Aquatic plant management refers to controlling nuisance species, to maximizing the beneficialaspects of plants in water bodies, and to restructuring plant communities.. This chapter disc
Trang 111 Macrophyte Ecology and Lake
Management
11.1 INTRODUCTION
“Macrophyte” refers to all macroscopic aquatic vegetation (vs microscopic plants like
phytoplank-ton), including macroalgae such as the stoneworts Chara and Nitella; aquatic liverworts, mosses,
and ferns; as well as flowering vascular plants Understanding aquatic plant biology is important
to the immediate problems of managing aquatic plants and aquatic ecosystems A thorough edge of macrophyte biology makes the development of new management techniques, the efficacy
knowl-of present techniques, and the assessment knowl-of environmental impacts more efficient Understandingmacrophyte biology also makes management results more predictable, especially when considered
in a long-term ecosystem context
Aquatic plant management refers to controlling nuisance species, to maximizing the beneficialaspects of plants in water bodies, and to restructuring plant communities As a natural part of thelittoral zone and of the entire lake, producing stable, diverse, aquatic plant communities containinghigh percentages of desirable species is a primary management goal
A single chapter cannot review all macrophyte biology that might be relevant to management.Potential topics range from subcellular biology as it relates to genetic engineering; to the physiology
of resource gain, allocation, and transport; and to plant relationships with their habitat and otherorganisms in the ecosystem This chapter discusses aquatic plant biology as it relates to otherchapters in this book; that is, types of aquatic plants, nutrient relationships, reproduction, phenology,the physiology of growth, and community and environmental relationships It briefly discusses theimportance of planning for aquatic plant management For more detailed information on topicsrelating to aquatic plant biology refer to Hutchinson (1975), Sculthorpe (1985), Barko et al (1986),Pieterse and Murphy (1990), Wetzel (1990, 2001), Adams and Sand-Jensen (1991), Hoyer andCanfield (1997), Jeppesen et al (1998), and the references contained within these publications.Two excellent resources for retrieving aquatic plant information, either “on-line” or by traditionalmethods are the Aquatic, Wetland, and Invasive Plant Information Retrieval System (APIRS) atCenter for Aquatic and Invasive Plants, University of Florida (http://plants.ifas.ufl.edu/) and theU.S Army Corps of Engineers, Aquatic Plant Control Research Program (www.wes.army.mil/el/aqua/)
Trang 2appropriateness; and public acceptability of all management options are considered and compared;(5) results are monitored to evaluate the effectiveness of management and to detect impacts to thelake ecosystem; and (6) a strong educational component keeps team members, opinion leaders,lake users, governmental officials, and others in the general public well informed When comparingcontrol techniques, a method should be discarded if it does not work or if it causes unacceptableenvironmental harm It may be discarded if it is more expensive than other suitable techniques.Aquatic plant management plans need not be complex and there is a variety of good advice onhow to develop a management plan (Mitchell, 1979; Nichols et al., 1988; Washinton Department
of Ecology, 1994; Hoyer and Canfield, 1997; Korth et al., 1997) Computer technology helpsdevelop and evaluate more complex aquatic plant management plans (Grodowitz et al., 2001a).Assessing the situation and evaluating and monitoring management practices are key compo-nents of an aquatic plant management strategy where aquatic plant sampling is needed Samplingschemes are many and a sampling method should be designed to answer specific managementquestions A number of references are available to help design a sampling program for assessment,evaluation, and monitoring (Dennis and Isom, 1984; NALMS, 1993; Clesceri et al., 1998)
The Wisconsin Department of Natural Resources gives grants for lake management planning scale lake planning grants of up to $3,000 are available for obtaining and disseminating basic lakeinformation, conducting education projects, and developing management goals Large-scale lakeplanning grants up to $10,000 per project are available for bigger projects that conduct technicalstudies for developing elements of, or completing comprehensive management plans In addition
