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Marsh Management Strategies: Management Strategies

This section describes management strategies most likely to be used in marshes along the lagoon. The list is by no means exhaustive, and the techniques described are subject to considerable modification to accommodate different marsh structures and management goals. Major alternatives from each category are described in the remainder of this section.

Index to Strategies

Unmodified

Modified Managed

Marshes under this category have not been impounded or ditched. Mosquito control is achieved by using larvicides when breeding becomes a problem.

Unmodified Managed

These are unmodified wetlands where no larvicides are used for mosquito control.

Closed

A closed impoundment is one which is not connected to the adjacent estuary at any time during the year. Impoundments maintained in this fashion may be actively flooded via pumps or wells, or passively flooded by trapping rainfall or stormwater runoff. Impoundments may be flooded year-round, or only seasonally.

Impoundment cells can be managed on a continuous flooding basis to provide suitable conditions for growth of waterfowl food plants and for feeding of resident and overwintering waterfowl. Although continuous flooding is not necessary to promote waterfowl utilization (see ¨Seasonal Drawdown¨), the technique may be used when there are no provisions for draining down the impoundment (no culverts), or when flooding must be maintained throughout the year for other reasons (e.g. for stormwater retention).

For best results for waterfowl, impoundments are flooded with 12 to 24 inches of water. Moderate salinities (about 10 ppt) are desirable but not imperative to grow Chara the primary waterfowl food plant. Depending upon the amount of freshwater inflow, the impoundment can be pumped-up with Indian River Lagoon water as needed to maintain desirable water levels and eliminate weeds.

Advantages:

  1. If the marsh is kept flooded, salt marsh mosquito (and sandfly) production from that site is eliminated year-round with no need for larviciding.
  2. If proper salinities can be maintained, the marsh may be managed for waterfowl habitat.
  3. Conducive to development of large populations of resident fish.

Disadvantages:

  1. Many of the natural marsh functions are eliminated.
  2. Not having any estuarine connection, means that benefits provided by the late summer/early fall rise in sea level that normally penetrate a marsh are eliminated.
  3. Biodiversity is reduced.
  4. May adversely affect marsh vegetation.
  5. Precludes the use of other management techniques.
  6. Long term maintenance of a closed system may degrade waterfowl habitat.
Open / No Hydrological Management

An open impoundment is one where a connection to the adjacent estuary is maintained year-round through breaches in the dike or through open culverts. By definition, open systems interfere the least with the natural pathways of material, energy, and organism exchange between wetlands and the lagoon. Several types of open systems can be envisioned:

  1. impoundments with breached dikes
  2. impounded marshes where the culverts remain open all year.
  3. connections maintained year-round by opening culverts only part way or by restricting spillage over riser boards. This is usually done to manipulate salinity, to control overdrainage of the marsh and/or to control water levels in all or parts of the marsh.

Open systems are best suited for areas where mosquito production is not a major problem such as areas removed from population centers, or in areas where other types of mosquito control can be effective. Open systems may also be appropriate as part of management schemes where management techniques in different members of an impoundment block alternate from year to year.

Advantages:

  1. The marsh is able to largely function in a natural fashion.
  2. If mosquito production is not a problem, continued management efforts are not required.

Disadvantages:

  1. It is impossible to accomplish mosquito source reduction through flooding thus requiring other forms of mosquito control such as OMWM or the application of chemicals as needed (larvicides directly to the marsh and adulticides in nearby areas).
  2. Large sand fly populations often develop along the marsh edges and along the edges of creeks.
Rotational Impoundment Management

Rotational Impoundment Management (RIM) is a technique that provides for source reduction mosquito control and the improvement of fish and wildlife habitat and water quality. It is a working compromise between totally open impoundments, where flooding is not possible, and totally closed impoundments, where many of the natural marsh functions are eliminated. RIM impoundments are minimally flooded from the late spring to early fall to prevent salt-marsh mosquito oviposition while protecting desirable vegetation. The impoundment is left open or partially open to the estuary for the remainder of the year to allow water, nutrient and organism exchange between the impoundment and estuary, and normal marsh drying. RIM water control structures may include: 1) flapgated culverts installed through the impoundment dike, and 2) a permanent or dedicated pump. General guidelines for RIM management have been developed by SOMM.

