7.5 Macrophyte management

Macrophytes, or aquatic plants, play crucial roles in freshwater ecosystems. They provide habitat, stabilize sediments, and improve water quality. However, excessive growth can lead to problems like reduced oxygen levels and impeded recreation.

Managing macrophytes involves balancing ecological benefits with human needs. Techniques include physical removal, herbicides, and water level manipulation. Successful management requires understanding plant biology, monitoring populations, and adapting strategies based on ecosystem responses.

Macrophyte characteristics and roles

Submerged vs emergent macrophytes

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  Frontiers | The Role of Macrophytes in Biogenic Silica Storage in Ivory Coast Lagoons View original

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Top images from around the web for Submerged vs emergent macrophytes

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  Frontiers | Seasonality and Species Specificity of Submerged Macrophyte Biomass in Shallow Lakes ... View original

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  Frontiers | Responses of Aquatic Plants to Eutrophication in Rivers: A Revised Conceptual Model ... View original

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  Frontiers | The Role of Macrophytes in Biogenic Silica Storage in Ivory Coast Lagoons View original

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  Frontiers | Seasonality and Species Specificity of Submerged Macrophyte Biomass in Shallow Lakes ... View original

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• Submerged macrophytes grow entirely underwater and include species like pondweeds (Potamogeton) and coontail (Ceratophyllum)
• Emergent macrophytes have roots in the sediment but leaves and stems that extend above the water surface such as cattails (Typha) and bulrushes (Scirpus)
• Floating-leaved macrophytes like water lilies (Nymphaea) have leaves that float on the surface while being rooted in the sediment
• Submerged macrophytes often have thin, flexible leaves to reduce drag in moving water while emergent macrophytes have rigid stems to support upright growth

Macrophyte adaptations for aquatic life

• Reduced cuticle thickness and stomata density on leaves to facilitate gas exchange underwater
• Aerenchyma tissue in stems and leaves provides air spaces for oxygen transport to roots
• Flexible stems and leaves allow plants to move with water currents and avoid damage
• Some species have heterophylly, producing different leaf types above and below the water surface (water buttercup, Ranunculus aquatilis)
• Adventitious roots along stems help absorb nutrients directly from the water column

Ecosystem services provided by macrophytes

• Provide habitat and refuge for fish, invertebrates, and other aquatic organisms
• Stabilize sediments and reduce shoreline erosion through root systems
• Improve water clarity by reducing turbidity and absorbing excess nutrients
• Oxygenate the water through photosynthesis, benefiting aerobic organisms
• Serve as a food source for waterfowl, herbivorous fish, and aquatic mammals
• Act as natural filters, removing pollutants and improving overall water quality

Factors influencing macrophyte growth

Light and temperature requirements

• Macrophytes require sufficient light penetration for photosynthesis, with different species having varying light requirements
• Water clarity and depth affect the amount of light available for submerged macrophytes
• Temperature influences germination, growth rates, and seasonal senescence of macrophytes
• Species have different optimal temperature ranges and may exhibit dormancy during cold periods

Nutrient availability and limitation

• Macrophytes require nutrients such as nitrogen and phosphorus for growth and reproduction
• Excess nutrients from anthropogenic sources (agricultural runoff, sewage) can lead to eutrophication and algal blooms that outcompete macrophytes
• Nutrient limitation, particularly of phosphorus, can restrict macrophyte growth in some systems
• Sediment nutrient content and water column concentrations both influence macrophyte nutrient uptake

Substrate type and stability

• Macrophytes anchor to substrates like sand, silt, or gravel depending on species preferences
• Soft, organic-rich sediments may limit growth of some species due to reduced anchoring ability and anoxic conditions
• Substrate stability affects the ability of macrophytes to establish roots and withstand water movement
• High sediment deposition or erosion rates can bury or uproot macrophytes, respectively

Water depth and clarity

• Water depth determines the extent of the littoral zone where macrophytes can grow
• Species have different depth tolerances based on their light requirements and ability to elongate stems or leaves to the surface
• Water clarity, influenced by suspended sediments and phytoplankton abundance, affects light availability for submerged macrophytes
• Fluctuating water levels can expose or inundate macrophytes, influencing their distribution and survival