Small-to monies supplied by the state, the grantee must supply 25% of the cost as cash or in-kind services.The grants are funded by a motorboat fuel tax
The White River Lake Management District, with the aid of a consultant, used lake planninggrant money to prepare an aquatic plant management plan in the year 2000 (Aron & Associates,2000) White River Lake has a surface area of 25.9 ha, a maximum depth of 8.8 m, and is located
in central Wisconsin The White River Lake Management District was created approximately 20years ago in response to growing water quality concerns The district acquired an aquatic plant
harvester approximately 15 years ago to control Chara sp They are also concerned about the invasions of the exotic species Eurasian watermilfoil (Myriophyllum spicatum) and curly-leaf pond- weed (Potamogeton crispus) The district desires to: (1) preserve native plants, (2) protect sensitive
areas, (3) control exotic and nuisance plants, (4) provide improved navigation, and (5) educatedistrict members on the value of aquatic plants and the threats to a balanced plant population TheTable of Contents (Table 11.1) shows the topics considered in the plan including goals and objectives,background and problem definition, and plant management alternatives From this and samplinginformation a plant management plan was developed that included a strong educational component.Macrophytes were sampled along 15 transects placed at approximately equal intervals aroundthe lake (Figure 11.1) Sampling points were randomly selected at approximately 0.5, 1.5, 3, and
4 m depths along each transect At each sampling location, the species present were noted and thedensity of each species was estimated on a 1–5 basis, with 5 representing the heaviest growth The
survey showed that Chara sp was dominant (Table 11.2) and that Eurasian watermilfoil occurred
in the lake Water star grass (Zosterella dubia), white water lily (Nymphaea sp.), and curly-leaf
pondweed were found in the lake but not at the sampling locations
The aquatic plant management plan recommendations are as follows (Aron & Associates, 2000):
RECOMMENDATIONS
White River Lake continues to have an excellent aquatic plant community with a wide range of diversity.Eurasian watermilfoil was only found in isolated patches Management efforts should be directed toward
Trang 3protection and maintenance of the resource with a focus on controlling Eurasian watermilfoil Small patches of Eurasian watermilfoil should be eradicated using hand-raking, pulling, or chemical treatment Additionally, signs should be placed at all access locations that describe this species and ask boaters
to remove all plant material from their boats and trailers prior to and after using White River Lake
OTHER RECOMMENDATIONS
Education and Information
The District should take steps to educate property owners regarding their activities and how they may affect the plant community in White River Lake Informational material should be distributed regularly
to residents, landowners, and lake users and local government officials A newsletter, biannually or quarterly, distributed to landowners and residents should be part of the plant management budget Topics
TABLE 11.1
Table of Contents for the White River Lake Aquatic Plant
Management Plan
Chapter I 2
Introduction 2
Goals & Objectives 2
Chapter II — Background 3
Shoreline Development 3
Recreational Uses 3
Value of Aquatic Plants 5
Current Conditions 12
Sensitive Areas 13
Fish and Wildlife 14
Chapter III — Problems 15
Chapter IV — Historical Plant Management 16
Chapter V — Plant Management Alternatives 17
Drawdown 17
Nutrient Inactivation 17
Dredging for Aquatic Plant Control 18
Aeration 18
Screens 18
Chemical Treatment 19
Native Species Reintroduction 21
Harvesting 21
Hand Controls 22
Biomanipulation 23
Chapter VI — Plant Management Plan 24
Recommendations 24
Other Recommendations 24
Education and Information 24
Chemical Treatment 24
Riparian Controls 24
Harvesting 25
Plan Reassessment 26
Finding of Feasibility 26
Chapter VII — Summary 27
Source: From Aron & Associates 2000 White River Lake — Aquatic Plant
Management Plan Unpublished report Wind Lake, WI With permission.
