New England Water Works Association
ORGANIZED 1882
VOL. CXXIX
June 2015
NO. 2
This Association, as a body, is not responsible for the statements or opinions of any individual.
Cyanobacteria in Reservoirs:
Causes, Consequences, Controls
By Robert W. Kortmann, Ph.D.*
Presented September 22, 2014
ABSTRACT
Cyanobacteria cause water treatment difficulties
including taste and odor episodes, chemical dose
fluctuation, shortened filter runs, and increased
disinfectant demand (increasing DBP formation).
Some Cyanobacteria produce hepatotoxins and
neurotoxins that have acute or chronic health
effects. Cyanobacteria genera occupy a variety of
habitat niches, including phytoplanktonic populations and benthic mats. Some genera have a
low light requirement, allowing them to inhabit
deeper strata, benthic habitat, and to grow during Fall and Winter. Some genera are capable of
fixing atmospheric nitrogen and can become
very buoyant. Reservoir features that stimulate Cyanobacteria include the N:P Ratio, Silica
Limitation of Diatoms, CO2 availability, pH shifts,
Light Penetration and Mixing Depth (De),and
Herbivore Grazing Rate. Reservoir management
approaches to reduce Cyanobacteria include
Artificial Circulation, Hypolimnetic and Layer
Aeration, Flow Routing, Reservoir Partitioning,
Depth-Selective Outflow and Supply Withdrawal,
Biomanipulation, and Algaecides.
Introduction to the
Ecology of Cyanobacteria
Cyanobacteria is a Phylum of Bacteria.
Cyanobacteria are protein-rich prokaryotes; they
have a cell structure like bacteria. It is important to distinguish the Cyanobacteria from Algae,
which are cellulose-rich eukaryotes.
• Cyanobacteria is a Phylum of Bacteria that
obtain energy via Photosynthesis
*Limnologist/President, Ecosystem Consulting Service,
Inc., P.O. Box 370, Coventry, CT 06238, (860) 742-0744,
[email protected]
• Cyanobacteria are Prokaryotic (Algae are
Eukaryotic)
• Cyanobacteria Have a High Protein Content
(Amine Groups) (Indeed, some Cyanobacteria
are sold as Health Food Supplements)
• Cyanobacteria have inhabited Earth for over
2.5 Billion Years
• Ancestral Cyanobacteria evolved the ability to use
Water as an Electron Donor in Photosynthesis,
creating our aerobic atmosphere.
•Cyanobacteria can use only Cyclic
Photophosphorylation (PSI) with Alternate
Electron Donors in Anaerobic Environments
(Low Light Requirement)
• Some Cyanobacteria can reduce elemental
sulfur (Anaerobic Respiration)
• Some Cyanobacteria can live heterotrophically
(like other Bacteria)
Cyanobacteria have long been known to produce compounds such as 2-Methylisoborneol
(MIB) and geosmin, which cause taste and odor
episodes. More recently, the Cyanobacteria
have become even more of a concern because
some species produce Cyanotoxins, such as
the recent Microcystis bloom in western Lake
Erie. Long-term exposure to cyanotoxins has
been implicated in neurodegenerative diseases such as Lou Gehrig’s, Alzheimer’s, and
Parkinson’s disease.
Over the several billion years that Cyanobacteria
have inhabited the Earth, they have evolved
numerous competitive strategies.
Light Intensity: Cyanobacteria have Accessory
Pigments. They can harvest green, yellow, and
orange wavelengths, and can live in environments with only green light. As a result, some
Cyanobacteria can grow under low light intensity
in deeper reservoir strata.
Journal NEWWA
June 2015
73
Cyanobacteria in Reservoirs: Causes, Consequences, Controls Cyanotoxins
Cyanobacteria genera
Affects
Ecostrategist
Categories
Natural Forcing Factors
Anatoxin-a
Anabaena, Aphanizomenon,
Planktothrix (Oscillatoria)
Nerve Synapse
Buoyant N-Fixers
N:P Ratio, pH, Temp, De, Light
Penetration, Grazing Rate
Aplysiatoxins
Planktothrix (Oscillatoria), Lyngbya,
Schizothrix
Skin Rash
Benthic, Stratifying,
Stratification Boundaries,
Light Penetration
Cylindrospermopsins
Cylindrospermopsis,
Aphanizomenon
Liver Function
Buoyant N-Fixers
N:P Ratio, pH, Temp, De, Light
Penetration, Grazing Rate
Lyngbyatoxin
Lyngbya
GastroIntestinal, Skin
Benthic, Stratifying,
Buoyant
Stratification Boundaries,
Light Penetration
Saxitoxins
Aphanizomenon ,
Cylindrospermopsis
Nerves
Buoyant N-Fixers
N:P Ratio, pH, Temp, De, Light
Penetration, Grazing Rate
Microcystins
Microcystis, Anabaena ,
Planktothrix (Oscillatoria), Nostoc
Liver Function
Buoyant N-Fixers
Nodularin
Nodularia
Liver Function
Brackish
TOXIN GROUP
Alkaloids
Cyclic Peptides
N:P Ratio, pH, Temp, De, Light
Penetration, Grazing Rate
Nitrogen Availability and
Form
Cyanotoxins are not produced by all species of a genera, and a
specific population may or may not be producing a toxin.
Figure No. 1. The Cyanobacteria can produce several potent toxins (WHO 1999). Common
“Ecostrategist Categories” and environmental forcing factors are listed for major genera in
New England Reservoirs.
Gas Vesicles: Some Cyanobacteria can control
cellular buoyancy. Buoyancy increases under low
light when the growth rate slows. When respiration exceeds photosynthetic production (e.g. during the dark cycle) cellular buoyancy increases.
When photosynthesis increases, exceeding respiration, buoyancy decreases. This results in diurnal vertical movements of some Cyanobacteria
populations.
High Affinity for Phosphorus and Nitrogen:
Cyanobacteria can out-compete other phytoplankton when N or P becomes Limiting. They
are able to use organic phosphorus and nitrogen, and have a high storage capacity for phosphorus. A low N:P Ratio favors Cyanobacteria,
especially N-Fixing genera like Anabaena sp. and
Aphanizomenon sp.
Growth Rate: Under Optimal Light at 20°C,
the growth rate of Cyanobacteria is slower
than other phytoplanktonic algae. Long water
retention time in a reservoir tends to favor
Cyanobacteria. Biomass accumulation tends to
be more problematic than rapid growth rate.
(Grazing Herbivores prefer cellulose-rich algae
over protein-rich Cyanobacteria cells.)
Carbon Source: Free carbon dioxide is needed
by phytoplankton for photosynthetic productivity. At pH exceeding 8.3 no free carbon dioxide
exists in solution (only carbonate and bicarbonate). Many Cyanobacteria are more capable of
using low free carbon dioxide concentrations
and Carbonate Inorganic Carbon (Shapiro,
1977). Some Cyanobacteria can also live as
Heterotrophs or Chemotrophs.
Temperature: Most Cyanobacteria have a high
optimum temperature (>25°C; higher than for
Diatoms and Green Algae). Some Cyanobacteria
produce dormant resting cells (Akinetes) that
survive extreme environmental conditions (cold
winters, desiccation), facilitating establishment of
future populations.
Silica and Inorganic Nitrogen (Nitrate):
Diatoms and Green Algae become limited by
Silica and Nitrate. Cyanobacteria are released
from competition. Some Cyanobacteria can fix
atmospheric nitrogen, and are not dependent
on a source of combined inorganic nitrogen in
the water.
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Journal NEWWA June 2015
Robert W. Kortmann, Ph.D.