Water Control Structures

Culverts

Research has shown that, to approach natural conditions, culverts should be made of a non-corroding material (most commonly aluminum), should be a minimum of 30 in. diameter, and be equipped with a structure that will allow trapping of tidal water when closed and passage of water when open. Culverts should be spaced at 500 m intervals or less and should be placed where natural tidal creeks, basins or sloughs intersect the dike. Invert elevations should be set at -1.0 ft. NGVD (National Geodetic Vertical Datum).

Pumps

A permanent or dedicated pump is necessary for optimal water level manipulations. Pumps can be electric or diesel, with automatic or manual operation.

RIM Variations

RIM is flexible, allowing for the incorporation of new techniques or management findings that prove beneficial. Some variations to a standard RIM protocol are discussed below.

Bleed Down or Bottom Water Release Structures

During the summer, when culverts are closed, water quality in some impoundments can deteriorate, particularly in deep perimeter ditches. In such situations, water control structures that allow a slow release of bottom waters have usefulness in improving water quality within the impoundment.

Periodic Drawdowns During the Closed Summer Period

Impoundments can be drained for a short duration during the normally flooded summer period to allow marsh soils to dry and oxidize. This technique may be used to reduce hydrogen sulfide build-up in marsh soils.

Constant Bleed

This technique requires pumping of estuarine water into the impoundment in excess of what is necessary for mosquito control. Excess water is allowed to either spill over riser structures or escape through bottom water release structures thus creating a flushing effect. Flushing may affect the whole impoundment, or may only be effective throughout the perimeter ditch.

Ditching

In some instances, shallow ditches constructed by hand or with a rotary ditcher can extend the open RIM period allowing the marsh to provide more natural functions. By connecting salt-marsh mosquito breeding areas to deeper ponds or ditches, mosquito production can sometimes be abated by improving drainage and/or allowing larvivorous fish improved high marsh access. However, ditching has the possibility to promote marsh drainage and may increase habitat for sandfly production (Culicoides spp.).

Specific Species Management

Variations of the standard RIM technique may be incorporated to manage for an individual or group of animal species. An example is a winter flood period with gradual water level drawdown to concentrate fish in shallow waters, thus providing improved feeding possibilities for wading birds.

Advantages:

  1. Many natural marsh functions are re-established (e.g., access to the marsh for transient organisms).
  2. Allows for seasonal source reduction mosquito control.
  3. By flooding to only minimal levels during the summer months with a winter dry-down, indigenous marsh vegetation has a better opportunity to survive and/or re-establish.
  4. The technique is economical.

Disadvantages:

  1. Because the marsh is not flooded year-round, some larvicide applications may be necessary.
  2. Flooding the marsh during the summer months is not a natural occurrence in this area and thus may cause problems with water quality and problems to some indigenous flora and fauna (e.g., displacement of Uca populations from the marsh surface).
Seasonal Drawdown

Seasonal drawdowns take advantage of the water management capabilities of impounded marshes to enhance habitat utilization by groups of species, to promote growth of desirable food plants, and to maximize management opportunities by following seasonal management needs.

The exact schedule of culvert opening and closing, and the time periods during which impoundments are in a drawdown stage can vary considerably depending upon local conditions. During open portions of the cycles, the opening can be complete or partial, and may be continuous or alternating. Some likely drawdown scenarios are described below.

Cyclic

Cyclic impoundment management involves the rotation of several different management techniques in different cells of an impoundment or in groups of impoundments. The rotation may be seasonal, or different techniques may be applied during succeeding years. Objectives may include wildlife enhancement (primarily waterfowl), runoff filtration, saltmarsh vegetation enhancement, and mosquito control.

The most common strategy involves a 3-year rotational cycle. Impoundments are prepared for one year before the rotation starts. The preparatory phase can include saltwater flooding of cells, burning of vegetation, and drawdown to eliminate weedy vegetation such as cattail. Those cells not in active cyclic impoundment management are usually maintained under RIM.