Trang 4should include information relating to lake use impacts, importance and value of aquatic plants, landuse impacts, etc Other issues that should be addressed may include landscape practices, fertilizer use,and erosion control Existing materials are available through the Wisconsin Department of NaturalResources (WDNR) and the University of Wisconsin Extension (UWEX) Other materials should bedeveloped as needed The District should also enlist the participation of the local schools The schoolscould use White River Lake as the base for their environmental education programs Regular commu-nications with residents will improve their understanding of the lake ecosystem and should lead to long-term protection.
FIGURE 11.1 Sampling transect locations in White River Lake, Wisconsin (From Aron & Associates 2000.
White River Lake — Aquatic Plant Management Plan Unpublished report Wind Lake, WI With permission.)
5
4
3 2
12 13
14
15 dam
8
z
Trang 5infestation of the areas by Eurasian watermilfoil and curly-leaf pondweed The native plants will alsohelp stabilize the sediments and minimize shoreline erosion.
Harvesting
The District may continue to harvest as needed to control the nuisances The equipment should bemaintained regularly Operators should be trained in aquatic plant identification to help protect nativenon-target plants
Plant management should be avoided in areas with species of special interest such as wild celery.Operators need to make sure that cutter bars and paddle wheels are kept out of the sediments or to cutone foot above the plant beds when possible
Operators should operate equipment at speeds only sufficient to harvest the plant material Excessivespeeds will increase the inefficiency of the harvester, causing plants to lay over rather than be cut, and
it will increase the numbers of fish trapped
Operators should work to aggressively control the number of “floaters” and if they do occur, should
be removed immediately Equipment should be operated so that cut plant material does not fall off theharvester
Plan Reassessment
The District should review or contract to review, the plant populations of White River Lake every 3–5years Eurasian watermilfoil removal efforts should be reviewed for effectiveness The managementplan should also be reviewed, and if necessary modified, every 3–5 years This will be especiallyimportant to determine the continued health of the aquatic plant population
TABLE 11.2
Aquatic Vegetation of White River Lake, Wisconsin for 2000
Species Frequency (%) Relative Frequency (%) Average Density a
a Average density of species rated on a 1–5 basis in sampling units where the species occurred.
Source: From Aron & Associates 2000 White River Lake — Aquatic Plant Management Plan.
Unpublished report Wind Lake, WI With permission.
Trang 6Finding of Feasibility
The harvesting program is necessary to maintain minimal recreational access to White River Lake It
is necessary to maintain a stable clear-water condition for the lake
The District has shown the ability to maintain and operate an effective harvesting program The Districtharvests approximately 50% (30 acres) of White River Lake Approximately 60 acres (94%) of the lakeare available for aquatic plant growth
In this plan the problem was defined, there was an assessment made of the underlying problem,management options were considered, and there is a strong educational component There arerecommendations for periodic monitoring of the plant community in the future Additional recom-mendations could include some periodic testing, even simple Secchi depth readings that monitorwater quality, to determine if habitat conditions in the lake are changing, or if plant managementmight be causing some unforeseen circumstance
11.3 SPECIES AND LIFE-FORM CONSIDERATIONS
Control tactics are often species-specific When devising a management plan it is important toknow each species’ identity, location, and abundance Each species has unique physiological,habitat, and ecological requirements The more known about the species of interest, the moresuccessful management will be The first step is identifying species Refer to Cleseri et al (1998)
to find taxonomic keys that are regionally appropriate There are computer programs that helpidentify aquatic plants (Grodowitz et al., 2001a, b) and The Center for Aquatic and Invasive Plants’website is an excellent place to find species-specific information, lists of taxonomic keys, and “on-line” help identifying plants
Depending on the definition of “aquatic” and “weed,” fewer than 20 of approximately 700aquatic species are major weeds (Spencer and Bowes, 1990) Because of their prolific growth andreproduction, they often interfere with utilization of fresh waters and may displace indigenousvegetation Much macrophyte research has been stimulated by the need to control nuisance plants
so there is a wealth of information about a limited number of species
Aquatic plants form four distinct groups based on life form: (1) submergent, (2) free-floating,(3) floating-leaved, and (4) emergent, that differ in habitat, structure and morphology, and themeans they obtain resources Plants in the same life-form group often have similar adaptations totheir environment By grouping species according to life-form, species that are well known may
be used as models for species that are less well known but have similar life-forms
Emergent macrophytes such as reeds (Phragmites spp.), bulrushes (Scirpus spp.), cattails (Typha spp.) and spikerushes (Eleocharis spp.) are rooted in the bottom, have their basal portion submersed
in water, and have their tops elevated into air This is ideal for plant growth Nutrients are availablefrom the sediment, water is available from the sediment and overlying water, atmospheric carbondioxide and sunlight are available to emergent portions of the plant
Floating-leaved macrophytes, such as waterlilies (Nymphaea spp.), spatterdock (Nuphar spp.), and watershield (Brasenia sp.), are rooted in the bottom with leaves that float on the water surface.
