11 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 knowl- edge of macrophyte biology makes the development of new management techniques, the efficacy of present techniques, and the assessment of environmental impacts more efficient. Understanding macrophyte 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 beneficial aspects of plants in water bodies, and to restructuring plant communities. As a natural part of the littoral zone and of the entire lake, producing stable, diverse, aquatic plant communities containing high 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 other organisms in the ecosystem. This chapter discusses aquatic plant biology as it relates to other chapters 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 the importance of planning for aquatic plant management. For more detailed information on topics relating 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 and Canfield (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 traditional methods are the Aquatic, Wetland, and Invasive Plant Information Retrieval System (APIRS) at Center for Aquatic and Invasive Plants, University of Florida (http://plants.ifas.ufl.edu/) and the U.S. Army Corps of Engineers, Aquatic Plant Control Research Program (www.wes.army.mil/el/aqua/) at Vicksburg, Mississippi. 11.2 PLANNING AND MONITORING FOR AQUATIC PLANT MANAGEMENT Without a plan, aquatic plant management is haphazard. Objectives remain undefined, leaving no way to gauge progress. Ineffective treatments are discarded without knowing why they failed. In short, the same failures are repeated every year. A successful aquatic plant management plan uses basic planning principles: (1) the problem is defined; (2) an assessment discovers the underlying cause of the problem; (3) plant ecology and the plant community relationships form the scientific basis for the plan; (4) the efficacy; cost; health, safety, and environmental impacts; regulatory Copyright © 2005 by Taylor & Francis appropriateness; 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 the lake 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 comparing control techniques, a method should be discarded if it does not work or if it causes unacceptable environmental 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 on how 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 helps develop 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. Sampling schemes are many and a sampling method should be designed to answer specific management questions. 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). 11.2.1 CASE STUDY: WHITE RIVER LAKE AQUATIC PLANT MANAGEMENT PLAN The Wisconsin Department of Natural Resources gives grants for lake management planning. Small- scale lake planning grants of up to $3,000 are available for obtaining and disseminating basic lake information, conducting education projects, and developing management goals. Large-scale lake planning grants up to $10,000 per project are available for bigger projects that conduct technical studies for developing elements of, or completing comprehensive management plans. In addition 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 planning grant 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 20 years 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) educate district members on the value of aquatic plants and the threats to a balanced plant population. The Table 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 sampling information a plant management plan was developed that included a strong educational component. Macrophytes were sampled along 15 transects placed at approximately equal intervals around the 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 the density 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 Copyright © 2005 by Taylor & Francis protection 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. Copyright © 2005 by Taylor & Francis should include information relating to lake use impacts, importance and value of aquatic plants, land use 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 Natural Resources (WDNR) and the University of Wisconsin Extension (UWEX). Other materials should be developed as needed. The District should also enlist the participation of the local schools. The schools could 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. Chemical Treatment If there is local public acceptance, the District may continue selective chemical treatment to control Eurasian watermilfoil. If conducted, a WDNR permit must be obtained and selective herbicides should be used to protect native aquatic plant species. Riparian Controls Riparians should be encouraged to use the least intensive method to remove nuisance vegetation. This could include minimal raking and pulling. If screens are considered by individuals, a WDNR permit will be required. Riparians should be encouraged to allow native plants to remain. This will help prevent 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 1 6 7 9 10 11 12 13 14 15 dam 8 z Copyright © 2005 by Taylor & Francis infestation of the areas by Eurasian watermilfoil and curly-leaf pondweed. The native plants will also help stabilize the sediments and minimize shoreline erosion. Harvesting The District may continue to harvest as needed to control the nuisances. The equipment should be maintained regularly. Operators should be trained in aquatic plant identification to help protect native non-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 cut one foot above the plant beds when possible. Operators should operate equipment at speeds only sufficient to harvest the plant material. Excessive speeds 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 