Slow Grazing Rate favors Cyanobacteria:
Cyanobacteria are not removed by grazing to
the same extent as other Phytoplankton (grazing
herbivores prefer cellulose-rich algae over protein-rich Cyanobacteria) (Shapiro, 1973). A poor
grazing rate favors Cyanobacteria, especially in
reservoirs that have a land-locked alewife population that impacts the zooplankton community.
Cyanobacteria Niches
Cyanobacteria can be “categorized” by the ecological niche occupied and function performed
(WHO 1999).
Scum-Forming Ecostrategists
e.g. Anabaena, Aphanizomenon, Microcystis
During high photosynthesis times carbohydrates accumulate in cells, which increases density
and causes cells to descend. Respiration consumes carbohydrate. When respiration exceeds
photosynthetic production (e.g. in the dark)
cells become buoyant and ascend. This results
in wind-drift accumulations of Cyanobacteria
that can concentrate toxins and taste & odor
compounds near supply intakes.
Nitrogen Fixing Ecostrategists
e.g. Anabaena, Aphanizomenon, Cylindrospermopsis,
Nodularia, Nostoc
N-Fixation requires intense light energy (substantial biotic energy is expended in order to
reduce atmospheric nitrogen gas into combined
Ammonia-N in the cells). However, when Total
Phosphorus and Light Energy are plentiful, while
inorganic nitrogen (Nitrate-N) is exhausted, these
Cyanobacteria will bloom.
Homogeneously Dispersed
Ecostrategists
Planktothrix, Limnothrix
These Cyanobacteria tend to be mixed throughout the Epilimnion, usually in shallow eutrophic
ecosystems.
Stratifying Ecostrategists
Some Cyanobacteria (e.g. Planktothrix –
Oscillatoria rubescens) create distinct vertical layers associated with density gradients
in the metalimnion. In several hardwater
“Marl Lakes” in Northwestern Connecticut,
Planktothrix-Oscillatoria rubescens turns
the water red under winter ice-cover, then
descends into the metalimnion during summer stratification.
Benthic Ecostrategists
e.g. Oscillatoria; Lyngbya
Cyanobacteria that form mats on the reservoir
bottom are more common in New England than
recognized (difficult to monitor without doing
SCUBA reconnaissance.) Benthic Cyanobacteria
are often found near the 1% light penetration
depth, typically at the warm-cool water transition
depth. The dominant Cyanobacteria within the
mat matrix are typically Oscillatoria sp. and often
produce MIB rather than geosmin.
Understanding the ecological characteristics of
Cyanobacteria is important for selecting reservoir
management strategies that can help reduce
or eliminate Cyanobacteria blooms. If conditions that provide a competitive advantage to
Cyanobacteria are avoided, blooms will decrease
and other, more desirable phytoplankton, can
compete. Among the limnological features that
can be managed to reduce Cyanobacteria are:
• Reduce Total Phosphorus Availability (TP, and
especially soluble reactive SRP)
• Expanding epilimnetic mixing to overcome
buoyancy control and light limitation.
• Enhancing the availability of Nitrate-N by
enhancing Nitrification or Importation.
•
Enhancing Diatoms during Fall, Winter,
and Spring.
• Maintaining pH below 8.3 to sustain Free CO2
Availability.
•Managing the Food-Web to enhance
Herbivorous Zooplankton.
• Managing respiration (aerobic and anaerobic)
to reduce TP and SRP availability.
Life Cycle of Common N-fixing,
Akinete-forming Cyanobacteria
The N-fixing Cyanobacteria that form resting
cells (akinetes) are common in nutrient-rich reservoirs of New England, especially Anabaena spp.
and Aphanizomenon spp. These genera can be
capable of producing taste and odor compounds
or cyanotoxins. Most studies have observed
that akinete germination is stimulated by light
intensity and warm temperatures. Akinetes, and
recently germinated cyanobacteria filaments,
rest and grow on the sediments, take up phosphorus, and transport it to surface waters during
their buoyant ascent in early summer. Akinete
Journal NEWWA
June 2015
75
Cyanobacteria in Reservoirs: Causes, Consequences, Controls Life Cycle of N-Fixing Akinete-Forming Cyanobacteria
(Gleotrichia, Anabaena, Aphanizomenon)
VEGETATIVE GROWTH
AKINETE
FORMATION
Exponential
Growth
Cell Differentiation
Senescence,
Scum Formation
RECRUITMENT
Buoyant
Migration
Germination
Growth, Nutrient Uptake
Sediments 0-4m
Vegetative Cells
Heterocysts, N-Fixation
Akinetes
Figure No. 2. Generalized life cycle of
Anabaena spp. Aphanizomenon spp. and
similar Cyanobacteria genera. (Modified after
Elfgren, 2003).
formation tends to be stimulated by green illumination after exponential growth as cell filaments
begin to senesce.
The life cycle begins with germination to filaments that begin to grow on sediments, typically
in the epilimnetic, photic zone depth range (warm
and illuminated). Recruitment of newly germinated filaments to the surface water can account
for a sizable portion of cell density increases
through early summer, and can be a significant
internal source of phosphorus to the epilimnion.
Managing to reduce blooms “this year” involves
managing vegetative growth and recruitment.
Akinete formation occurs after exponential
growth, as the filaments mature and approach
senescense. Managing to reduce “future year
blooms” involves managing to reduce akinete
formation and minimize germination next year.
Critical Limnological Features of
Water Supply Systems
Primary productivity by phytoplankton is usually limited by the availability of Total Phosphorus
(TP) in reservoir ecosystems (Schindler, 1977).
The amount of phosphorus in reservoir water is
the result of P loading from the watershed, direct
atmospheric deposition, and internal loading from
bottom sediments. A number of simple empirical
models can estimate external and internal phosphorus loads, and forecast reservoir productivity,
from observed Spring TP and core incubation
(Kortmann, 1980). Although TP availability often
controls the amount, or rate, of primary productivity, other ecological factors tend to dictate what
76
Journal NEWWA June 2015
types of organisms perform the primary productivity: Macrophytes, Phytoplanktonic Algae, or
Cyanobacteria. Although phosphorus availability
usually limits overall productivity in freshwater
ecosystems, other factors can become limiting
to primary producers at times including nitrogen,
inorganic carbon, light (intensity and wavelength),
and silica.
Sustaining inorganic nitrogen availability can
help reduce N-Fixing Cyanobacteria. Minimizing
nitrogen limitation can also reduce akinete formation. Reducing the direct source of readily
available soluble reactive phosphorus (SRP) from
reservoir sediments and avoiding deep nutrientrich releases from upstream storage reservoirs
can reduce phytoplankton productivity. Source
water management needs to control external
watershed sources of nutrients and organics
to “Protect the Future of the Resource”, while
targeting the sources that are most cost-effectively reduced to “Improve Water Quality and the
Expression of Trophic State”.
Understanding the limnology of the source water
reservoir system, and how natural features influence phytoplankton abundance and species composition, can help to identify reservoir management
approaches that effectively reduce Cyanobacteria.
The temperature of greatest density of water is at
4oC. Water density decreases as it gets colder or
warmer than that “anomaly temperature”, and density change per degree C is not linear, but increases
faster the further temperature deviates from 4
degrees. Relative Thermal Resistance to Mixing
(RTRM) accounts for that non-linearity (Kortmann
and Rich, 1994). In warm water reservoirs, stratification becomes strong (large density difference
per oC) but can be unstable (small temperature
changes result in large density change).
Relative Thermal Resistance to Mixing (RTRM).