Floating leaves live in two different habitats, water on the bottom, air on top A thick, waxy coatingprotects the upper leaf surface from the aerial environment Floating leaves do not have the structuralsupport of emergents so they can be ravaged by wind and waves Floating-leaved species are usuallyfound in protected areas
Submergent species include such varied groups as quillworts (Isoetes spp.), mosses (Fontinalis spp.), stoneworts, and numerous vascular plants like the many pondweeds (Potamogeton spp.), wild celery (Vallisneria americana.), and watermilfoils (Myriophyllum spp.) They face special problems
obtaining light for photosynthesis and they must obtain carbon dioxide from the water where it is
Trang 7much less available than it is in air They invest little energy in structural support because they aresupported by water and water accounts for about 95% of their weight.
Free-floating macrophytes float on or just under the water surface Their roots are in water,
not in sediment Small free-floating plants include duckweeds (Lemna spp.), mosquito fern (Azolla caroliniana), and water fern (Salvinia sp.) Water hyacinth (Eichornia crassipes), and frog’s bit (Limnobium spongia) are examples of larger free-floating plants They depend on the water for
nutrients and their leaves have many characteristics of floating-leaved species Their location is atthe whims of wind, waves, and current so they are usually found in quiet embayments
11.4 AQUATIC PLANT GROWTH AND PRODUCTIVITY
The aquatic habitat moderates extremes of temperature and water stress that commonly limitsterrestrial plant productivity Water, however, exerts a high resistance to solute diffusion andselectively attenuates the quality and quantity of light, which can limit aquatic productivity Species
of a similar life-form, although taxonomically diverse, encounter the same habitat limitations Somespecies have traits that allow them to exploit conditions in an opportunistic and competitive manner.These species are more productive and thus more likely to become aquatic nuisances
11.4.1 LIGHT
The quality and quantity of light in aquatic systems have important influences on the growth anddevelopment of submergent species The quality and quantity of light depend upon dissolvedmaterials and suspended particulate matter in the water, and upon water depth Light becomes morelimited and the quality changes with increasing depth and with turbidity from algae, silt, andresuspended bottom sediments Zonation of macrophytes along depth gradients can be caused bythe light regime (Spence, 1967) and increased turbidity can decrease the maximum depth of plantgrowth (Spence, 1967; Nichols, 1992) Light may also play an important role in seasonal changes
in macrophyte dominance and interspecific competition
Emergent, free-floating, and floating-leaved plants grow in atmospheric sunlight They are sunplants Each leaf can potentially utilize all the solar energy it receives for growth (Spencer andBowes, 1990) Their productivity, at least for emergents, is similar or even greater than terrestrialsun plants
Submergent species are shade plants Leaf photosynthesis is saturated by a fraction of fullsunlight The light compensation point (i.e., where the photosynthetic rate equals the respirationrate) for some species is as low as 0.5 percent of full sun (Spencer and Bowes, 1990) Some ofthe most important nuisances have the lowest compensation points This may give them a slightbut decided advantage over other species for accumulating energy resources
Light generally limits the lakeward edge of the littoral zone and there is evidence that increasedturbidity decreases maximum plant biomass (Robel, 1961) Clear water lakes usually have deeperlittoral zones Nichols (1992) found a 1.2–7.8 m range of maximum plant growth depths for a suite
of Wisconsin lakes This depth range is similar to those reported by Hutchinson (1975), is broaderthan the 1.0–4.5 m range reported by Lind (1976) for eutrophic lakes in southeastern Minnesota,and is more shallow than the 12 m maximum depth for Lake George, New York (Sheldon andBoylen, 1977) and the 11 m for Long Lake, Minnesota (Schmid 1965) All these depths are
