Blue-green algae (cyanobacteria) have had a profound and unparalled impact on the aquatic environment because of the phenomenon of bloom formation. In many countries, water management is threatened with extensive and persistent noxious blooms of blue-green algae in surface and near-surface mesotrophic and eutrophic waters. In view of this, ecological and physiological factors responsible for blue-green algal dominance are discussed. The implications of cyanobacterial blooms are highlighted and recommendations made to combat this menac

Akin-Oriola, G.A.

Jeje, C.Y.

English

Aquatic Commons

ECOP SIOLOGICAL PERSPECTIVES OF BLUE-GREEN ALGAL BLOOMS
BY
AKIN-ORIOLA, G.A.' AND JEJE, C.Y.2
I. Department of Fisheries and Zoology, Lagos State University, P.M.B. 1087, Apapa,
Lagos.
2. Department of Zoology, University ofIbadan, Madan.
ABSTRACT
Blue-green algae (cyanobacteria) have had a profound and unparalled impact onthe aquatic
environment because of the phenomenon of bloom formation. In many countries, water
management is threatened with extensive and persistent noxious blooms of blue-green algae in
surface and near-furfacemesotrophic and eutrophic waters.
In view of this, ecological and physiological factors responsible for blue-green algal
dominance are discussed. The implications of cyanobacterial blooms are highlighted and
recommendations made to comba this menace.
INTRODUCTION
Phytoplankton comtnunities are an essential component of all aquatic environments because
primary production by phytoplankton forms the base of the food chain in water. At moderate
standing crops, phytoplankton are beneficial as net producers of dissolved oxygen and assimilators f
ammonia as a nitrogen source for growth. Despite these beneficial aspects, most water quality
problems are the result of unmanaged growth of phytoplankton communities (Smith 1988).
In many countries, regular blooms of toxic algae occur and these have a significant impact
on the utilizafion of aquatic resource (Graneli et. al., 1990). The major groups of phytoplank-ton
responsible for bloom formation are dinoflagellates and blue-green algae.
What are blue-green algae?
Blue-green algae are prokaryotes which lack a membrane-bound nucleus and other
membrane-hound organielles. They are unique in their ability to fix free nitrogen inthe atmostphere.
In some genera such as Anabaena, Aphaizomen.on, Nostoc and Nodularia nigrogen fixatin is
confined to metabilically specialize thick-walled cells known as heterocysts. Several non-
heterocystous genera are also c,apable of nitrogen fixation.
Blue-green algae possess differentiated cells referred to as akinetes which are perennating
structures that can accumulate storage polymers and en.cyst. Thus, they aid in survival of the
species under adverse environmental conditions (Fogg et al., 1973). All bloom forming
cyanobacteria have intracellular gas vacuoles which are involved in regulating cell buoyancy (Fig.
1). They are widely distributed and include at least 22 genera atad over 90 species (Gibson a,nd
Smith, 1982). They can be separated into unicellular, colonial and filamentous forms.
Factors Favouring Algal Bloom Formation
Despite the ubiquity and persistence of algal blooms, there is little information linking the
physical, chemical and biological conditions in the environment with physiological adaptations that
such algae have evolved to optimize growth and persistence during bloom periods.
Ecophysiological factors responsible for blooms are nutnerous and complex (Table 2). However,
physical factors are frequently of overriding importance in determining which genera and species
become established and dominant in specific ecosystems (Paerl and Tucker, 1995).
152
Implications of Bloom Formation
Cyanobacterial blooms lead to some of the most ominous symptoms of water quality
degradation including bottom water anoxia and hypoxia; odour, taste and toxicity problems;
destruction of the aesthetic and recreational values of water and structural shifts in plankton
communities However, the most economically important consequence of bloom fonnation
involves mortality and morbidity in fisheries re,sources (Van der Ploeg and Boyd, 1991).
Generally, cyanobacteria are k-selected organisms with relatively slow growth rates but
high cotnpetitive ability for limited resources (IClham and Hecky, 1988). Hence, be low biomass-
specific rates of net carbon fixation translates into reduced fish yields. The efficiency of food
transfer may be further reduced when blooms are present because they are poorly utilized as food
