MACROPHYTES AND HUMAN AFFAIRS
Aquatic macrophytes are rarely
managed seriously in the sense of nuturing desirable spp.; usually the only
effort is to eradicate undesirable "weed" spp., e.g. Myriophyllum spicatum, which outcompetes
other spp. and is not eaten by waterfowl or fish.
Much of the early knowledge of
aquatic plant ecology was generated by waterfowl biologists. Other animals ranging from songbirds to
muskrats and moose use aquatic plants for various purposes, but aquatic plants
never manipulated in this context. Sculthorpe
Ornamental spp. such as water
lillies (Nymphaea spp.), water
hyacinth (Eichornia crassipes) are
heavily favored, although just within the last century the latter has spread
from its native South America throughout the (sub) tropics of all continents,
and is arguably the greatest nuisance
aquatic plant globally.
Different interest groups may have
conflicting ideas of what plants are desirable in what locations. Swimmers, waterskiers and boaters like NO
plants, whereas fisherman may see a modest lilypad stand as good bass habitat.
A major concern near populated
areas is that macrophytes increase mosquito populations, both by providing
stagnant water and shelter from predatory fish. Some mosquitoes actually obtain their O2 from
macrophyte lacunae! Esp. in tropics,
mosquitoes are vectors for malaria, yellow fever, etc., so aquatic plant
reduction is a public health concern.
Snails are another disease vector
(e.g. schistosomiasis, causing 2-4 million deaths annually) that depend on
aquatic macrophytes.
Fisheries managers desire
negligible macrophyte growth in hatchery impoundments, but natural lakes should
have some plants for shelter and spawning.
Dense stands of certain aquatic weeds,
e.g. water hyacinth in Florida, can completely prevent navigation of canals,
requiring constant clearing by the Army Corps of Engineers. Similarly, weeds such as Potamogeton pectinatus (sago pondweed)
can choke out irrigation systems.
Macrophyte
Control, Eradication and Prevention
Control = decrease population density
and/or vigor to an "acceptable" level.
Eradication = total
elimination of a weed sp. from a body of water; us. impractical for all but the
smallest ponds.
Prevention =
avoidance of innoculation events and design of pond bathymetry (e.g. steep
slopes), etc. to discourage colonization.
1. Biological Control
Requirements for a biological
control agent is that it must:
attack only the target plant(s),
not desirable spp. (this criterion is rarely met)
be able to survive in the
introduced habitat, but not itself become a pest
be capable of decreasing the
target organism to an acceptable level; also rarely achieved, at least not
consistently (sometimes there are cyclic, out-of-phase oscillations)
An often overlooked problem is the
up-front research cost (time and $) to identify and test potential control
agents for effectiveness and safety
a) pathogens -
very limited and as yet marginally successful research in this area; a
population crash of M. spicatum in
Chesapeake Bay in 1960's was attributed (not definitively) to an unknown virus
b) competitor species
intentional eutrophication to increase
phytoplankton that shade submersed macrophytes
Eleocharis
coloradoensis, a low-growing, submersed spike rush, controls pondweeds
obstructing irrigation canals in California, and helps to prevent erosion and
itself does not obstruct flow.
allelopathy has not
been demonstrated yet in aquatic plants, but is a possibility
c) grazers
Alligator weed (Alternanthera philoxeroides) is an
introduced (S. America native) pest in the southeastern U.S. and California,
and can grow anywhere from moist soils to free-floating mats. In the 1960's a flea beetle (Agasicles hygrophila) that is an
obligate grazer on alligator weed was discovered and successfully
introduced. A few years later, 2 other
insects were introduced to assist in conditions where the beetle was marginally
effective because of climate, etc.
Other insects have been tried with
variable success, mainly in the tropics, to control Salvinia and Eichornia
crassipes, and research continues.
In the 1960's, the tropical snail Marisa cornuarietis was studied for its
voracious grazing of aquatic weeds.