considerably more shallow than the 18 m maximum depth for Utricularia geminiscapa in Silver
Lake, New York (Singer et al., 1983), the 20 m maximum depth for bryophytes in Crystal Lake,Wisconsin (Fassett, 1930), and the approximately 150 m maximum depth for charophytes andbryophytes in Lake Tahoe, California (Frantz and Cordone, 1967) Even shallow lakes, if they areturbid enough, will have sparse aquatic plant growth (Engel and Nichols, 1994; Nichols and Rogers,1997)
Trang 8Hutchinson (1975), Dunst (1982), Canfield et al (1985), Chambers and Kalff (1985), Duarteand Kalff (1990), and Nichols (1992) found a significant regression between Secchi depth and themaximum depth of plant growth (Table 11.3) In many cases these regressions are similar (Duarteand Kalff, 1987) and are used as models to predict the maximum depth of plant growth formanagement such as dredging depth to eliminate plant growth (see Chapter 20).
Light also affects a number of morphogenetic processes in submerged aquatic plants includingthe germination of fruits, anthocyanin production in stems and leaves, the positioning of chloro-plasts, leaf area, branching, and stem elongation (Spence, 1975) The most important for manage-
ment purposes may be stem elongation For some of the worst nuisance species like Hydrilla verticillata, Egeria densa, and M spicatum, low light stimulates substantial increases in shoot
length (Spencer and Bowes, 1990) These species quickly form a surface canopy so they are nolonger light limited, they can shade out slower growing competitors, and they greatly restrict wateruse by forming a tangled mass of stems and leaves on the water surface
11.4.2 NUTRIENTS
Submergent macrophytes use both aqueous and sedimentary nutrient sources, and sites of uptake(roots vs shoot) are related at least in part to nutrient availability in sediment versus the overlyingwater In other words, submergent plants operate like good opportunistic species should operate;they take nutrients from the most available source
Rooted macrophytes usually fulfill their phosphorus (P) and nitrogen (N) requirements directlyfrom sediments (Barko et al., 1986) The role of sediment as a source of P and N for submergentmacrophytes is ecologically significant because available forms of these elements are normally low
in the open water during the growing season This is important knowledge because there is acommon misconception that excessive external nutrient loading directly to the water column causesmacrophyte problems External nutrient loading usually produces algal blooms, shading and reduc-ing macrophyte biomass The availability of micronutrients in open water is usually very low, butrelatively available in sediments However, the preferred source of potassium (K), calcium (Ca),magnesium (Mg), sulfate (SO4), and chloride (Cl) appears to be the open water (Barko et al 1986).Free-floating species obtain their nutrients from the water column and may compete directly withalgae for available nutrients
There are few substantiated reports of nutrient related growth limitation for aquatic plants(Barko et al., 1986) Nutrients supplied from sediments, combined with those in solution aregenerally adequate to meet nutritional demands of rooted aquatic plants, even in oligotrophicsystems There are exceptions to this statement so there is not a clear consensus on the relationship
of nutrient supplies to plant productivity under natural conditions In Lake Memphremagog bec-Vermont border), Duarte and Kalff (1988) demonstrated that biomass increases averaged 2.1
MD 0.5 = 1.51 + 0.53 ln SD Wide variety Duarte and Kalf, 1987
MD = 0.61 log SD + 0.26 Finland; Florida; Wisconsin, Canfield et al., 1985
MD = 2.12 + 0.62 SD Wisconsin Nichols, 1992
MD 0.5 = 1.33 log SD + 1.40 Quebec and the World Chambers and Kalf, 1985
Note: MD = maximum depth of plant growth in meters; SD = Secchi depth in meters.