Macrophyte management techniques

Physical removal methods

• Hand-pulling or raking can be effective for small-scale removal of nuisance macrophytes
• Mechanical harvesters cut and collect macrophytes, but may fragment and spread some species
• Benthic barriers or bottom screens can be installed to prevent macrophyte growth in targeted areas
• Dredging removes sediments and associated macrophyte roots and tubers, but can have significant ecosystem impacts

Chemical control with herbicides

• Herbicides like glyphosate, 2,4-D, and fluridone can be applied to selectively control problematic macrophyte species
• Timing and dosage of herbicide applications are critical to minimize non-target impacts and ensure effectiveness
• Some species may develop herbicide resistance over time, requiring alternative management approaches
• Potential for negative effects on water quality and non-target organisms must be considered

Biological control using herbivores

• Introducing grass carp (Ctenopharyngodon idella) can control submerged macrophytes through grazing, but may have unintended ecosystem consequences
• Native herbivores like waterfowl, insects, and snails can help regulate macrophyte populations
• Promoting the growth of native herbivores through habitat management may provide a more sustainable control option
• Sterile grass carp are often used to prevent uncontrolled reproduction and spread

Drawdown and water level manipulation

• Lowering water levels exposes macrophytes to desiccation and freezing, reducing their abundance
• Drawdowns can also promote the germination of desirable native species from the seed bank
• Timing and duration of drawdowns must consider the life histories of target and non-target species
• Refilling after a drawdown must be managed to prevent rapid recolonization of nuisance species

Impacts of excessive macrophyte growth

Reduced water quality and clarity

• Dense macrophyte beds can trap sediments and organic matter, leading to increased turbidity when plants senesce
• Decomposition of macrophyte biomass can release nutrients back into the water column, fueling algal blooms
• Reduced water mixing in heavily vegetated areas can lead to stratification and anoxic conditions
• Invasive macrophyte species like Eurasian watermilfoil (Myriophyllum spicatum) can form dense mats that shade out native species and degrade habitat quality

Decreased dissolved oxygen levels

• Respiration by dense macrophyte stands can consume oxygen, particularly during night or under ice cover
• Decomposition of senescent macrophyte biomass by bacteria also depletes oxygen levels
• Fish kills and shifts in invertebrate communities can occur under hypoxic conditions
• Anoxic sediments can release nutrients and toxins (hydrogen sulfide) that further degrade water quality

Altered aquatic community structure

• Excessive macrophyte growth can lead to shifts in fish and invertebrate species composition
• Dense vegetation may favor smaller, littoral-dwelling fish species over larger, open-water species
• Invasive macrophytes can outcompete native species, reducing biodiversity and altering food web dynamics
• Changes in macrophyte community structure can affect the distribution and abundance of waterfowl and other wildlife

Impediments to recreation and navigation

• Dense surface mats of floating macrophytes like water hyacinth (Eichhornia crassipes) can hinder boat access and clog waterways
• Submerged macrophytes can entangle boat propellers and interfere with swimming and fishing activities
• Accumulation of macrophyte debris along shorelines can be aesthetically displeasing and produce unpleasant odors
• Economic impacts can result from reduced recreational value and increased management costs for affected waterbodies

Macrophyte monitoring and assessment

Mapping and quantifying macrophyte coverage

• GPS and GIS technologies can be used to map the spatial extent and distribution of macrophyte beds
• Transect surveys or quadrat sampling can provide quantitative estimates of macrophyte abundance and biomass
• Hydroacoustic methods (echosounders) can measure submerged macrophyte height and density
• Aerial photography or drone imagery can be used to assess macrophyte coverage in shallow, clear waters

Species identification and diversity

• Accurate species identification is critical for understanding macrophyte community composition and detecting invasive species
• Taxonomic keys, field guides, and expert consultation can aid in species identification
• Diversity indices (Shannon, Simpson) can be calculated to assess macrophyte community structure and compare across sites or time
• DNA barcoding and molecular techniques are increasingly used for precise species identification and detection of cryptic species