Density Layer 1 temp - Density Layer 2 Temp
RTRM =
Density 4º C - Density 5º C
The greater the RTRM, the steeper the density
gradient. Reservoirs gain heat near the surface
primarily due to solar inputs. Warm water is less
dense than colder water. Hence, as a reservoir
gains heat in the Spring, the warm water at the
surface floats on the deeper cold water. The
cold water layer on the bottom of the reservoir
becomes isolated from atmospheric oxygen
input; dissolved oxygen becomes depleted by
Robert W. Kortmann, Ph.D.
respiration and decomposition but is not replenished from the atmosphere. The loss of dissolved
oxygen builds upward from the bottom. When
totally devoid of dissolved oxygen, OxidationReduction Potential (ORP) is used to diagnose
environmental conditions. The lower the ORP (or
more highly negative), the greater the concentration of chemically reduced substances such as
ferrous iron, manganese, and hydrogen sulfide.
The relationship between thermal stratification boundaries and light penetration through
the vertical water column plays an important
role in determining how much primary productivity occurs, and the composition of the
primary producer community that performs it.
Transparency is measured by an eight inch diameter black and white disk, called a Secchi disk
(after its inventor). If you double the Secchi depth
(depth where the disk disappears from view) that
is the approximate depth to which adequate
light penetrates for net photosynthetic oxygen
production (Compensation Depth). That is the
Anatomy of Stratification
The position of the Compensation Depth
relative to Thermocline Boundaries effects
Phytoplankton Abundance and Composition.
SD= Secchi Depth
CD= Compensation Depth;
(estimated as 2X Secchi Depth)
Sunlight
0
EPILIMNION
4
DEPTH (m)
Figure No. 3. Sources of Nitrogen and
Phosphorus in Water Supply Reservoirs.
Trophogenic Zone (net Oxygen Production, net
Organic Production). Below the Compensation
Depth (approximated by doubling the Secchi
depth) is the Tropholytic Zone (net Breakdown
of Organics, net Oxygen Consumption). When
the Tropholytic Zone extends up into the mixed
surface layer (epilimnion) Cyanobacteria tend to
be stimulated as the primary producers.
Light penetration into the reservoir can help
maintain deep oxygen concentrations via photosynthesis. When the Secchi Depth exceeds ½ the
depth of epilimnetic mixing, the Compensation
Depth penetrates the thermocline. Light penetration controls the upward ascent of the
aerobic-anaerobic boundary in such reservoirs
(Wetzel, 1975). The “Trophogenic Zone” (net productivity) extends below the thermocline and
contains the “Tropholytic Zone” (net breakdown
of organic matter) in deeper strata. This creates
conditions for aerobic cold water in the middle
of the water column and very good water quality. Phytoplanktonic Cyanobacteria tend not
to be stimulated (however, light penetration
may stimulate benthic Cyanobacteria in deep
strata). When the Compensation Depth occurs
within the metalimnion where density gradients
exist, Cyanobacteria can establish distinct layers (Golterman, 1975). Such layers can often be
SD
Trophogenic Zone
Net Productivity
O2 Production
METALIMNION
8
EPILIMNION
De
METALIMNION
Thermocline
CD
SD < ½ De tends
to favor
Cyanobacteria
Thermocline
HYPOLIMNION
12
Temperature
Tropholytic Zone
HYPOLIMNION
RTRM
Dissolved
Oxygen
Net Respiration
CO2 Accumulation
Figure No. 4. Reservoirs in New England
exhibit vertical thermal stratification during
the Summer, characterized by a warm and
mixed surface layer (epilimnion), a middepth layer in which temperature decreases
rapidly with depth (metalimnion), and deep
uniformly cold layer (hypolimnion). The depth
of light penetration, relative to stratification
boundaries, plays a role in determining
phytoplankton community structure.
Journal NEWWA
June 2015
77
Cyanobacteria in Reservoirs: Causes, Consequences, Controls avoided by vertical depth-selective raw water
gate operation (Kortmann and Karl, 2011).
When the Secchi Depth decreases below ½ the
depth of epilimnetic mixing, the Compensation
Depth is within the mixed surface layer. When
the anoxic boundary ascends into the bottom of
the surface epilimnion, it stimulates the buoyant
N-fixing Cyanobacteria such as Anabaena and
Aphanizomenon. The depth of the aerobic-anaerobic interface is no longer controlled by light
penetration. The ascent of the anoxic boundary
becomes controlled by downward diffusion of
dissolved oxygen (Diffusional Control of Anoxic
Depth; Wetzel, 1975). As a reservoir becomes
more eutrophic, Secchi transparency decreases
and the Compensation Depth ascends into the
mixed epilimnion (stimulating Cyanobacteria).
All genera of Cyanobacteria are able to perform photosynthetic primary productivity. All
Cyanobacteria perform aerobic respiration.
Some Cyanobacteria have been shown to live
heterotrophically by aerobic, and in some cases
anaerobic, respiration. Some Cyanobacteria can
take advantage of “residual energy” contained in
the chemically reduced products of anaerobic
respiration (e.g. sulfide) to reduce CO2 to Organic
Matter, with or without low intensity light. Benthic
Detrital Electron Flux becomes an energy source
for additional “primary productivity” (although the
energy came from respiration of previously produced organic matter, Kortmann and Rich, 1994).
The Cyanobacteria have evolved mechanisms to perform Photosynthesis (PS I, PSII),
Aerobic Respiration, and Anaerobic Respiration
(Kortmann and Rich, 1994).
The Iron Cycle performs an important role in
most New England Reservoirs. Iron typically carries most of the electron transport from anaerobic respiration in deep strata to aerobic waters
nearer the surface (Benthic Detrital Electron Flux;
Rich, 1979). Oxidized ferric iron complexes are
also what give sediment its phosphorus-binding
capacity in most New England reservoirs. When
Ferric Iron complexes are reduced to Ferrous
Iron, both iron and phosphorus accumulate in
deep anoxic waters (and the phosphorus load
from sediments is readily available SRP). As the
Compensation Depth ascends, more reservoir
bottom area functions via anaerobic respiration,
releasing more soluble reactive phosphorus (SRP)
which stimulates more phytoplankton productivity. Eutrophication becomes an accelerating
78
Journal NEWWA June 2015
Figure No. 5 Important Biological Processes
in Source Water Reservoirs.
cycle when the Compensation Depth ascends
above the thermocline. The internal phosphorus
load from sediments stimulates late summer
Cyanobacteria blooms in many New England reservoirs. When anaerobic respiration becomes
even more intense, sulfur is reduced to hydrogen
sulfide (imparting a “rotten egg odor”) which
can permanently remove iron as Ferrous Sulfide
(Doyle, 1968). The iron deposited as ferrous sulfide no longer participates in benthic detrital
electron transport. Sediment-Phosphorus binding capacity can become depleted by many years
of anaerobic respiration.
Managing the iron cycle (enhancing oxidized ferric iron complexes) can significantly
lower P availability and reduce Cyanobacteria.
Maintaining low Fe and Mn concentrations,
and good raw water quality in deeper strata
by managing aerobic respiration (e.g. hypolimnetic aeration) can provide a cold, high quality
raw water source during late summer when
surface blooms occur (raw water withdrawal
from under Cyanobacteria).
The Carbonate Buffering System is also an
important feature of freshwater systems. The
Carbonate Buffering System consists of Free
Carbon Dioxide, Carbonate, and Bicarbonate, in
proportions that are dictated by pH. When the
pH exceeds 8.3 no free Carbon Dioxide exists
in solution, giving Cyanobacteria a competitive
advantage. Cyanobacteria are favored when pH
exceeds 8.3 and free CO2 becomes limiting to
phytoplankton in the trophogenic zone.