However, it was found to be a generalist feeder (incl. rice, etc.) and
dies at 9°C, so no further work has been done on this or other snails.
Silver dollar fish (Metynnis roosevelti and Mylosomma argenteum) are small tropicals
that clip plants near the base, then graze them at the surface; limited
application due to partial effectiveness and warm water requirement.
Tilapia spp.,
widely cultivated warm water food fish, are herbivorous but also have the
potential to outcompete native fish; T.
zillii is used in the Sonoran desert (Calif.) to control weeds in
irrigation canals, but must be restocked each spring.
Grass carp (Ctenopharyngodon idella), another food fish native to northern
China, has great potential for aquatic weed control in temperate waters because
of their wide tolerance for temperature (0-40°C, feeding to 12°), low [O2]
(survival to ~ 0.5 ppm, feeding to 2.5 ppm), salinity (to ~ 15 ppt = ½
seawater), and pH up to at least 10.8.
It is usually a generalist feeder, but only partly digests plant
material and so excretes nutrient-rich organic matter that can cause algal
blooms. Debate rages over the potential
for disrupting native fish, waterfowl and wildlife populations; reproduction
reportedly occurs only under very specific conditions. It is also a good food and sport fish (up to
> 1 m and ~ 45 kg; willow leaves for bait!)
Manatees (Trichechus manatus) has been proposed for tropical aquatic plant
control, but their endangered status and low reproductive capacity, and
susceptibility to poaching all argue against a serious potential.
2. Chemical Control (Herbicides)
Considerations for herbicides are:
Safety: teratogenicity
and chronic & acute toxicity to humans, livestock, pets, wildlife and
inverts; effect on irrigated crops, domestic & ornamental plants.
Efficacy: required
dosage (amount, frequency, timing & interfering conditions).
Environmental
fate: solubility, adsorption to sediment, residence time,
bioaccumulation, breakdown conditions and products?
Spectrum
& mode of activity: which plants does it affect and why? Some are broad, others narrow-spectrum. Understanding specific biological effect is
imperative.
Formulation
effects: safety, efficacy and fate are all affected by the exact
chemical form of the parent compound (acid, salt or other derivative) as well
as the physical and chemical composition of the carrier (liquid, granular,
powder, emulsion, specific properties of each).
Economics: is it
cost-effective and commercially viable (and patentable)?
Specific herbicides approved for
aquatic use include:
2,4-D: 20% by
weight in granular form specifically for aquatic use (submersed plants), or
liquid form may be sprayed on emergent shoreline vegetation; effectiveness increases
at low pH; only recreational activities may proceed immediately.
Endothall: broad
spectrum, liquid or granular; waiting times specified for various subsequent
uses of the water.
Diquat: liquid
at 2 pounds/gallon; similar spectrum to endothall; 10 day wait before swimming
or agricultural use; adsorbs to clay & organics -- do not use in turbid
waters.
Simazine: white
powder, must by mixed w/ water before use; slow acting, specific for algae
& some vascular plants; recreation immediately, 12 months wait for
consumption!
Dichlobenil:
granular, esp. useful for Characeae; cannot be used in potable or livestock
water, and fish cannot be consumed for 90 days following application.
Fenac: requires
that entire pond or lake is first drained, then applied to bottom.
Dalapon: soluble
powder dissolved in water before use; not for aquatic application but may be
used on shoreline grasses/reeds.
Amitrole: similar
restrictions/use to dalapon but broader spectrum
Selection of the
"proper" herbicide(s) (a philosophical problem as well!) requires
correct identification of the problem organism(s) & consideration of all
possible uses of the pond/lake.
Application is usually during
spring growth period before surface mats form (impedes dispersal of herbicide)
and before killed biomass is high enough to be a significant BOD (could cause
fish kills). For high biomass/volume
situations, only part of the lake may be treated.
Application mechanisms range from
handheld garden sprayers or granule spreaders for small ponds to aircraft,
airboats or barges in larger bodies of water.