the harvester. Plan Reassessment The District should review or contract to review, the plant populations of White River Lake every 3–5 years. Eurasian watermilfoil removal efforts should be reviewed for effectiveness. The management plan should also be reviewed, and if necessary modified, every 3–5 years. This will be especially important 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 Chara sp. 92 35.5 3.8 Myriophyllum spicatum 72.71.3 Potamogeton zosteriformis 42 16.2 1.9 Vallisneria americana 10 3.9 2.2 Potamogeton richardsonii 51.93.3 Najas flexilis 12 4.6 2.6 Potamogeton pectinatus 33 12.7 1.6 Ceratophyllum demersum 17 6.6 2.1 Ranunculus longirostris 83.11.2 Myriophyllum heterophyllum 20 7.7 1.3 Elodea canadensis 20.81.3 Potamogeton amplifolius 72.72.0 Polygonum amphibium 20.81.3 Utricularia vulgaris 20.82.0 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. Copyright © 2005 by Taylor & Francis Finding 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 District harvests approximately 50% (30 acres) of White River Lake. Approximately 60 acres (94%) of the lake are 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 are recommendations for periodic monitoring of the plant community in the future. Additional recom- mendations could include some periodic testing, even simple Secchi depth readings that monitor water quality, to determine if habitat conditions in the lake are changing, or if plant management might 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 to know each species’ identity, location, and abundance. Each species has unique physiological, habitat, and ecological requirements. The more known about the species of interest, the more successful 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 help identify 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 700 aquatic species are major weeds (Spencer and Bowes, 1990). Because of their prolific growth and reproduction, they often interfere with utilization of fresh waters and may displace indigenous vegetation. 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 the means they obtain resources. Plants in the same life-form group often have similar adaptations to their 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 available from the sediment, water is available from the sediment and overlying water, atmospheric carbon dioxide 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 coating protects the upper leaf surface from the aerial environment. Floating leaves do not have the structural support of emergents so they can be ravaged by wind and waves. Floating-leaved species are usually found 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 Copyright © 2005 by Taylor & Francis much less available than it is in air. They invest little energy in structural support because they are supported 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 at the 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 limits terrestrial plant productivity. Water, however, exerts a high resistance to solute diffusion and selectively 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. Some species 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 and development of submergent species. The quality and quantity of light depend upon dissolved materials and suspended particulate matter in the water, and upon water depth. Light becomes more limited and the quality changes with increasing depth and with turbidity from algae, silt, and resuspended bottom sediments. Zonation of macrophytes along depth gradients can be caused by the light regime (Spence, 1967) and increased turbidity can decrease the maximum depth of plant growth (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 sun plants. Each leaf can potentially utilize all the solar energy it receives for growth (Spencer and Bowes, 1990). Their productivity, at least for emergents, is similar or even greater than terrestrial sun plants. Submergent species are shade plants. Leaf photosynthesis is saturated by a fraction of full sunlight. The light compensation point (i.e., where the photosynthetic rate equals the respiration rate) for some species is as low as 0.5 percent of full sun (Spencer and Bowes, 1990). Some of the most important nuisances have the lowest compensation points. This may give them a slight but decided advantage over other species for accumulating energy resources. Light generally limits the lakeward edge of the littoral zone and there is evidence that increased turbidity decreases maximum plant biomass (Robel, 1961). Clear water lakes usually have deeper littoral 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 broader than 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 and Boylen, 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 and bryophytes in Lake Tahoe, California (Frantz and Cordone, 1967). Even shallow lakes, if they are turbid enough, will have sparse aquatic plant growth (Engel and Nichols, 1994; Nichols and Rogers, 1997). Copyright © 2005 by Taylor & Francis Hutchinson (1975), Dunst (1982), Canfield et al. (1985), Chambers and Kalff (1985), Duarte and Kalff (1990), and Nichols (1992) found a significant regression between Secchi depth and the maximum depth of plant growth (Table 11.3). In many cases these regressions are similar (Duarte and Kalff, 1987) and are used as models to predict the maximum depth of plant growth for management such as dredging depth to eliminate plant growth (see Chapter 20). Light also affects a number of morphogenetic processes in submerged aquatic plants including the 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 no longer light limited, they can shade out slower growing competitors, and they greatly restrict water use 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 overlying water. 