“It is concluded that initiation of the blue-green
maximum does not depend upon conditions of low
CO2 concentration or high pH. However, once the
Robert W. Kortmann, Ph.D.
Figure No. 6 The iron cycle in New England Reservoirs and relationships to internal P loading.
cupric ion remains in solution. In hardwater
systems, a chelating agent or increased copper
sulfate dose is required for effective treatment.
Copper sulfate is more toxic to the herbivorous
zooplankton (grazers) than to the target phytoplankton. Hence, repeated treatments are
often needed because removal of phytoplankton biomass by grazing is decreased.
Carbon dioxide is the common product of respiration, both aerobic and anaerobic. By quantifying
the rate of dissolved oxygen consumption and
inorganic carbon accumulation, the Respiratory
Quotient (RQ) of a reservoir can be computed. The
higher the RQ, the greater the proportion of total
respiration that is carried out anaerobically (often
greater than aerobic respiration) (Rich, 1979).
Carbonate Buffering System in Fresh Water
100
% of Total DIC
blue-greens become abundant they ensure their
dominance by reducing concentrations of CO2 to
levels available only to themselves” (Shapiro, 1973).
In hardwater reservoirs, the carbonate buffering system tends to regulate internal P cycling.
Photosynthesis at the surface removes CO2 ,
calcium carbonate precipitates to re-establish
equilibrium, which precipitates P (adsorbed by
calcium carbonate). Respiration at the bottom
(aerobic and anaerobic) produces CO2; calcium
carbonate dissolves to re-establish equilibrium
(liberating bound P) (Wetzel, 1975). In soft water
reservoirs, where iron carries most electron
transport from anaerobic respiration, sedimentP release is due to the chemical reduction of
ferric iron to soluble ferrous iron. In hard water
reservoirs, sediment-P release results from
accumulation of carbon dioxide and dissolution of carbonate-P complexes. Both sediment
flux processes are driven by respiration in the
tropholytic zone. Free CO2 often becomes limiting in the trophogenic zone, and accumulates
in the tropholytic zone as the common product
of aerobic and anaerobic respiration. Managing
the carbonate buffering system in hardwater
systems to stimulate calcite formation in the
trophogenic zone and reduce the accumulation
of carbon dioxide in the tropholytic zone can
reduce TP availability and decrease phytoplanktonic productivity.
Copper sulfate treatments to control
Cyanobacteria are less effective in hardwater reservoirs because much of the copper
precipitates as carbonate complexes; less
DECALCIFICATION PHOSPHORUS REMOVAL
60
HCO3=
Free CO2
CO3=
Free HCO2
20
4
6
Photosynthesis Removes
Free Carbon Dioxide
8
> pH 8.3
10
12
Calcium Carbonate
Precipitates to
Re-establish Equilibrium
Many algae are more dependent on Free Carbon Dioxide
as a carbon source than Cyanobacteria
Figure No. 7 The Carbonate Buffering
System of freshwater reservoirs.
Journal NEWWA
June 2015
79
Cyanobacteria in Reservoirs: Causes, Consequences, Controls Respiratory Quotient
Carbon Dioxide Released (molar)
RQ =
Oxygen Taken Up (molar)
Measuring CO2 accumulation, as well as oxygen
consumption, accounts for all respiration (aerobic
and anaerobic), and the Respiratory Quotient of a
reservoir can be used to accurately size aeration
system oxygen input requirements and other
reservoir management techniques (Kortmann
and Rich, 1994).
The Nitrogen Cycle is critically important in
New England reservoirs that experience blooms
of N-Fixing, Scum-Forming Ecostrategists like
Anabaena and Aphanizomenon. Nitrate plays an
important role. Most think of Nitrate as a “polluting nutrient” because it does cause degradation
in estuarine and marine environments. However,
in freshwater ecosystems too little Nitrate-N can
pose problems. Nitrate-N is the first alternate terminal electron acceptor (ATEA) used in anaerobic
respiration after oxygen is depleted (Figure 4).
That anaerobic respiration process (denitrification
/ dissimilatory Nitrate reduction) is performed by
facultative anaerobes (which use oxygen when it is
available) (Golterman, 1975). As long as Nitrate-N is
available for anaerobic respiration, ferric iron will
not be reduced to ferrous iron, and phosphorous
will not be liberated from the sediments. Nitrate-N
availability also helps maintain Green Algae in the
phytoplankton, which are more readily removed
by grazing, and don’t produce taste and odor
or toxic substances. Nitrate-N availability can be
beneficial in freshwater systems, especially if total
phosphorus is abundant. N-fixing Cyanobacteria
can fix atmospheric nitrogen if inorganic nitrogen
compounds are not available (Figure 2). Nitrate-N
availability can be enhanced by increasing nitrification of Ammonia-N (via aeration or circulation methods) or by selective transfers of source
water. Enhancing nitrification requires temperature above approximately 10-12oC (for enzyme
activation) as well as dissolved oxygen. (Nitrifying
bacteria will do the rest.)
Macrophytes and Cyanobacteria
Phytoplankton (Algae and Cyanobacteria) are
the photosynthetic primary producers free-floating in open water (the “Pelagic Zone”). Rooted
macrophytes are “higher plants” that inhabit
areas where adequate sunlight penetrates to the
reservoir bottom (the “Littoral Zone”). Reservoirs
tend to exhibit dominance by one of these two
modes of primary productivity. Rooted macrophytes cause few quality problems for raw supply
water composition. In some cases, Cyanobacteria
can grow on macrophytes (epiphytic). Loss of
rooted vegetation due to a turbidity episode
(reducing light penetration to the bottom) can
result in a “switching of productivity state” from
macrophyte dominance to phytoplankton dominance. Cyanobacteria often flourish when this
shift occurs.
Several “macrophytes-higher plants” occur
free-floating in reservoirs (e.g. Bladderwort and
Coontail). These rarely cause adverse raw water
supply impacts. Indeed, they can compete with
phytoplanktonic algae and Cyanobacteria for
available nutrients and light energy. Some species also produce substances that can inhibit
phytoplankton (alleopathic substances). In
most source water reservoirs, macrophyte
productivity poses fewer water quality issues
than phytoplankton and epibenthic or epiphytic
Cyanobacteria.
Limnology of Reservoir Systems
Figure No. 8 In hardwater reservoirs the
carbonate buffering system regulates
internal P cycling.
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Journal NEWWA June 2015
Although the fundamental principles of
Limnology apply to reservoirs, there are also several very important and unique characteristics of
water supply reservoir systems. Many reservoirs
exhibit longitudinal zones from the “river inflow
end” to the deeper lake-like “lacustrine zone”
near the dam. Reservoir Management Methods
that focus on the “Transition Zone” where oxygen demand is high, and where Cyanobacteria
Robert W. Kortmann, Ph.D.
blooms often originate, can help to improve
reservoir quality.
The river inflow can have a profound effect
on how a reservoir stratifies thermally, and
can enter the reservoir as an overflow (mixing
with the surface epilimnion), underflow (mixing into the deep hypolimnion), or interflow
(layering on the thermocline) depending on the
relative flow volume and temperature/density
of river vs. reservoir water. The relationships
between river inflow and reservoir stratification
can be managed to optimize raw source water
quality at a selected supply intake. When river
water is colder (more dense) than the reservoir surface, a “plunge point” will exist where
river water descends to a depth of equivalent
density in the reservoir. Enhancing the plunge
of river inflow below the trophogenic zone can
decrease nutrient load impacts that stimulate
Cyanobacteria.