The first active year, a cell is flooded by June and flooding continues with closed culverts through December, at which time the culverts are open. The second year the cell is flooded with Indian River Lagoon water in November and then drawn down in February; therefore there is no flooding for mosquito control during the summer of that year. Some restriction to marsh-estuary water flow may be imposed during this period for salinity or overdraining control. The third year, after the culverts are opened in February, the cell follows standard RIM.

Other cells in the group begin the same cycle in sequence one year behind. The whole process can then be repeated every three years.

Advantages:

  1. A great deal of variety can be introduced in a modest area.
  2. Complementary strategies can be implemented in neighboring impoundment groups.
  3. Management can be adjusted to optimize seasonal utilization of the area.
  4. Some strategies that may be beneficial, but which need not be implemented continuously (e.g. drawdowns for sediment oxidation) can be incorporated into the regular management schedule without precluding use of other strategies at other times.
  5. If proper salinities can be maintained the area will be attractive to waterfowl, and provide waterfowl food.
  6. The impoundment is unsuitable for mosquito and sandfly production during 2 out of 3 years.

Disadvantages:

  1. May be labor intensive and expensive to implement.
  2. If not carefully designed, may result in fragmented management of areas too small to produce maximum benefits.
  3. Access to the marsh by aquatic organisms is limited during 2 out of 3 years.
  4. The impoundment is suitable for mosquito and sandfly production during 1 out of 3 years.
Open Marsh Water Management (Impounded)

This mosquito control method reduces immature mosquito production by establishing more regular hydroperiods that are less conducive to mosquito oviposition, and by enhancing water exchange and circulation to allow recruitment and maintenance of larvivorous fish populations. It utilizes ponds, ditches and canals connected to tidal sources.

In areas with a heavy concentration of mosquito-producing depressions, ponds are constructed so as to eliminate the depressions. The ponds serveas reservoirs for predatory fish during low water periods, and as water sources for nearby areas. Radial ditches may extend from the ponds to eliminate breeding in areas near the ponds. Ditchesare also used to eliminate scattered breeding depressions and to\interconnect the ponds, and deeper ditches are used to connect the system with a tidal source. Ditches are often silled at their connection to the lagoon to prevent over-drainage of the high marsh during low tide periods. Technique is often used in impounded marshes to allow the marshes to remain open for longer periods of time, often throughout the whole year.

Detailed standards and protocols for OMWM have been produced for New Jersey (Bruder 1980; Proc. N.J. Mosq. Control Assoc. 67:72-76), Maryland (Lesser 1982; Proc. N.J. Mosq. Control. Assoc. 70:29-34), and Delaware (Meredith et al. 1985; Wetlands 5:119-133), and various modifications of OMWM have been devised for dealing with slight tidal ranges and unpredictable tidal events (closed systems, ditch sills, etc.). In Florida, the method has been modified to suit the Indian River's low tidal amplitudes. The use of both open and closed systems, variations in the placement and height of silled connectors, and the use of graded, meandering ditches, are used to accomplish specific management needs.

Because of the small tidal amplitude in the Indian River Lagoon, and other limitations imposed by vegetation structure and equipment, opportunities for OMWM may be restricted along the lagoon. However, the technique may offer significant benefits in specific cases, or when used in combination with other techniques to achieve primary or secondary management goals.

Advantages:

  1. Reduces pesticide use.
  2. May increase suitable habitat for the endangered Atlantic salt marsh snake along the edges of ditches.
  3. May allow breaching of dikes in some impoundments, or maintaining culverts open for longer periods of time in others.
  4. Minimal system maintenance can be expected for up to ten years.
  5. Provides a passive management system that requires little personnel for operation.
  6. Increases edge habitat.
  7. Variations in techniques provide flexibility for different situations and/or objectives.