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 directly from sediments (Barko et al., 1986). The role of sediment as a source of P and N for submergent macrophytes 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 a common misconception that excessive external nutrient loading directly to the water column causes macrophyte 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, but relatively available in sediments. However, the preferred source of potassium (K), calcium (Ca), magnesium (Mg), sulfate (SO 4 ), 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 with algae 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 are generally adequate to meet nutritional demands of rooted aquatic plants, even in oligotrophic systems. 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 (Que- bec-Vermont border), Duarte and Kalff (1988) demonstrated that biomass increases averaged 2.1 TABLE 11.3 Regression Equations of Secchi Depth versus Maximum Depth of Plant Growth Equation Region Reference MD = 0.83 + 1.22 SD Wisconsin Dunst, 1982 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. Copyright © 2005 by Taylor & Francis times greater for fertilized plants (fertilized with 3:1:1, N to P to K ratio) than paired controls. The biomass 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 nutrient deficiencies appear to diminish growth on extremely low density and extremely high density (usually meaning highly organic or highly sandy) substrates (Barko and Smart, 1986). Plant tissue analysis suggested to Gerloff (1973) that the elements most likely to limit macrophyte growth differed by lake and that nitrogen, phosphorus, calcium and copper were growth limiting or close to growth limiting in different Wisconsin lakes. When available, plants take up nutrients well above their physiological 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 covering nutrient 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 by controlling 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) is improved water clarity that improves macrophyte growth. Although this information suggests that nutrients do not limit aquatic plant growth, oligotrophic lakes generally maintain less total plant biomass and usually contain different species than more nutrient rich lakes. Many species found in oligotrophic lakes have the ability to seasonally conserve both biomass and nutrients. 11.4.3 DISSOLVED INORGANIC CARBON (DIC), pH, AND OXYGEN (O 2 ) 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 CO 2 transport and it is even more critical in submersed species. Carbon dioxide diffusion is much slower in water than in air. Free CO 2 is the most readily used carbon form for photosynthesis. Some species can utilize bicarbonate, but they do so less efficiently and they expend more energy doing so. The ability 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. The ratio of CO 2 to bicarbonate to carbonate is determined by the alkalinity and pH of the water, and by CO 2 uptake by plants. In dense plant beds free CO 2 and bicarbonate can be depleted in a few hours of photosynthesis. This shifts the carbon equilibrium toward carbonates that are not used for photosynthesis and increases O 2 concentration and pH. These water conditions cause O 2 inhibition of photosynthesis and photorespiratory CO 2 loss (Spencer and Bowes, 1990). All three conditions lower net photo- synthesis. In addition to the utilization of bicarbonate, submergent macrophytes have a number of anatomical, morphological, and physiological mechanisms to enhance carbon gain (Spencer and Bowes, 1990; Wetzel, 1990). Emergent, free-floating, and floating-leaved plants use atmospheric CO 2 so photosynthesis is not hampered by the slow diffusion rates of gases in water. In addition, lack of water stress allows their 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 of oxygen hinders nutrient acquisition. Some species, especially emergents, produce aerenchyma that Copyright © 2005 by Taylor & Francis allows oxygen diffusion from the aerial environment to submerged organs (Wetzel, 1990). Even dead stems are capable of conducting oxygen to rhizomes (Linde et al., 1976). Cutting off emergent plant stems (including dead stems) so they remain below the water surface, thus depriving rhizomes and 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 where another nutrient becomes growth limiting. Smart (1990) described laboratory experiments where a reciprocal relationship was found between inorganic C supply and sediment N availability. High levels 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 of inorganic C and high sediment N availability (see section above on nutrients). 11.4.4 SUBSTRATE Substrates provide an anchoring point for rooted plants and, as explained above, are the nutrient source for critical nutrients like N and P. Some sediments (e.g., rocks or cobble) are so hard that plant roots cannot penetrate them; others are so soft, flocculent, and unstable that plants cannot anchor in them. Coarse textured sediments can be nutritionally poor for macrophyte growth. Small accumulations of organic matter stimulate plant growth on these sediments. Low sediment oxygen concentrations, or high concentrations of soluble reduced iron and manganese or soluble sulfides, can be toxic to plants. High soluble iron concentrations interfere with 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 plants protect themselves from these toxins with oxygen release from their roots. This eliminates the anaerobic conditions that create toxic substances in the rhizophere surrounding the root. Also, as explained above, sediment density has important impacts on nutrient acquisition by plants. Consolidating flocculent sediments using drawdown is one method of improving the habitat for aquatic plant restoration (see Chapter 12). 