How water exits a reservoir also affects stratification structure and water quality. Many New
England source water reservoirs were constructed “in series”, with up-gradient storage
reservoirs feeding terminal withdrawal reservoirs downstream. Storage reservoirs are often
designed to maximize usable volume; release
gates are located at the bottom. Phosphorus,
Ammonia-N, CO2, and anaerobic respiration
products accumulate in deep strata (Figures
5 and 7). Releasing deep, nutrient-rich water
from the bottom of a storage reservoir into the
productive surface waters of downstream reservoirs can cause nutrient enrichment that stimulates Cyanobacteria blooms. As source water
passes through up-gradient storage reservoirs
(Modified after Thornton, K.W. et.al., 1990)
Riverine
High Flow, Velocity
High NTU
Zp < Zm
Allochthonous
Light Limited
Transition
Overflow
Lacustrine
Interflow
Underflow
Reduced Flow, Velocity
Reduced NTU
Increased Light
Low Flow, Velocity
Alloch- and Autochthonous
Low NTU
High Productivity
Zp > Zm
Autochthonous
Storage Reservoir
More Nutrient Limited
Terminal Distribution Reservoir
Source Water Reservoir Systems
Bottom releases from storage reservoirs can be rich in nutrients and anaerobic respiration
products, and can stimulate Cyanobacteria in downstream reservoirs. Nitrate is often
exhausted in Storage Reservoirs, favoring N-Fixing Cyanobacteria in downstream reservoirs.
Figure No. 9 Longitudinal zones of runof-river reservoir impoundments, and
source water reservoirs in series.
and flows downstream to a terminal source reservoir Nitrate-N is consumed by assimilation and
denitrification. Nitrogen limitation can develop
in downstream reservoirs, which can stimulate
N-fixing Cyanobacteria. Deep releases from upgradient storage reservoirs can be enriched with
anaerobic respiration products and nutrients.
These factors can set the “habitat stage” for
blooms of N-fixing Cyanobacteria in downstream
reservoirs.
However, withdrawal or release by vertical gate
scheduling can modify mixing depth and anaerobic boundary depth. In some multi-reservoir
systems, the deep anoxic volume is small relative to the sediment area it covers. Continuous
controlled deep release can avoid anaerobic conditions and SRP and CO2 accumulation in deep
strata in some reservoirs, which can help to maintain water quality (lower P, Mn, Fe). Quantifying
and comparing feasible flushing rates to the measured oxygen deficit rate can identify effective
operations for management of multiple reservoir
source systems. Submerged dams (often dams
existing before reservoir expansion), can create
isolated stagnant bottom layers that become
anoxic nutrient-release sources.
Management Principles for
Reducing Cyanobacteria
Management of nutrient and contaminant
loads from the land draining to a reservoir (the
watershed) can help reduce primary productivity
and Cyanobacteria. Watershed management is
always beneficial for protecting resource quality.
However, there are many cost-effective management approaches that can increase a reservoir’s
capacity to receive watershed loads without
exhibiting poor water quality and Cyanobacteria
blooms. Reducing the contribution of internal
phosphorus loading (SRP) can reduce primary
productivity, improve trophic state, and reduce
Cyanobacteria blooms. Maintaining availability
of nutrients that become limiting to algae (but
not to Cyanobacteria) such as Silica, Nitrate-N,
and CO2, can maintain a more desirable phytoplankton community. Some management
strategies for improving water quality in storage and terminal reservoirs, and for reducing
Cyanobacteria, include:
• Source Selection and Sequencing
Scheduling use of a more eutrophic reservoir early in the season, reserving higher
Journal NEWWA
June 2015
81
Cyanobacteria in Reservoirs: Causes, Consequences, Controls quality sources for later in the Summer, can
help reduce impacts to raw water quality and
treatment.
• Depth-Selective Withdrawal and Release
Selecting vertical source water intakes can manage stratification structure and avoid the highest Cyanobacteria cell densities.
• Bloom Containment
Installing surface or submerged weir partitions
in strategic locations can avoid impacts of surface Cyanobacteria blooms and hypolimnetic
anaerobic respiration products (Fe, Mn, Sulfide)
at raw water intakes.
• Aeration or Oxygenation to Reduce Internal
SRP Loading
Can reduce phytoplankton density (esp.
Cyanobacteria) and improve the removal rate of
phytoplankton biomass by grazing. Maintaining
aerobic respiration in more reservoir volume,
covering more bottom area, can reduce readily
available soluble reactive phosphorus (SRP),
and can facilitate depth-selective withdrawal
to avoid Cyanobacteria without elevated Fe
and Mn.
• Maintaining Nitrate Availability,
Nitrification Enhancement
Enhancing the microbial conversion of ammonia to nitrate, or importing nitrate, can help to
avoid stimulating the N-fixing Cyanobacteria
(Anabaena and Aphanizomenon)
• Increasing Epilimnetic Mixing Depth
Increasing the depth of epilimnetic mixing can
help avoid the competitive advantage of buoyancy control in Cyanobacteria, and can reduce
the anoxic area that releases SRP.
• Prolonged Diatom Maximum during Spring
Enhancing the Spring Bloom of Diatoms
has been effective at delaying and reducing
Cyanobacteria later in the summer.
• Carbonate Buffer System Management,
maintaining pH< 8.3, CO2 Availability
CO2 accumulates in deep strata due to respiration, and can be a source of CO2 to maintain pH
below 8.3 and avoid favoring Cyanobacteria over
other phytoplankton genera.
• Light Penetration
Maintaining Secchi transparency > ½ the epilimnetic mixing depth can help avoid Cyanobacteria
blooms.
• Enhance the Grazing Rate
Increasing the removal rate of phytoplankton
biomass by enhancing grazing by herbivorous
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Journal NEWWA June 2015
zooplankton can favor Green Algae and Diatoms
over Cyanobacteria.
Case Study Examples
The following case study summaries illustrate
a variety of reservoir management techniques
that have been used to reduce Cyanobacteria
blooms, DBP precursors, and anaerobic respiration products in raw source water.
Saugatuck – Hemlocks Reservoir
System, Aquarion Water Co.,
Connecticut
Hemlocks Reservoir is the direct source terminal reservoir. It is maintained full by transfers
from a large storage reservoir through a very
small river impoundment. Saugatuck Reservoir
receives water from a “hardwater geologic area”
of Connecticut. The Aspetuck River and Hemlocks
Reservoir are located in an area where runoff is
of lower conductivity and hardness.
To reduce phytoplanktonic and benthic
Cyanobacteria, and to control raw water Fe and
Mn, a management system was developed consisting of:
• Depth-Selective Release from the upper hypolimnion in Saugatuck Reservoir
• Re-Aeration of Saugatuck Water as it is transferred through Aspetuck Reservoir
• Layer Aeration of Hemlocks Reservoir (creating
a mixed mid-depth layer)
• A Small Hypolimnetic Aerator in the deepest
water layer in Hemlocks Reservoir
•Depth-Selective Supply Withdrawal from
Hemlocks Reservoir (from the Layer)
• Monitoring of Phytoplankton and Benthic
Cyanobacteria Mats (to guide treatments)
Phytoplanktonic Cyanobacteria are often
abundant in the eiplimnion of Saugatuck
Reservoir; release from below the thermocline avoids the Cyanobacteria. The cold water
releases from Saugatuck Reservoir flow through
Aspetuck and into Hemlocks Reservoir, and
plunge to depth due to its greater density than
the warmer epilimnion of Hemlocks Reservoir.