Disadvantages:

  1. Works best in areas with high tidal energy.
  2. Limited applicability in areas dominated by mangroves because the necessary modifications would be difficult to construct.
  3. Marsh hydrology changes with lowering of water table a distinct possibility.
  4. May require maintenance and, possibly, continued modification as conditions in the marsh change. Periodic inspections are needed to keep track of changing marsh conditions.
  5. May foster the spread of mangroves into salterns and herbaceous halophyte areas and of invasion of salterns by herbaceous halophytes.
  6. Depends partially upon weather and tidal events, which may cause control failures in part of the marsh. Larval inspections are still necessary.
Continuous Bleed

This type of management is generally accomplished by pumping enough water into the managed system so that its water level is higher than the surrounding estuary. This allows for a constant flow of water out of the managed area (through bottom water release culverts or over flapgate risers) into the estuary. Many modifications of this technique can be done. For example, water levels could be kept at a level that would allow for high tides to augment the input from the pumps. This modification would allow tidal water to enter the marsh at selected tidegates giving the manager much flexibility in determining inflow and outflow points. Because tides tend to be variable, this technique can be combined with tidal trapping allowing the manager to use the pumps when tides are low and natural tides when tides are high. Like many other management schemes, this method can be combined with other techniques for increase management versatility.

Advantages:

  1. Creates a flow through system, giving the manager a choice of options on the amount of water released and the location of release points.
  2. Can easily vent water from top or bottom by using various tidegate configurations.
  3. Tends to improve water quality by continually circulating water in the system.
  4. May compensate for the absence of natural tidal flushing which is found in many natural systems.
  5. Allows for great versatility in management.

Disadvantages:

  1. Running pumps for long periods of time is expensive.
  2. Pump failure may cause large fish mortalities because of sudden decrease in water quality.
  3. May cause the managed system to depart too much from the type of system which would normally be found in a marsh/swamp environment (are we creating an aquarium?).
Merritt Island NWR Strategy

This technique is a special case of a cyclical management strategy, specially suited for the Merritt Island National Wildlife Refuge goals and mandates. It encompasses four different strategies that alternate as needs and conditions dictate as follows:

  • Open all year.
  • Open fall, winter, spring and closed and flooded in summer with pumping or passive water capture and retention.
  • Open spring, flooded summer, fall and winter for waterfowl habitat and mosquito control.
  • Open spring and summer, flooded fall and winter for waterfowl.

The Refuge strives to maintain impoundments open as much as possible and to alternate management so that nearby impoundments are not all managed the same at any particular time. In some cases, impoundments are left open for several years to help oxidize sediments and consolidate the substrate, or are closed for 1 to 2 years (with periodic flushing) to control exotic vegetation.

Advantages:

This technique gives the MINWR the needed flexibility to manage the impoundments to satisfy their mandate of providing habitat and food for migratory birds and waterfowl while maintaining as much marsh-lagoon connection as possible both in a geographical and temporal sense.

Disadvantages:

Some impoundments remain closed longer than necessary for mosquito control.

Other (Impounded)

A variety of other techniques may be utilized, alone or in combination to attain specific management objectives. These techniques may represent a temporary change in management (e.g. leaving an impoundment open during a specified period of time to oxidize the sediments and then resuming normal management), or they may be permanent management strategies (e.g. stormwater retention). Some (e.g. exotic removal, prescribed burns) may be applied in conjunction with other strategies.

Many variations on the standard management options can be envisioned; a few possibilities include:

  1. prescribed burning,
  2. stormwater processing,
  3. overflooding,
  4. scheduled exotic removal, and
  5. wastewater retention.

Specific techniques will vary depending upon the objectives and the structure of the impoundment(s) in question.

Open Marsh Water Management (Unimpounded)

This mosquito control method reduces immature mosquito production by establishing more regular hydroperiods that are less conducive to mosquito oviposition, and by enhancing water exchange and circulation to allow recruitment and maintenance of larvivorous fish populations. It utilizes ponds, ditches and canals connected to tidal sources.

In areas with a heavy concentration of mosquito-producing depressions, ponds are constructed so as to eliminate the depressions. The ponds serve as reservoirs for predatory fish during low water periods, and as water sources for nearby areas. Radial ditches may extend from the ponds to eliminate breeding in areas near the ponds. Ditches are also used to eliminate scattered breeding depressions and to interconnect the ponds, and deeper ditches are used to connect the system with a tidal source. Ditches are often silled at their connection to the lagoon to prevent over drainage of the high marsh during low tide periods.