11.4.5 TEMPERATURE Water buffers temperature extremes for plant growth but submerged plants can be exposed to temperature extremes from near zero to as much as 40C (Spencer and Bowes, 1990). Some submerged 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 are generally 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 and light appear to cause opposing response in shoot length (Barko et al., 1986). Different metabolic processes show differing responses to temperature so growth represents an integration of temper- ature responses. In thermally stratified lakes, depth related temperature decreases could reduce the length 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 cool water strategists. Although optimum photosynthetic temperatures for both species appear to be between 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 their maximum rate and higher than some other species (Nichols and Shaw, 1986). For milfoil, the Copyright © 2005 by Taylor & Francis [...]... to the point of extreme eutrophy or high turbidities, as lakes and reservoirs lose depth to internal processes and to additions of allochthonous material from runoff; and when exotic plants invade A thorough understanding of macrophyte biology is the basis for developing innovative management approaches Continued research and development will improve our understanding of the relationship of aquatic plants... impacts of management For a more detailed description of these relationships Carpenter and Lodge (1986), Engel (1990), Wetzel (1990), and Jeppesen et al (1998) are recommended In general, aquatic plants are a natural and desirable part of the aquatic ecosystem and the shallow waters of lakes and reservoirs are ideal habitats for plant growth The desire to have a “weed-free” lake is both naïve and unreasonable... Surface area and shape significantly influence the effect wind has on wave size and current strength Large lakes have large fetches and thus have greater wave and current energy than small lakes Wave action and current erode shorelines The directions and strength of the wind, slope, and lake shape determine sediment movement Points and shallows are swept clean by wind and waves; bays and deep spots fill... species, and (5) non-pathogenic to animals Aquatic plant management or the potential for aquatic plant management using plant pathogens is discussed in Chapter 17 11. 9 THE EFFECTS OF MACROPHYTES ON THEIR ENVIRONMENT Habitat and environment influence macrophyte distribution and productivity Macrophytes also impact the lake ecosystem How? — The effects are physical, chemical, and biological Dense stands of. .. large aquatic plants on algae and rotifers Ecology 30: 359–364 Hoyer, M.V and D.E Canfield 1997 Aquatic Plant Management in Lakes and Reservoirs NALMS, Madison, WI and Lehigh, FL Hutchinson, G.E 1975 A Treatise on Limnology-Limnological Botany John Wiley, New York James, W.F and J.W Barko 1990 Macrophyte influences of the zonation of sediment accretion and composition in a north-temperate reservoir Verh... limitations in Lakes: bacterioplankton, phytoplankton, epiphytes-snail consumers, and rooted macrophytes In E Jeppesen, M Sondergaard, M Sondergaard and K Christoffersen (Eds.), The Structuring Role of Submerged Macrophytes in Lakes Ecol Studies 131, Springer-Verlag, New York, NY pp 318–325 Moss, B., J Madgwick and G.L Phillips 1996 A Guide to the Restoration of Nutrient-Enriched Shallow Lakes Broads... die and decay until fall Eurasian watermilfoil and hydrilla, however, slough off leaves during the warm season; curly-leaf pondweed typically dies in early summer and Eurasian watermilfoil autofragments during the summer, making nutrients Copyright © 2005 by Taylor & Francis Translocation of N and P back to rhizomes at the end of summer Loss of litter to water in autumn - some goes to sediment - some... Duarte, C.M and J Kalff 1990 Patterns in the submerged macrophyte biomass of lakes and the importance of the scale of analysis in the interpretation Can J Fish Aquatic Sci 47: 357–363 Dunst, R.C 1982 Sediment problems and lake restoration in Wisconsin Environ Int 7: 87–92 Elakovich, S.D and J.W Wooten 1989 Allelopathic Aquatic Plants for Aquatic Plant Management; A Feasibility Study Tech Rept A-8 9-2 U.S... N and P of which much is lost to overflow Influx from land Uptake by roots Chemical and bacterial mobilisation of P in sediments Accelerated by large supply of organic matter by plants to sediments creating anaerobic conditions at the sediment surface FIGURE 11. 3 Nutrient transfers that occur between plant beds and open water in lakes (From Moss, B et al 1996 A Guide to the Restoration of Nutrient-Enriched... basin size, shape and depth determine the distribution of sediments in a lake and therefore the distribution of plants In shallow water the direct physical forces of wind, waves, and ice also determine plant distribution (Duarte and Kalff, 1988, 1990) For management purposes, macrophytes are likely to establish and proliferate in lakes with large areas of shallow, warm water; rich, fine-textured, moderately . plants and aquatic ecosystems. A thorough knowl- edge of macrophyte biology makes the development of new management techniques, the efficacy of present techniques, and the assessment of environmental. 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 developing from an understanding of the competition between macrophytes and algae (and a variety of other factors including nutrient status and fish and zooplankton populations), and the alternate stable