Moderate evenness required.
Depending on the particular
herbicide, dosage is determined on either a surface area or volume basis, so
both must be known or estimated. For
flowing water (usually irrigation canals & drainage ditches, not natural
streams), application is either gradual over several days (chronic low level
exposure) or massive at one time (acute action).
3. Mechanical Control
Most primitive method involves
hand-pulling, -raking or -cutting.
Obviously limited to small areas and cheap labor! Cutting of cattails below the water level prevents regrowth (anoxia).
Early mechanized means incl.
modified farm machinery and dragging logging chain ("chaining") or
various cutting blades through irrigation canals or ponds.
Modern harvesters variously
utilize a shallow-draft, flat, bargelike paddlewheel boat with adjustable-depth
cutting bars and a conveyor belt to collect the cut weeds on the deck of either
the mowing boat or an accessory transport barge. Recutting frequency varies.
Mechanical removal of weeds also
has the advantage of permanently removing nutrients from the water, esp.
effective in (shallow) lakes w/ high % area cover by macrophytes, as a means to
counter eutrophication. Riemer
Cost-effectiveness varies greatly;
few uses for the removed vegetation have been adopted, so it is generally
placed in landfills.
Dredging (or draining &
bulldozing) not only decreases plants but also deepens the water thus lower PFD
at bottom. Can also be used to reduce
shallow shoreline zones favorable to regrowth.
However, it is very expensive and disposal of sediments is a problem.
Shading with opaque plastic film,
coated fiberglass mesh, or dyes also are sometimes used to decrease
macrophytes, the latter mainly in small, slowly flushed ponds (e.g. Theta pond
at OSU!).
Especially in colder climates,
winter draining (partial or total) for 2-6 months and consequent freezing for
several weeks is often very effective, although winter draining is also partly
effective in controlling Eurasian milfoil (M.
spicatum) even in warm Florida waters.
PROBLEM ALGAL BLOOMS
We have already studied the role of nutrient loading and
vertical mixing/stratification on algal biomass. A "problem bloom"
is simply an unusually high density of algae that is harmful or a nuisance to
humans or the ecosystem. The manner of
nuisance depends on the context, and may include:
Reduced water clarity that is
unsightly to recreational users.
Increased fouling of pumps,
filters, pipes, increased costs of water treatment, etc.
Taste and odor problems in
drinking water supplies.
Fish kills which are unsightly,
foul smelling and presumably harmful to the ecosystem as well as sport or
commercial fisheries.
Possible changes in ecosystem
functioning (e.g. diversity).
Toxicity to humans and other
animals.
In addition to microalgal blooms, multicellular algae often
form nuisance blooms, either attached or floating. In the latter case, the algae normally start out on the
sediments, then rise due to trapped O2 bubbles. The major culprits are primarily green algae
(esp. Spirogyra, Cladophora, Pithophora)
and charophytes, plus mat-forming cyanobacteria (Oscillatoria, Lygbya, Phormidium) and in some cases the xanthophyte
Vaucheria and the rhodophyte Bangia.
Macroalgae play some positive roles in aquatic ecosystems, e.g. they may
be a minor but critical dietary supplement to fish and waterfowl, and normal
densities of Chara have been found to
harbor 1.5-5x the density of inverts as submersed vascular plants. Some negative impacts of macroalgal blooms
include:
Fouling of plumbing.
Negative impact on recreational
use, and danger to swimmers (slippery surfaces).
Competition with phytoplankton,
impacting plankton-based fisheries food chains. However, little is known about positive roles of macrophytes as
fish habitat/shelter.
Reduced waterfront real estate
values.
Reduced flow rate/capacity of
irrigation canals.
Less so than microalgae, possible
contributors to taste/odor problems in potable water.
Possible shelter for flies and mosquitoes,
as well as snails causing schistosomiasis (tropics)
Charophytes produce allelopathic
chemicals inhibiting epiphytes, phytoplankton and zooplankton.