(Paerl 1988b). The slow growth rates of blue-green algae also means that they are poor
oxygenators of water on a per unit biomass basis compared to other phytoplankton species. In
addition, surface bloom communities reduce net input of dissolved oxygen to water because
oxygen production is restricted to the upper few centimetres. massive die-offs of cyanobacterial
communities results in oxygen depletin as dead algal cells are decomposed. Fish kills in tropical
lakes as a consequence of blooms oecur but are rarely reported (Belay and Wood, 1982; Ochumba,
1990).
The production of highly odourous and/or toxic metabolites by some species causes great
economic loss in fisheries. Two metabilites of blue-green algae geosmin and 2-methylisoborneol
cause earthy-musty odours and off-flavours in pond-cultured fish (Van der Ploeg et al., 1992).
Several blue-green algae produce toxins which have poten cyto-hepato - and neuro-toxic effects on
fish, shellfish, invertebrates, birds and mammals (Carmichael, 1986; Haney et al., 1995).
Blue-green algal dominance in aquatic environments results in a structural shift in the
plankton community structure and hence in the dynamics of the ecosystem
Control
In proposing control strategies against algal blooms, water column turbulanece of mixing
asumes a prominent role. This realization has fostered the application of artificial mixing strategies
in reducing and eliminating bloom populations. However, to be effective mixing must operate
more or less continually during favourable bloom growth periods.
Close monitoring of the ambient nutritive conditions in water bodies is necessary.
Decreased phosphate availability can lead to declines in algal bloom potentials particularly for
nitrogen-fixing genera.
There is the possibility of biological control using amoeboid protozoans which are active
grazers on blue-green algae even when higher-ranked crustacean zooplankton find such
cyanobacteria either inedible, distasteful or toxic (fulton & Paerl 1988). However, bloom
eradication and mitigation
153
Table 2: Factors influencing algal bloom formation
{PRIVATE }ECOLOGICAL EFFECTS OR IMPACTS
FACTORS
Physical
Temperature Generally warm water temperatures accompanied by a
stratified water column and high nutrient loading rates
promote blooms.
Large Scale Vertical Mixing Prevents surface bloom accumulations and forces competition for
light and nuthents with non-buoyant eukaryotic taxa
Small Scale turbulence May disrupt filaments, colonies and mutualistic associations with
other micro-organisms.
High water transparency Promotes bloom formatin because milike other eukaryotic
phytoplankton bloom-forming genera can withstand long periods of
exposure to high photosynthetically active radiation (PAR).
Low hydraulic retention time/flushing Prevents bloom establishment or removes bloom if flushing rate
exceeds growth rate of bloom taxa.
Cheinical
Eutrophication/Nutrient input Favours bloom formatin especially if the ratio of N to P loading is
low.
Salinity
Environmental pH Trace metals
Biological
Salinity in excess of a few ppt may be an effective barrier to the
development and persistence of blooms.
Generally high pH favours blue-green algal growth. Under high N
and P loading, restricted availability of Fe may favour
cyanobacteria.
Absence OT scarcity of herbivorous zooplankton and larger filter
feeding species that can ingest blue-green algae.
SOURCE: Paerl and Tucker, 1995.
Approaches utilizing biological control measures require careful and critical evaluation prior to deployment on
thenatural community.
More recently advancements in biotechnology offer prospects for the use of "bloom" species. It has been
discovered that many micro-organisms are a promising new source of bioactive substances which could be used int he
manufacture of industrial chemicals, marine biomedicals and non-polluting energy sources. Notable among these, is
the discovery of antileukaemia activity in blue-green algae of the family Oscillatoriacea. Thus in the near future, blue-
green algal blooms may be harvested and exploited for the production of novel compounds (Ninewe, 1995).
154
Ninawe, A.S. (1995):
Ochumba, P.B.0 (1990):
Paerl, H.W. (1988b):
Pacrl, H.W. and Tucker, C.S. (1995):
Smith, D.W. (1988):
Van der Ploeg, M.and Boyd, C.E. (1991):
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Ecophysiological perspectives of blue-green algal blooms

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Ecophysiological perspectives of blue-green algal blooms

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