
open ocean environments), three symbiotically
associated algae and three unicellular free-living
organisms (phylotypes). Algal distributions were
examined in terms of the presence of the nif H gene,
which encodes for the iron protein of the nitrogenase
enzyme (responsible for nitrogen fixation) and is
highly conserved. The presence of species-specific
nif H gene sequences was determined using DNA
quantitative polymerase chain reaction (QPCR)
technology.
The studies showed that one particular algal sym-
biont ( Richelia ), associated with the diatoms Hemi-
aulus hauckii and Rhizosolenia clevei , was dis-
tributed within the freshwater lens of the Amazon
plume - with abundances of H.hauckii-Richelia nif H
genes being particularly prominent. The QPCR study
showed the dominance of H. hauckii-Richelia sym-
bioses in the Amazon plume waters, implying that
these associations had an ecological advantage over
other diazotrophs and establishing them as indica-
tor organisms of the plume environment. Free-living
unicellular blue-green diazotrophs were particularly
abundant in the saline waters outside the plume,
where inorganic nutrients were at minimal levels of
detection.

1600 km from the river mouth (Borstad,
1982). The Amazon plume brings silica, phosphorus,
nitrogen and Fe into the WTNA, promoting high lev-
els of phytoplankton productivity within the plume
area. This productivity tends to be nitrogen lim-
ited, promoting the growth of nitrogen-fixing (dia-
zotrophic) blue-green algae.

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Molecular detection The detection of dia-
zotrophic blue-greens can be used to map regions
of eutrophication resulting from freshwater outflow.