Trang 9times greater for fertilized plants (fertilized with 3:1:1, N to P to K ratio) than paired controls Thebiomass increase was greatest in shallow water (1 m depth) and with perennial plants In Lawrence
Lake, Michigan Scirpus subterminalis and Potamogeton illinoensis biomasses increased with
nitro-gen and phosphorus fertilization (Moeller et al., 1998) Nutrient limitation also reduced productivity
in plants such as wild rice (Zizania spp.) that annually produce high biomasses (Dore, 1969; Carson,
2001) There is evidence that nitrogen needs to be replenished to sustain annual macrophyte growth
in infertile sediments (Rogers et al., 1995) The nitrogen can be supplied by non-point sources such
as sedimentation from shoreline erosion and silt loading, or from lawn fertilization Multiple nutrientdeficiencies appear to diminish growth on extremely low density and extremely high density (usuallymeaning highly organic or highly sandy) substrates (Barko and Smart, 1986) Plant tissue analysissuggested to Gerloff (1973) that the elements most likely to limit macrophyte growth differed bylake and that nitrogen, phosphorus, calcium and copper were growth limiting or close to growthlimiting in different Wisconsin lakes When available, plants take up nutrients well above theirphysiological needs (e.g., luxury consumption), which confounds the analysis of the direct rela-tionship between nutrients and growth (Gerloff, 1973; Moeller et al., 1998)
Attempts to control plant growth by limiting sediment nutrients through dredging or coveringnutrient rich sediments, or chemically making nutrients unavailable with alum have been unsuc-cessful (Engel and Nichols, 1984; Messner and Narf, 1987) Attempts to control macrophytes bycontrolling nutrients in the water column are counter-productive Phytoplankton obtain their nutri-ents exclusively from the water column so the first response to nutrient limitation (primarily P) isimproved water clarity that improves macrophyte growth
Although this information suggests that nutrients do not limit aquatic plant growth, oligotrophiclakes generally maintain less total plant biomass and usually contain different species than morenutrient rich lakes Many species found in oligotrophic lakes have the ability to seasonally conserveboth biomass and nutrients
Dissolved inorganic carbon (DIC) most likely limits submergent macrophyte photosynthesis (Barko
et al., 1986; Spencer and Bowes, 1990) Photosynthesis in terrestrial plants is limited by CO2
transport and it is even more critical in submersed species Carbon dioxide diffusionis much slower
in water than in air Free CO2 is the most readily used carbon form for photosynthesis Some speciescan utilize bicarbonate, but they do so less efficiently and they expend more energy doing so Theability to use bicarbonate has adaptive significance in many fresh water systems because the largest
fraction of inorganic carbon may exist as bicarbonate Eurasian watermilfoil (M spicatum), a
notorious nuisance species, has a substantial capacity to use bicarbonate for photosynthesis Theratio of CO2 to bicarbonate to carbonate is determined by the alkalinity and pH of the water, and
by CO2 uptake by plants
In dense plant beds free CO2 and bicarbonate can be depleted in a few hours of photosynthesis.This shifts the carbon equilibrium toward carbonates that are not used for photosynthesis andincreases O2 concentration and pH These water conditions cause O2 inhibition of photosynthesisand photorespiratory CO2 loss (Spencer and Bowes, 1990) All three conditions lower net photo-synthesis In addition to the utilization of bicarbonate, submergent macrophytes have a number ofanatomical, morphological, and physiological mechanisms to enhance carbon gain (Spencer andBowes, 1990; Wetzel, 1990)
Emergent, free-floating, and floating-leaved plants use atmospheric CO2 so photosynthesis isnot hampered by the slow diffusion rates of gases in water In addition, lack of water stress allowstheir stomata to remain open so photosynthesis proceeds unhindered during daylight hours.Oxygen concentrations determine redox conditions and thus nutrient release from sediments.The underground biomass of rooted species may be living in an anaerobic environment Lack ofoxygen hinders nutrient acquisition Some species, especially emergents, produce aerenchyma that
