Environmental Engineering Reference
In-Depth Information
organisms in the main water channel are subjected
to wide periodic fluctuations in salinity, and biota
occurring on the surface of mudflats are exposed to
extremes of salinity change, desiccation and irradia-
tion. The typically high nutrient level of river inflow
also signals a further difference between freshwater
and saline organisms in terms of productivity, with a
marked increase in C-fixation from freshwater algae
when saline water is displaced (Section 3.5.2).
sediment and nutrient loads to downstream estuarine
and coastal systems. This results in deterioration of
water quality and an overall decline in the ecological
andeconomicconditionofthecoastalzone,including
loss of fisheries habitat and resources.
3.5.2 Algae as estuarine bioindicators
Algae have been used to monitor changes in water
quality of estuaries and coastal systems in relation to
two main aspects - changes in salinity and increases
in inorganic nutrient levels (eutrophication). The
potential role epipelic diatoms as monitors of local
pore water salinity/nutrient concentrations has been
noted earlier, and diatom bioindicators of coastal wet-
land salination discussed in Section 3.3.
Detection of large-scale eutrophication is a more
pressing and general problem, since it is a feature of
almost all estuaries. Nutrient increases are accelerat-
ing with rising levels of human population and have
important economic impacts in relation to fisheries.
The use of algae as bioindicators of general estuarine
eutrophication is considered in relation to two main
aspects - monitoring algal populations using pigment
concentrations and molecular detection of freshwater
species in river plumes.
Algaldiversity:themudlatbioilm
Organism
diversity within the complex estuarine environment
is illustrated by the area of mudflats, where localised
drainage and inflow (Fig. 3.8) lead to numerous
small-scale salinity and nutrient gradients (Under-
wood
et al
., 1998). Epipelic diatoms are the main
group of algae inhabiting these intertidal sediments,
forming surface biofilms that have high levels of pro-
ductivity. Algae associated with estuarine sediments
are also important in substrate stabilisation (Section
2.7.2)andareindynamicinterchangewithplanktonic
populations (Section 1.1.4).
Although many epipelic diatoms are cosmopoli-
tan in their distribution, indicating broad tolerance
of environmental variations, others are more specific.
Analysis of diatom distribution within local salin-
ity gradients led Underwood
et al
. (1998) to classify
some diatoms as oligohaline (0.5-4% of full salin-
ity:
Navicula gregaria
), mesohaline (5-18%:
Navic-
ulaphyllepta
) and polyhaline (18-30%:
Pleurosigma
angulatum
,
Plagiotropis vitrea
). Variations along a
nutrient gradient indicated some species (
Nitzschia
sigma
) typical of high nutrient conditions, while oth-
ers (
N. phyllepta, Pl. angulatum
) were more com-
mon of low nutrient levels. Similar studies by Agatz
etal
. (1999) on tidal flat epipelic diatoms also demon-
strated nutrient-tolerant (
Ni. sigma
) and nutrient-
intolerant (
Achnanthes
,
Amphora
) taxa. The above
algae serve as bioindicators for micro-environmental
(local) conditions of pore water quality.
Eutrophication: analysis of pigment
concentrations
Analysis of algal populations as bioindicators in estu-
arine and related coastal systems typically involves
monitoring a large area of water surface. The speed
with which environmental changes can occur also
means that relatively frequent sampling may be nec-
essary. Because of these requirements, many estu-
arine monitoring programmes have combined rapid
sample analysis (using pigments as diagnostic mark-
ers) with automated collection (from fixed sites or by
remote sensing).
The combined use of HPLC (pigment separa-
tion), spectrophotometry (pigment quantitation) and
a matrix factorisation programme such as ChemTax
®
(calculation of biomass proportions) is described
in Section 2.3.3 and provides a speedy and accu-
rate assessment of the algal community. Although
Human activity
Human activity has a major and varied effect on estu-
aries and is a dominant source of stress and change.
At least half the world's population resides in estuar-
ine watersheds (Paerl
et al
., 2005), greatly increasing
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