High quality Saugatuck water flows through the
mid-depth layer in Hemlocks Reservoir (created
by Layer Aeration) to the depth-selective vertical
raw water intakes. The conductivity signature
of Saugatuck Reservoir is readily observed in
profiles from Hemlocks Reservoir.
Robert W. Kortmann, Ph.D.
Figure No. 10 The Saugatuck-AspetuckHemlocks Source Water System lies in both
hard and soft water areas and the reservoirs
exhibit typical longitudinal zones.
Lake Shenipsit (Supply Reservoir),
Connecticut Water Company
Lake Shenipsit is a large recreational fishery lake used as a major source of water
supply to north-central Connecticut. During
the late 1970s, blooms of Anabaena sp. and
Aphanizomenon sp. were stimulated by oxygen
loss and internal cycling of nutrients (especially
sediment-released SRP). Nearly all cold-water
habitat for fish and herbivorous zooplankton
refuge was lost during the summer. The blooms
caused water supply treatment problems including taste and odor episodes, early exhaustion
of activated carbon beds, and elevated chlorine
demand (increasing DBP formation potential). A
two stage Layer Aeration System, driven by 200
SCFM compressed air capacity, was designed
and installed. Within three years the summer
Cyanobacteria blooms subsided, over 3000
acre-ft of cold water habitat was restored, and
zooplankton grazer populations increased.
Summer transparency increased from 4 ft to 4
meters (13 ft). Oxygen has been maintained to
14 m (45 ft) since 1990.
Layer Aeration is a depth-selective circulation
method that adjusts how a reservoir stratifies
in order to improve raw water quality. The temperature of the Layer, and locations of induced
thermoclines, are designed from monitoring
temperature profiles and computing a vertical
heat budget. Oxygen already in the reservoir
water is redistributed from above the compensation depth throughout the layer, and concentrations can be predicted from observed depth
profiles. The temperatures and dissolved oxygen
concentrations observed during diagnostic studies were used with stage-volume data to forecast
the structure of stratification and distribution
of oxygen during Layer Aeration (assumes no
oxygen input from gas phase in bubbles to solute
phase in reservoir water). The Layer boundaries were selected by iteration of circulation scenarios. Following Layer Aeration the respiratory
quotient approached 1.0, indicating that much
more of the lake’s total respiration was aerobic.
Layer Aeration decreased anoxia and reduced
total phosphorus availability by substantially
decreasing sediment-P release. Habitat for herbivorous grazers was improved and the grazing
rate on algae biomass removal increased. These
changes resulted in elimination of the summer
Cyanobacteria blooms, significant improvement
in water clarity and light penetration, and deepening of the aerobic-anaerobic boundary. Raw
water turbidity decreased from 4 -5.5 NTU to
below 2 NTU. Raw water TOC decreased from
approximately 6 mg/L to less than 4 mg/L.
As a result of Layer Aeration at Lake
Shenipsit, Total Phosphorus concentrations
decreased throughout the water column,
resulting in an overall decrease in phytoplankton (especially Cyanobacteria). Water supply
improvements included extended GAC substrate longevity, elimination of summer taste
and odor episodes, and reduced turbidity and
TOC in raw source water.
LAYER AERATION
Depth-Selective Circulation
Vented excess air can be
re-diffused at a selected
depth to expand
epilimnetic mixing De
MEAN DO=
∑ (DOz x Vz) / ∑ Vz
Layer Aeration reduces the volume and area of
the deepest hypolimnion, which can then be
aerated very efficiently to prevent a negative
Oxidation-Reduction Potential (low Eh)
Shenipsit Lake
523 acres
(212 ha)
WTP
MEAN TEMP=
∑ (Tz x Vz) / ∑ Vz
Water that is blended and aerated from
multiple depths is returned at an
intermediate depth and density.
Aerated Layers are typically designed
around raw water intake elevations.
The Layer DO, Temperature, and Thermocline Formation can be predicted using
strata volumes and observed dissolved oxygen and temperature profiles
Figure No. 11 Layer Aeration has been
performed annually since 1986 to reduce the
internal loading of readily available soluble
reactive phosphorus (SRP), and maintain
grazing by herbivorous zooplankton.
Journal NEWWA
June 2015
83
Cyanobacteria in Reservoirs: Causes, Consequences, Controls Figure No. 14 Profiles from Glendola
Reservoir prior to Layer Aeration.
Figure No. 12 Dissolved oxygen profiles
from before and during Layer Aeration,
measured annual Secchi transparency and
Compensation Depth, and Raw Water Total
Organic Carbon (TOC) and Turbidity (NTU).
Glendola Reservoir, New Jersey
American Water Company
Glendola Reservoir is a bowl-shaped off-line
storage reservoir that receives diversion from
the Manasquan Reservoir, or Manasquan River,
in coastal New Jersey. A Layer Aeration System
manages “how stratification develops” while
aerating three vertical depth zones: Enhanced
Epilimnetic Mixing, Mid-Depth Layer, and the
Hypolimnion. Vertical aerated layer boundaries
were selected relative to the vertical position of
supply raw water intakes.
Before implementing Layer Aeration,
Cyanobacteria blooms in surface water, and
deep accumulations of Fe and Mn, made
Figure No. 13 Changes in in Total
Phosphorus in the Epilimnion, Metalimnion,
and Hypolimnion, and Phytoplankton
response to Layer Aeration since 1986.
(Dashed lines represent 20 micrograms P
per liter, an approximate “threshold” for
stimulating Cyanobacteria).
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Journal NEWWA June 2015
treatment difficult using any of the vertical
intakes. The Mid-Depth Layer Aeration Tower
was the primary focus for providing the highest
quality water to the middle supply gates (under
Cyanobacteria, above Fe and Mn accumulation;
isolated by thermoclines). A smaller hypolimnetic aerator maintains high oxidation-reduction
potential in the deepest layer to decrease the
accumulation of anaerobic respiration products
and internal SRP loading. An epilimnetic mixer
ensures a well-mixed surface layer and deepening of the epilimnion (for avoiding buoyant
Cyanobacteria species).
Total phosphorus (TP) concentrations
decreased, especially during Fall due to reduction in internal SRP loading. Prior to management
of stratification and respiration, Cyanobacteria
dominated the phytoplankton composition during late Summer and Fall. The phytoplankton
community structure shifted from dominance by
Cyanobacteria (Anabaena and Aphanizomenon)
to Greens and Diatoms. Layer aeration has significantly reduced the internal accumulation of
phosphorus, iron, and manganese related to
oxygen loss in Glendola Reservoir. Ongoing monitoring by NJ American resulted in changes in
operations (for example, shifting airflow delivery
between aerators seasonally). This has resulted
in continued improvement, as exemplified by
decreased maximum Fe and Mn concentrations,
and strong shift to Diatom dominance.
Brick Township Municipal
Reservoir, New Jersey
During planning and design of a new reservoir to store river water to meet peak summer
demands, water quality modeling indicated
that oxygen loss, internal nutrient cycling, and
Cyanobacteria blooms were expected to cause
treatment difficulties.
Robert W. Kortmann, Ph.D.
Enhanced Mixing
Deepened De
Layer
Aeration
Anoxia
Anoxia
Full Circulation (seasonal)
Layer Aerator
Brick Off-Line Storage Reservoir
Brick Township, NJ
Full Circulation Module
Figure No. 15 Cyanobacteria and Diatom
densities, Iron, Manganese, and seasonal TP
concentrations observed at Glendola Reservoir.