Displacement of vascular macrophytes
by epiphytic algae (e.g. Najas and Elodea by Spirogyra and Cladophora
in England) under eutrophic conditions.
High densities of e.g. Cladophora may decrease invert diversity
and fish spawning. In Lake Erie, mats
of Cladophora averaged 100-400 gDW m-2, and
individual filaments were 30-50 cm.
Algal Management Practices
For both micro- and macroalgae, the most effective and
lasting treatment is reduction of nutrient loading by point and nonpoint
sources. However, this requires
considerable time and political will, so temporary quick fixes are still
common, especially in smaller bodies of water.
In addition to diquat, simazine and endothall presented
under macrophyte control, other approved algicides include copper (sulfate or organic
forms), acrolein and chlorine. Only Cu
and diquat have federally established tolerances for potable water (1 and 0.01
ppm, respectively).
CuSO4 is by far
the most widely used. It is considered
safe for all water uses except in trout habitat, and is highly specific for
algae, particularly cyanobacteria, being largely ineffective for vascular
plants. Many bloom forming macroalgae,
incl. Pithophora, Cladophora, Vaucheria,
Hydrodictyon and Lyngbya, are frequently cited as Cu-tolerant while Spirogyra and Oedogonium are Cu-sensitive.
Tolerance to Cu and other algicides may in some cases be related to mat
thickness, w/ thick mats shielding the lower filaments from exposure.
Mechanical and biological control of algae also is used
sometimes; waterfowl, snails, crayfish, Tilapia,
and especially grass carp all eat algae to some extent.
Algal Toxins
Marine algae are known to produce compounds highly toxic to
vertebrates, such as domoic acid (a neuroexcitatory amino acid causing
amnesic shellfish poisoning (ASP); from diatoms, concentrated in bivalves or
anchovies*), ciguatoxin
(Ciguatera poisoning; from dinoflagellates, bioconcentrated in reef fish) and saxitoxin
(paralytic shellfish poisoning (PSP); from dinoflagellates, esp. Gonyaulax, concentrated in bivalve
mollusks), and others. The freshwater
cyanophyte Aphanizomenon flos-aquae
also has been shown to produce PSP components, though this should not be a
public health concern.
[* Fritz, L. et al. 1992.
An outbreak of domoic acid poisoning attributed to the pennate diatom Pseudonitzschia australis. J. Phycol. 28:439-442.]
Toxic freshwater algae are largely limited to a few species
of cyanobacteria, particularly Microcystis
aeruginosa (microcystin
and cyanoginosin,
monocyclic heptapeptide hepatotoxins),
Anabaena flos-aquae (anatoxin, an alkaloid neurotoxin, and microcystin), and Aphanizomenon flos-aquae (aphantoxin, an alkaloid neurotoxin). Codd
& Poon Table 15.1
All of these are primarily a problem for livestock and
pets. A local veterinarian reported a
cattle kill in early summer 1993 in the north Texas area, attributed to Anabaena. For more information:
Codd, G.A.
& G.K. Poon. 1988. Cyanobacterial
toxins. In: Rogers, L.J. & J.R.
Gallon [Eds.] Biochemistry of the Algae and Cyanobacteria. Clarendon Press, Oxford, pp. 283-296. 589.31 B516
Gorham,
P.R., & W.R. Carmichael. 1988.
Hazards of freshwater blue-green (cyanobacteria). In: Lembi, C.A. & J.R. Waaland
[Eds.] Algae and Human Affairs.
Cambridge Univ. Press, pp. 403-431. 589.3
A3935
World
Health Org. 1984. Aquatic (Marine and Freshwater) Biotoxins. 589.460469 A656.
Cyanobacterial toxicity episodes are unpredictable in
occurrence and duration, although they tend to occur in warmer months during
stratification, and certainly are favored by eutrophication. Potentially toxic blooms probably occur much
more frequently than reported livestock kills, based on a survey of mouse LD50 values
for British cyanobacterial blooms. It
may be necessary for the cells to be concentrated at the surface along the
shore for an effective oral dose in livestock.