Trang 10allows oxygen diffusion from the aerial environment to submerged organs (Wetzel, 1990) Evendead stems are capable of conducting oxygen to rhizomes (Linde et al., 1976) Cutting off emergentplant stems (including dead stems) so they remain below the water surface, thus depriving rhizomesand roots of oxygen for a long period of time is an effective technique for controlling cattails(Beule, 1979) and possibly other emergent species
Increased levels of a single nutrient are likely to increase plant growth only to the point whereanother nutrient becomes growth limiting Smart (1990) described laboratory experiments where
a reciprocal relationship was found between inorganic C supply and sediment N availability Highlevels of both factors stimulated plant growth, increasing the demand on the other factor until one
of them limited growth High levels of aquatic plant production required both an abundance ofinorganic C and high sediment N availability (see section above on nutrients)
Substrates provide an anchoring point for rooted plants and, as explained above, are the nutrientsource for critical nutrients like N and P Some sediments (e.g., rocks or cobble) are so hard thatplant roots cannot penetrate them; others are so soft, flocculent, and unstable that plants cannotanchor in them Coarse textured sediments can be nutritionally poor for macrophyte growth Smallaccumulations of organic matter stimulate plant growth on these sediments
Low sediment oxygen concentrations, or high concentrations of soluble reduced iron andmanganese or soluble sulfides, can be toxic to plants High soluble iron concentrations interferewith sulfur metabolism Sediments containing excessive organic matter may contain high concen-trations of organic acids, methane, ethylene, phenols, and alcohols that can be toxic to vegetation(Barko et al., 1986)
The above conditions are most common in eutrophic lakes To some degree, aquatic plantsprotect themselves from these toxins with oxygen release from their roots This eliminates theanaerobic conditions that create toxic substances in the rhizophere surrounding the root
Also, as explained above, sediment density has important impacts on nutrient acquisition byplants Consolidating flocculent sediments using drawdown is one method of improving the habitatfor aquatic plant restoration (see Chapter 12)
11.4.5 TEMPERATURE
Water buffers temperature extremes for plant growth but submerged plants can be exposed totemperature extremes from near zero to as much as 40 C (Spencer and Bowes, 1990) Somesubmerged plants can grow at temperatures as low as 2°C (Boylen and Sheldon, 1976) and it is
not unusual to find some species in a green condition living under ice cover Weed problems aregenerally most severe in the 20–35°C range
Water temperature interacts with light to affect plant growth, morphology, photosynthesis,respiration, chlorophyll composition, and reproduction (Barko et al., 1986) High temperatures,within the thermal tolerance range, promote greater chlorophyll concentration and productivity,with a concomitant increase in both shoot length and shoot number Increasing temperature andlight appear to cause opposing response in shoot length (Barko et al., 1986) Different metabolicprocesses show differing responses to temperature so growth represents an integration of temper-ature responses In thermally stratified lakes, depth related temperature decreases could reduce thelength of the growing season if plant growth reaches the thermocline or below (Moeller, 1980).Eurasian watermilfoil and curly-leaf pondweed, two aquatic nuisances, are examples of coolwater strategists Although optimum photosynthetic temperatures for both species appear to bebetween 30 and 35°C, which is high when compared to terrestrial plants and suggests a preference
for warm climates, their photosynthetic rate at low temperatures is a higher percentage of theirmaximum rate and higher than some other species (Nichols and Shaw, 1986) For milfoil, the