Indicators of ecosystem health

• Macrophyte species composition and abundance can serve as indicators of water quality and ecosystem condition
• Presence of sensitive or intolerant species may indicate good ecosystem health, while dominance of tolerant or invasive species suggests degradation
• Macrophyte-based indices (e.g., Floristic Quality Index) integrate species tolerance values to assess overall ecosystem integrity
• Changes in macrophyte community structure over time can reveal trends in ecosystem health and response to management actions

Remote sensing and GIS applications

• Satellite imagery (Landsat, Sentinel) can be used to map macrophyte distribution over large spatial scales
• Multispectral and hyperspectral sensors can differentiate macrophyte species based on their unique spectral signatures
• Aerial drones equipped with high-resolution cameras can provide detailed imagery for macrophyte mapping and monitoring
• GIS software allows for the integration of macrophyte data with other spatial layers (bathymetry, land use) for comprehensive analysis and management planning

Balancing macrophyte management goals

Maintaining biodiversity and habitat

• Preserving a diverse assemblage of native macrophyte species supports overall aquatic biodiversity
• Macrophytes provide critical habitat structure for fish spawning, juvenile development, and predator avoidance
• Waterfowl and other birds rely on macrophytes for food, nesting sites, and migratory stopover habitat
• Management plans should prioritize the protection and restoration of native macrophyte communities

Controlling invasive species spread

• Early detection and rapid response are key to preventing the establishment and spread of invasive macrophytes
• Targeted control efforts (physical, chemical, biological) can reduce the abundance and distribution of invasive species
• Preventing the introduction of invasive species through public education and boat cleaning regulations is crucial
• Collaborative management across jurisdictional boundaries is necessary to control the regional spread of invasive macrophytes

Enhancing aesthetic and recreational value

• Maintaining clear, navigable waters with diverse macrophyte communities can improve the aesthetic appeal of lakes and rivers
• Balancing macrophyte control with habitat conservation can support recreational activities like fishing, boating, and wildlife viewing
• Engaging stakeholders in the development of management goals can help align ecological and social priorities
• Communicating the benefits of macrophytes and the rationale behind management decisions can foster public support

Integrating stakeholder interests and concerns

• Involving diverse stakeholder groups (landowners, recreational users, conservation organizations) in the management planning process
• Conducting public meetings, surveys, and workshops to gather input and address concerns
• Developing consensus-based management objectives that balance ecological, social, and economic considerations
• Establishing ongoing communication channels to keep stakeholders informed and engaged throughout the management process

Best practices for sustainable management

Developing site-specific management plans

• Tailoring management strategies to the unique characteristics, uses, and challenges of each waterbody
• Setting clear, measurable objectives based on the desired ecological and social outcomes
• Incorporating adaptive management principles to allow for flexibility and adjustment based on monitoring results
• Prioritizing prevention, early detection, and rapid response to minimize the need for long-term control efforts

Implementing integrated control strategies

• Combining multiple control methods (physical, chemical, biological) to maximize effectiveness and minimize adverse impacts
• Timing control efforts to target the most vulnerable life stages of nuisance macrophytes
• Selectively treating problematic areas while preserving native macrophyte communities in others
• Considering the potential interactions and trade-offs among different control methods and their effects on non-target species

Monitoring and adapting to ecosystem responses

• Establishing baseline data on macrophyte community composition, abundance, and distribution before implementing management actions
• Conducting regular post-treatment monitoring to assess the effectiveness of control efforts and detect any unintended consequences
• Adapting management strategies based on monitoring results and new scientific information
• Sharing monitoring data and lessons learned with other lake managers and researchers to advance the field of macrophyte management

Engaging community participation and education

• Developing volunteer monitoring programs to involve local citizens in data collection and promote stewardship
• Organizing community events (lake clean-ups, invasive species removal) to raise awareness and encourage participation
• Partnering with schools, universities, and youth organizations to integrate macrophyte education into curricula and service-learning projects
• Creating educational materials (brochures, websites, social media) to inform the public about macrophyte ecology and management efforts