Moreover, toxin content per unit cyanobacterial biomass varies widely
both spatially (few meters) and temporally within a bloom. Other animals may also be affected by
bioaccumulation through the food chain.
At present there is no rapid field test for toxicity, only lab mouse
toxicity assays.
Phycotoxins are almost certainly toxic to humans, although
we do not directly or indirectly (via consumer organisms) ingest algae. However, microcystin (and possibly other
toxins) is not completely destroyed by normal potable water treatment, and it
may be released from CuSO4-treated cells, so activated
carbon and sand filtration may be required prior to CuSO4-treatment
of infested reservoirs. Small epidemics
of gastroenteritis have been ascribed tentatively to dissolved
phycotoxins. Contact dermatitis and
hay-fever-like symptoms also have been linked to Anabaena and other cyanobacteria.
ALGAE IN WASTEWATER TREATMENT
[The following is based largely on: Oswald, W.J. 1988. The role of microalgae in liquid waste treatment and
reclamation. In: Lembi, C.A. & J.R.
Waaland [Eds.] Algae and Human Affairs.
Cambridge Univ. Press, pp. 403-431. 589.3
A3935]
The average American uses >100
gallons (380 L) of potable water daily, most of which becomes wastewater.
Industry water discharge ranges
from 10 to 100x the weight of manufactured product.
Waste treatment methods vary
widely, but are physical, chemical, biological or combinations of all
three. Oswald Table 11-1
Newer design algal-bacterial systems
can equal or exceed conventional wastewater treatment at lower energy input and
~ 5x lower cost. This involves, as combined 2° and 3° treatment, photosynthetic oxidation of organic material
in continuous flow high-rate ponds,
which removes >90% of the C-BOD and up to 80% of the N & P in a few
days. This compares to 85-90% C-BOD but
a much smaller % of N/P in 5-8 hours with traditional activated sludge or biofiltration
2° treatment techniques, which then require complex & expensive chemical 3°
treatment to remove N/P.
Additional benefits of the
high-rate pond system include pH > 9 (due to Ps) that kills coliform
bacteria; removal of heavy metals by adsorption and high pH precipitation;
precip. of CaPO4 and Ca-/MgCO3 at high
pH. Oswald Fig. 11-1, Table 11-2
Increasingly stringent EPA
regulations for discharged wastewater means soaring treatment costs using
traditional methods, so the high-rate pond system is a feasible alternative.
Rate limitation is controlled by
the mutually dependent cycling of CO2 from
decomposers and O2 from photosynthesis, and by the
amount of sunlight, a function of pond depth.
Economic, efficient and
sustainable algal productivity in high-rate ponds is on the order of 15-20 g DW
m-2 d-1,
comparable on an annual basis to highly productive natural ecosystems. Pilot plants must be established at each
site to customize the design for local climate.
A possible expensive problem is
algal harvesting. Pond mixing is
required both for efficiency and to promote growth of forms that tend to settle
out/flocculate for easy harvest when mixing is suddenly stopped. Nonsettling genera such as Chlorella, Euglena,Chlamydomonas, and Oscillatoria are undesirable.
Almost all well-mixed ponds usually dominated by large, relatively
fast-growing, spined, poorly grazed spp. of Scenedesmus
and/or Micractinium.
Future possible applications
include:
The capture of methane for energy
generation from the 1° pond and from fermentation of the algal harvest.
Removal of toxins such as selenium
from contaminated irrigation and industrial effluent.
Nutrient-stripping and rinsewater
recycling in integrated feedlot systems, which cuts nutrient effluent to
watersheds and doubles the efficiency of N conversion to food products (meat,
milk, eggs); although feasible, has not been implemented to date because of the
sustained depressed farm economy.