Detailed standards and protocols for OMWM have been produced for New Jersey (Bruder 1980; Proc. N.J. Mosq. Control. Assoc. 67:72-76), Maryland (Lesser 1982; Proc. N.J. Mosq. Control. Assoc. 70:29-34), and Delaware (Meredith et al. 1985; Wetlands 5:119-133), and various modifications of OMWM have been devised for dealing with slight tidal ranges and unpredictable tidal events (closed systems, ditch sills, etc.). In Florida, the method has been modified to suit the Indian River's low tidal amplitudes. The use of both open and closed systems, variations in the placement and height of silled connectors, and the use of graded, meandering ditches, are used to accomplish specific management needs.

Because of the small tidal amplitude in the Indian River Lagoon, and other limitations imposed by vegetation structure and equipment, opportunities for OMWM may be restricted along the lagoon. However, the technique may offer significant benefits in specific cases, or when used in combination with other techniques to achieve primary or secondary management goals.

Advantages:

  1. Reduces pesticide use.
  2. May increase suitable habitat for the Atlantic salt marsh snake along the edges of ditches.
  3. In open systems, a connection can be maintained year round between the marsh and the estuary. Usually results in improved water quality and more stable water levels.
  4. Minimal system maintenance can be expected for up to ten years.
  5. Provides a passive management system that requires little personnel for operation.
  6. Increases edge habitat.
  7. Variations in techniques provide flexibility for different situations and/or objectives.

Disadvantages:

  1. Works best in areas with high tidal energy.
  2. Limited applicability in areas dominated by mangroves because the necessary modifications would be difficult to construct.
  3. Marsh hydrology changes, with lowering of water table a distinct possibility.
  4. May require maintenance and, possibly, continued modification as conditions in the marsh change. Periodic inspections are needed to keep track of changing marsh conditions.
  5. May foster the spread of mangroves into salterns and herbaceous halophyte areas and invasion of salterns by herbaceous halophytes.
  6. Depends partially upon weather and tidal events, which may cause control failures in part of the marsh. Larval inspections are still necessary.
Ditched

This category includes marshes that were ditched in the past (particularly during the 1940s and 1950s) in attempts to control mosquitoes; It does not include ditches constructed as part of OMWM systems or shallow ditches that may be used to complement RIM. The usual strategy was to construct an evenly-spaced grid system of deep ditches that would drain the marsh and often lowered water tables. This approach was not successful because considerable breeding still occurred between the ditches. Furthermore, spoil deposition in the marsh interfered with sheet flow and provided sites for invasion by exotic species and upland plants.

Many of these ditches have fully or partially closed with time, particularly at the marsh-estuary edge. Although new ditch systems of this type are not likely to be constructed, maintenance of old ditches may be advisable in some cases as some of the ditches may promote water circulation and may be used as part of other management strategies. Partially closed ditches are often heavy mosquito producers, and may require maintenance to prevent mosquito production and water stagnation. Finally, spoil piles or berms created by this technique often block water movement and may have to be removed.

Advantages:

In some marshes where the deep ditches have remained functional, mosquito breeding has been reduced, thus reducing the need to use pesticides for mosquito control.

Disadvantages:

  1. Frequently not effective for long term mosquito control.
  2. Spoil piles created by the deep ditching interfere with sheet flow, block circulation, and promote invasion by exotics and upland plants.
  3. Often lowers marsh water table.
  4. Has made traversing some marshes virtually impossible, thereby hindering inspections for larval mosquitoes.
  5. Ditch banks can be suitable habitats for sandflies.
Other (Unimpounded)

As with impounded marshes, a variety of other techniques may be utilized, alone or in combination to attain specific management objectives. These techniques may represent a temporary change in management or they may be permanent management strategies. Some (e.g. exotic removal, prescribed burns) may be applied in conjunction with other strategies.

Many variations on the standard management options can be envisioned. Specific techniques will vary depending upon the objectives and the structure of the marshes in question.