Environmental Engineering Reference
In-Depth Information
Hypertrophic lakes. These include artificially fer-
tilised fish ponds (Pechar et al. , 2002) and
lakes with sewage discharges, and are dominated
throughout the season by small unicellular algae
with short life cycles. The algae form a succes-
sion of dense populations, out-competing larger
colonial organisms which are unable to establish
themselves.
concentration in surface waters) or Secchi depth
(mean/maximum annual value). Examples of total
volumes of planktonic algae at different trophic lev-
els (Norwegian lakes) are also given in Table 3.2,
together with characteristic bioindicator algae.
Algae as bioindicators of inorganic trophic
status
2. Species diversity . The use of algal counts
to characterise species richness (Margalef index),
species evenness/dominance (Pielou index, Simp-
son index) and a combination of richness and
dominance (Shannon-Wiener index) is discussed in
Section 2.5.5.
Analysis of species richness is particularly useful,
combining data on the total number of species iden-
tified and total number of individuals in the popu-
lation (Equation 2.10). During the summer growth
phase, species diversity is typically low in olig-
otrophic lakes, rising progressively in mesotrophic
and eutrophic lakes, but falling again in some
eutrophic hypertrophic lakes where small numbers
of species may out-compete other algae. The effects
of increasing nutrient levels on algal diversity ( d )are
illustrated by Reynolds (1990), with summer-growth
values of 3-6 for the nutrient-deficient North Basin
of Lake Windermere (UK) - falling to levels of 2-4
in a nutrient- rich lake (Crose Mere, UK) and 0.2-2
for a hypertrophic water body (fertilised enclosure,
Blelham Tarn, UK).
Planktonic algae within lake surface (epilimnion)
samples can be used to define lake trophic status in
terms of their overall productivity (Table 3.2) and
species composition (Table 3.3). Species composi-
tion can be related to trophic status in four main
ways - seasonal succession, biodiversity, bioindica-
tor species and determination of bioindices.
1. Seasonal succession . In temperate lakes, the
development of algal biomass and the sequence
of phytoplankton populations (seasonal succession)
directly relate to nutrient availability. In all cases, the
season commences with a diatom bloom, but subse-
quent progression (Reynolds, 1990) can be separated
into four main categories (Table 3.3).
Oligotrophic lakes. In low-nutrient lakes, the
spring diatom bloom is prolonged, and diatoms
may dominate for the whole growth period. Chrys-
ophytes ( Rhizosolenia ) and desmids ( Staurastrum )
may also be present, and in some lakes, Ceratium
and Gomphosphaeria may be able to grow in the
nutrient-depleted waters by migrating down the
water column higher nutrient conditions.
3. Bioindicator species . Some algal species and
taxonomic groups show clear preferences for par-
ticular lake conditions and this can act as potential
bioindicators. In a broad comparison of oligotrophic
versus eutrophic waters, desmids (green algae) tend
to occur mainly in low-nutrient waters while colo-
nial blue-green algae are more typical of eutrophic
waters. Such generalisations are not absolute, how-
ever, since some desmids (e.g. Cosmarium menegh-
inii , Staurastrum spp.) are typical of meso- and
eutrophic lakes, while colonial blue-green algae such
as Gomphosphaeria are also found in oligotrophic
waters.
Although it is not possible to pin-point individual
algal species in relation to particular trophic states, it
Mesotrophic lakes. These have a shorter diatom
bloom (dominated by Asterionella ), often fol-
lowed by a chrysophyte phase then mid-summer
dinoflagellate, blue-green and green algal blooms.
Eutrophic lakes. In high-nutrient lakes, the spring
diatom bloom is further limited, leading to a
clearwaterphase(dominatedbyunicellularalgae),
followed by a mid-summer bloom in which
large unicellular ( Ceratium ), colonial filamen-
tous ( Anabaena ) and globular ( Microcystis )algae
predominate.
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