[The following is largely based on/plagiarized from: Reddy, K.R. & T.A. DeBusk. 1987. State-of-the-art
utilization of aquatic plants in water pollution control. Wat. Sci. Tech. 19(10):61-79.]
In addition to high-rate oxidation ponds that utilize algae,
vascular hydrophytes are also promising for wastewater and even polluted
natural water purification. Pilot and
some operational municipal macrophyte-based systems demonstrate exceptional
nutrient and toxin removal efficiencies at a fraction of the initial
cost of traditional activated sludge or biofiltration systems, despite little
research on optimization to date. While
not popular with profit-minded engineers, this may be an affordable way for
industries/municipalities to meet increasingly stringent EPA effluent
standards.
A limiting factor in
implementation on a large scale has been the lack of practical uses for the harvested
macrophyte biomass.
Another limitation is the greater
land area required for macrophyte-based systems.
In general, systems consist of
either natural wetlands, or mono- or polyculture (latter should be more stable
to perturbations such as temperature extremes) in shallow ponds or raceways,
with a long residence time (Tr) relative to traditional wastewater systems to enhance:
solids settling
plant uptake of contaminants
bio- and physico-chemical
transformations
System design depends on climate,
influent characteristics, effluent quality requirements, e.g.:
To remove suspended solids, BOD
and N, systems may involve macrophytes mainly as high surface area substrates
for microbial activity.
For removal of P, metals and some
organics, systems must optimize uptake by the plants.
The biological dynamics of such
systems are poorly understood, with most research to date involving "black
box" trial and error optimization of inflow/outflow characteristics.
The criteria for optimization
should include the plants':
productivity and growth rate
adaptability to local climate and
tolerance to climatic extremes
high O2 transport
capacity (shoots to roots): increases aerobic bacterial/fungal decomposition,
thus more quickly depleting organic BOD Reddy
& DeBusk Table 6
tolerance to high concentrations
of pollutants
pollutant assimilative capacity;
this incl. both short-term storage, i.e. the amount present in the
plants at any given time (plants must be harvested regularly to avoid decomposition
in situ; for emergents, much of the
biomass and pollutant reserves may be in the rhizomes/ roots, thus
unharvestable), plus uptake rate Reddy & DeBusk Tables 1, 2, 7
for micronutrient/toxin removal
(e.g. metals), it is often necessary to provide excess macronutrients (esp. N)
to ensure efficient uptake
resistance to pests and disease
ease of management
Species most suitable include:
floating plants such as Eichhornia crassipes (water hyacinth, w/
a potential yield of ~ 200 dry metric tons·ha-1·y-1), esp. in
(sub)tropical locations; Lemna spp.
(duckweeds) and Hydrocotyle
(pennyworts) in temperate areas
emergents such as Phragmites (reeds) and Typha (cattails) in temperate areas
submerged plants such as Elodea
Types of systems that are being
studied include:
floating aquatic macrophyte
systems (FAMS): 1° Eichhornia in warm
climates
artificial wetland treatments
(AWT): broader temperature range than FAMS
1.
root zone method (RZM): growth of emergents in sediment
2.
gravel bed treatment (GBT): growth of emergents in gravel
3.
nutrient film technique (NFT): plants grown in shallow layer of wastewater, which
stimulates a dense root mat that efficiently removes BOD and suspended solids;
high potential in colder climates
Operational and maintenance costs
are unavailable for macrophyte systems, but construction costs are 2-8x lower
than conventional systems. Any of these
systems could be integrated with existing conventional systems to enhance
capabilities at reasonable cost. Reddy & DeBusk Fig. 2, Table 9
Much research is necessary to optimize
systems for any given application; in additon to the plant selection criteria
above:
1.
system optimization:
hydrology/hydraulic loading
effects
pond size, shape, aspect ratio
water depth (FAMS)
sediment characteristics
hydraulic properties of
soil/gravel in AWT
wastewater characteristics and
effects