A Layer Aeration System was designed and built
into the new reservoir infrastructure. It has several treatment capabilities, including:
• A full depth circulation system which maintains
a well-mixed reservoir through May to extend
the Spring conditions and delay a shift from
Diatoms to Cyanobacteria,
• A Layer Aeration system that maintains a
well-mixed and highly aerobic deep layer to
minimize raw water iron, manganese, nutrients,
algae, and organic content,
• Vented air released from the Layer Aeration
Towers is re-diffused to expand the mixed
surface water layer down to approximately
20 ft deep, which inhibits the development of
buoyant Cyanobacteria.
Chemical feed lines were installed to each of
the vertical aeration/circulation system diffusers
to use the aeration system as a rapid mix apparatus if treatments became necessary. To date,
no algaecide treatments have been required.
The system was designed specifically around
the water supply intake depths and locations,
so the Utility can select the highest quality water
during any season. If Cyanobacteria become
abundant in the surface waters, raw water can
be withdrawn from under the thermocline,
while aeration maintains low concentrations
of anaerobic respiration products (Fe and Mn).
External nutrient loads are accurately quantified from pumping logs and concentration
data. River diversion is selected by hydrograph
flow stages. The rising arm of the hydrograph
is avoided because it contains the highest river
TP concentrations. The receding hydrograph is
diverted to reservoir storage to help maintain
Figure No. 16 Pump Storage Reservoir
located in Coastal New Jersey (Kortmann and
Karl, 2011).
Nitrate-N availability and avoid buoyant N-fixing
Cyanobacteria. Research continues to follow raw
water quality, the establishment of biological
communities, and ecological interactions, in
this “new lake”.
Research conducted at the Brick Township
Reservoir identified a “Seasonal Approach” to the
operation of aeration systems to reduce Summer
Cyanobacteria. During 2009 and 2010 the diffuser modules were operated after ice-out to
maintain a well-mixed condition as the isothermal
water column warmed. This was done to prevent
intermittent episodes of temporary stratification
as the lake heating season progressed, and to
keep Diatoms suspended in the water column
as the water became warmer. Once the isothermal mixed temperature reached approximately
)
Figure No. 17 Reservoir profiles during Layer
Aeration and Diatom densities during full
circulation in April and May.
Journal NEWWA
June 2015
85
Cyanobacteria in Reservoirs: Causes, Consequences, Controls Figure No. 18 Shifts in the seasonal
phytoplankton community structure
following spring diatom enhancement by
artificial circulation. (Dashed vertical line
separates two years prior to seasonal mixing
from two years following initiation.)
15oC, compressed airflow was redirected to the
Layer Aeration Towers which manage how stratification develops relative to water supply intake
elevations, and to maintain aerobic conditions
to prevent increases in nutrients and anaerobic
respiration products in deep strata.
Peaks in Total Phytoplankton abundance
decreased significantly. Abundance of summer
Cyanobacteria decreased by approximately 75%,
and the occurrence was delayed approximately 6-8
weeks. Abundance of Green Algae also decreased,
and exhibited a delay to later in the summer stratification season. Use of artificial circulation during
the Spring-Summer transition manages the timing
of phytoplankton seasonal succession. Later in the
summer Microcystis sp. increases (scum-forming
ecostrategist, not N-fixing). Densities remain relatively low at the deep raw water intake.
Figure No. 19 The Supply Intake was
extended into the Northern Bay, Isolated
from Agricultural Runoff.
avoided water quality impacts related to agricultural runoff and Cyanobacteria blooms. A section
of the reservoir was partitioned off by a surfaceto-bottom weir curtain at a narrow location. The
raw water intake was extended from the existing
location, anchored on the reservoir bottom, to
the northern bay which was now isolated from
the stream carrying runoff from the agricultural
use area of the watershed.
As seen in the photo, intense Cyanobacteria
blooms continue to occur in the lower basin
exposed to agricultural runoff. However, the
source water withdrawn from the isolated
northern bay exhibits much better water quality (significantly lower NTU, Coliform, Nutrients,
Cyanobacteria, Taste & Odor, Chlorine Demand,
etc.) (Kortmann, 1994).
Cyanobacteria densities are much lower in the
isolated northern bay than in the lower basin.
Comparison of Upper and Lower Stafford Reservoir 2
Lower Basin Total Algae
Lower Basin Cyanobacteria
Upper Bay Total Algae
Upper Bay Cyanobacteria
A dairy farm located upstream of a source
water reservoir resulted in very high nutrient
loading, coliform, Cyanobacteria blooms, turbidity, TOC/DOC, and other contaminants. At times
oxidant demands were so high that it was difficult
to add enough chlorine to maintain adequate disinfection and residuals, DBPs were problematic,
as well as taste and odor episodes. Diagnostic
research identified a Reservoir Partitioning
and Flow Routing approach which substantially
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Journal NEWWA June 2015
% Lower Concentration
Stafford Reservoir, Connecticut
Water Company
Upper vs. Lower Basin
Figure No. 20 Comparison of phytoplankton
densities, TP concentrations, and source
water composition between the Upper and
Lower reservoir basins.
Robert W. Kortmann, Ph.D.
Iron, manganese, turbidity, and other water quality parameters are much lower in the isolated
northern bay where the intake was extended to.
During early July, releases from the headwater
reservoir help to further reduce water quality
degradation in the Lower Basin and maintain storage volume in the terminal reservoir. Although
agricultural runoff from the dairy farm land-use
continues to stimulate Cyanobacteria blooms in
the Lower Basin of the terminal reservoir, raw
water quality impacts have been controlled and
Cyanobacteria densities avoided.
Pennichuck Source Water
Management System
The source water system to Pennichuck Water
in New Hampshire provides an example of a
run-of-river reservoir system in series. Water is
also imported from the Merrimack River during
extended dry periods. A system of Surface Booms
and Submerged Weir Partitions was designed and
installed to manage how water flows through the
source water reservoirs (containing hypolimnetic
anoxia and surface Cyanobacteria away from raw
supply intakes). The direct withdrawal reservoir
basin is managed by Layer Aeration to prevent
Fe and Mn accumulation in deep strata, and to
maintain depth-selective withdrawal capability. Mechanical aeration is only needed in the
lower portion of Harris Reservoir; flow-routing
baffles provide passive management further
“up-reservoir”.
Broadbrook Reservoir,
Meriden, Connecticut
An integrated reser voir management
approach was implemented at Broadbrook
Reservoir. A surface containment partition and
submerged weir partition baffle were installed
at a narrow reservoir location. The surface
partition contains buoyant Cyanobacteria “upreservoir” in the riverine and transition zones.
The submerged weir partition contains the
anaerobic hypolimnion “up-reservoir”. Only
water from middle depths is allowed to pass
down-reservoir toward the dam and raw water
supply intakes. Two small hypolimnetic aerators
and two artificial circulation diffuser lines (at two
different elevations) are operated seasonally to
maintain raw water quality in the lower reservoir
(lacustrine zone) nearest the supply intakes. In
this approach, the passive reservoir flow routing
Pennichuck Water – Source Water Reservoir System
Harris Reservoir: Anaerobic Bottom Area Before and After Management
Aerobic
Anoxic
BEFORE
AFTER
Aerate
Figure No. 21 Floating and submerged weir
partitions manage how water passes through
the series of source water reservoirs to
contain Cyanobacteria and hypolimnetic Fe
and Mn in up-reservoir zones. Only the direct
source water basin is aerated.
Figure No. 22 Surface and submerged weir
baffles contain Cyanobacteria and anaerobic
respiration products in the up-reservoir
areas; only the direct raw water withdrawal
basin is aerated.
partitions reduce the area and volume needing
aeration; hence improving cost-effectiveness of
aeration facilities.
Reservoir monitoring indicated that the surface
and submerged weir baffles effectively contained
Cyanobacteria, and isolated the deep anaerobic
hypolimnion. The lacustrine zone, from which raw
water is depth-selectively withdrawn, was aerated
and circulated to maintain aerobic conditions and
reduce accumulation of Fe and Mn.
Putnam Reservoir Aquarion Water Company
Putnam Reservoir is a terminal distribution reservoir serving the Greenwich Connecticut area.
Journal NEWWA
June 2015
87
Cyanobacteria in Reservoirs: Causes, Consequences, Controls Figure No. 23 Land-Based Off-Grid Solar Powered Layer Aeration System.
Raw water typically had elevated iron and manganese during the Summer and early Fall, as well as
episodes of taste and odor related to both benthic and phytoplanktonic Cyanobacteria. A Layer
Aeration System was designed specifically for the
stratification structure of the reservoir and elevations of available water supply intakes. The system
is an “off-grid solar powered system”: photovoltaic
panel arrays, battery bank, load controls, and DC
compressors provide the compressed air to drive
the air-lift pumping system. The in-reservoir components blend and aerate water from a selected
depth range, redistributing available ambient
dissolved oxygen, produced by photosynthesis
above the compensation depth, throughout the
layer depth range to offset deeper oxygen deficits.
Manganese and iron concentrations decreased
by 80% and 60%, respectively. Geosmin and MIB
have been kept below the 10 ng/L odor threshold
most of the time.
An extensive monitoring program collects water
quality data from the distribution reservoir as well
as upstream storage reservoirs. Ongoing research
is focusing on reservoir management approaches
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Journal NEWWA June 2015
to improve the water quality entering the terminal
reservoir, reducing nutrient concentrations and
anaerobic respiration products.
Ledyard Reservoir, Groton
Utilities, Connecticut
The Groton Source Water System consists of
a terminal distribution reservoir (Poquonock
Reservoir) and two headwater storage reservoirs (Morgan and Ledyard Reservoirs). Ledyard
Reservoir receives water from Morgan Reservoir,
and delivers water to Poquonock Reservoir. A small
hypolimnetic aeration system was installed in
Ledyard Reservoir to maintain more aerobic
conditions over the bottom sediments to reduce
Mn accumulation. Two Vertical Axis Wind Turbine
(VAWT) Circulators were installed in Ledyard
Reservoir to “push oxygen-rich water from above
the compensation depth” to the bottom of the
reservoir to provide additional deep oxygen input
without increasing compressor capacity.
Reservoir monitoring data indicates that the
two small VAWT Circulators transport water to
the bottom, increasing oxygen concentrations
Robert W. Kortmann, Ph.D.
Figure No. 24 A direct-drive vertical
axis wind turbine circulator deployed
in a Groton Source Water Reservoir
and resulting temperature and
dissolved oxygen profiles.
and reducing Fe and Mn in over-bottom water.
An increase in over-bottom temperature indicates
that the VAWT Circulators transport warmer water
from above the compensation depth to the reservoir bottom without destratifying the reservoir.
Summary
Total Phosphorus (TP) will dictate “how much”
primary productivity will occur in most New
England reservoirs. Higher TP concentration
results in more phytoplanktonic productivity.
When TP exceeds approximately 20-25 µg/L,
Cyanobacteria tend to become dominant in the
summer phytoplankton community. TP sources
include atmospheric deposition, watershed runoff (and waterfowl in some cases), and internal
loading. Most reservoirs in New England are soft
water systems in which the Iron Cycle dominates
electron transport from anaerobic respiration,
and provides P-binding capacity of sediments.
Internal loading as a result of iron reduction can
result in increased availability of soluble reactive
phosphorus (SRP) that stimulates Cyanobacteria.
Few New England reservoirs are hard water systems (some coastal reservoirs and in northwestern Connecticut and western Massachusetts).
Although TP dictates “how much productivity” other ecological features will dictate “what
kind” of primary productivity will occur, and
which producer will dominate. Where adequate
light reaches the bottom aquatic macrophytes
or attached benthic algae and Cyanobacteria
will dominate. Where light doesn’t reach the
reservoir bottom, phytoplankton will dominate
primary productivity. When adequate Silica
and Nitrate-N are available Diatoms tend to
be favored in the phytoplankton community.
When water temperature increases (decreasing density and viscosity), Silica is depleted
(below approximately 0.5 mg/L), and Nitrate-N
remains available, Green Algae will tend to be
favored. Avoid watershed management practices that decrease nitrate loading disproportionately to decreased TP loading. If external
nitrogen loading is reduced more than TP,
there may be less phytoplanktonic productivity but more N-fixing Cyanobacteria. Maintain
a molar N:P ratio greater than 10, especially
in systems where TP is much higher than 25
µg/L. When inorganic nitrogen is exhausted,
silica depleted, light penetration decreases, and
the Compensation Depth ascends above the
thermocline, Cyanobacteria have a competitive advantage. Cyanobacteria productivity then
tends to increase pH, and decrease CO2 availability to levels available only to themselves.
The higher the TP becomes, the more likely that
Cyanobacteria will dominate the phytoplankton,
and internal SRP loading occurs during summer
stratification when Cyanobacteria dominate. The
goals of source water reservoir management to
control Cyanobacteria include:
• Reduce TP to below 20-25 µg/L (especially
reduce SRP)
• Maintain Silica availability and Enhance Diatoms
during Spring
• Maintain Nitrate-N availability for denitrification (to decrease internal SRP loading), and to
favor Green Algae and Diatoms
• Increase the grazing rate by herbivorous
zooplankton
Management approaches include:
Journal NEWWA
June 2015
89
Cyanobacteria in Reservoirs: Causes, Consequences, Controls •
•
•
•
•
•
•
•
•
•
Source Selection and Sequencing
Depth-Selective Withdrawal and Release
Bloom Containment
Aeration or Oxygenation
• Reduce Internal P Loading
• Maintain Zooplankton Refuge
• Piscivorous Habitat
Maintaining Nitrate Availability
• Import Nitrate
• Enhance Nitrification
Increase Epilimnetic Mixing Depth
Prolong the Diatom Maximum during Spring
Carbonate Buffer System Management
• Maintain pH< 8.3; CO2 Availability
Light Penetration Comp Depth >> De
Enhance the Grazing Rate
• Zooplanktivore Reduction (e.g. Alewife)
Conclusion
Reservoir management approaches can
help to reduce or avoid Cyanobacteria
blooms and improve raw water quality. In
some cases, operational decisions such as
source reservoir use scheduling can reduce
Cyanobacteria. In some cases, passive management systems such as Flow Routing Baffles
can manage physical conditions to maintain
raw water quality and reduce Cyanobacteria. In
other cases, circulation or aeration systems can
help reduce Cyanobacteria. Methods need to be
designed in relation to the limnology of the specific source water system (all reservoirs and the
watershed), the infrastructure (intakes, release
gates), seasonal supply demands, and the ecological features that a provide a competitive
advantage to Cyanobacteria. The first step is
to determine “what needs to be accomplished”.
Then a management plan can be developed
and implemented for raw water quality control
and Cyanobacteria avoidance.
Acknowledgements
I wish to acknowledge and thank the many
Water Utilities that have contributed to the
advancement of Source Water Reservoir
Management over the past 30 years. I am especially thankful to Peter H. Rich, Ph.D. for helping me understand the principles of ecosystem
ecology and how to apply them to diagnostic
limnology; and to Kevin Walsh for helping me
understand water treatment processes and raw
water composition needs.
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Journal NEWWA June 2015
References Cited
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