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Salt Marshes & Mangroves

Mosquito Impoundments

IRL Mangroves

Impounding of salt marshes and mangrove forests is likely the most broadly reaching and destructive disturbance in these coastal Florida ecosystems (Montague & Wiegert 1990).
Before mosquito control methods like impoundments were in place, mosquito landings were among the highest densities ever recorded in the continental United States (Provost 1949), reaching 500 per person per minute in some areas of Florida (Leenhouts 1983).

Ditching in marshes in the 1930s was ineffective, and the use of DDT pesticide in the 1940s had adverse effects on wildlife (Montague & Wiegert 1990). In the 1950s, impounding marshes became an effective and seemingly less invasive way to control mosquitoes. Using this method, salt marshes and intertidal mangroves are closed off to tidal influences and the water is pumped out, allowing mosquitoes to lay eggs on exposed sediment. The impoundments are then flooded to kill eggs and larvae. Today, 192 impoundments are active on the east coast of Florida alone (Rey & Kain 1989). Many of these areas are still closed systems that allow influx of freshwater from precipitation and runoff, creating wide variations in salinity. This, along with the flooding process, can cause dieback of natural vegetation and establishment of more oligohaline species. Breeding and spawning behaviors of fishes and invertebrates can also be restricted in closed systems.

One of the most devastating effects of impounding was to the dusky seaside sparrow, Ammodramas maritimus nigrescens, which was driven to extinction in 1987 (Kale 1996). Currently, several areas have begun operating on a rotational impoundment management (RIM) approach (David 1992). The RIM plan allows impoundments to be open to regular tidal flows and wildlife migrations during non-breeding months.
From May to August, impoundments are closed to control mosquito reproduction. The addition of culverts to enhance tidal cycles in open impoundments (known as breached-RIM or restored areas) have helped to increase biodiversity, correct chemical values for sediments and to re-establish common vegetation (Brockmeyer et al. 1997, Klassen 1998, Poulakis et al. 2002, Rey & Cain 1993).

Mangrove species seem to respond differently to RIM and restored areas. For example, the red mangrove, Rhizophora mangle, reaches higher densities in RIM and restored impoundments; whereas, the black mangrove, Avicennia germinans, grows best in natural undeveloped areas (Middleton et al. 2008). More information on mosquito impoundments can be found by navigating to the impoundment link on the main habitat page.

Shoreline Development

Land use changes from the growing population and urbanization in Florida and throughout the world have altered coastal ecosystems.
In Florida, mangroves have been removed and both these areas and salt marshes have been filled with dredged material to create roads, residential communities and businesses.
Habitat fragmentation has occurred from this development, fracturing animal communities and leaving ecosystems more vulnerable to other habitat disturbances (Larson 1995). Fortunately, substantial mangroves and salt marshes lay within protected areas such as the Merritt Island National Wildlife Refuge and property managed by the Florida Department of Environmental Protection. Proper management provides conservation from further development and encourages restoration programs that work to increase habitat acreage.

Muck & Nutrients

Accumulations of fine-grained, organic-rich clays and silt known as muck are introduced to coastal environments from terrestrial and industrial runoff. Muck generally settles into depressions in the sediment, and can reach up to 2 m deep in some areas of the IRL (Trefry et al. 1990). Disturbance from boat traffic, wind and waves can suspend muck, creating particulates that cloud the water, reducing sunlight penetration and retarding plant and algal growth (Trefry et al. 2007).
Transported in the water column by currents, muck can settle in salt marshes and mangrove forests, possibly smothering young vegetation. Muck accrual in the IRL has been ongoing for the past 40 to 60 years. Although less than 10% of the IRL bottom was covered in muck in 1990, coverage continues to grow (Trefry et al. 1990, Trefry et al. 2007).

In addition to the physical stresses caused by this sediment accumulation, muck carries large quantities of nutrients and toxic substances that can create health problems or death for a variety of aquatic organisms. Excess nitrogen and phosphorous can alter the dominant plants in marshes and mangroves, allowing some species to thrive outside of their natural elevation (Levine et al. 1998). Recently, dredging projects in isolated areas of the lagoon, including the St. Sebastian River, Turkey Creek and Crane Creek, have successfully removed thousands of cubic meters of muck, along with harmful chemicals like pesticides that are incorporated into the sediment (Trefry & Trocine 2002).
Coupled with decreases in terrestrial and industrial runoff, further dredging projects could result in long-term reductions of muck throughout the IRL.

Invasive Species

Bioinvasions have become high profile issues affecting ecosystem dynamics in both aquatic and terrestrial environments. Because salt marshes and mangroves are unique mixtures of both habitats, invasive species from land and sea pose threats to biodiversity and ecosystem health. In Florida, the introduced nutria, Myocastor coypus, contributes to the loss of marsh acreage by foraging on vegetation (Ford & Grace 1998). Changes in water flow around salt marshes and mangroves have allowed for expansion of the invading Brazilian pepper, Schinus terebinthifolius, and the Australian pine, Casuarina equistifolia. Closing portions of these habitats for mosquito impoundments has reduced the salinity, allowing the invasion of more oligohaline vegetation and animals (FWS 1999) such as the blackchin tilapia, Sarotherodon melanotheron (Faunce et al. 1999, Poulakis et al. 2002). Furthermore, disturbed or barren areas will often be colonized by invasives before native plants can become established. Efforts are ongoing to remove invasive plants from terrestrial areas, but aquatic invasions of fishes and invertebrates are often difficult or impossible to reverse, and can only be managed to prevent further range expansion.


Salt marshes, mangroves and other coastal ecosystems can usually recover quickly from natural disturbances such as fire and hurricanes. However, when disturbance events occur in close succession, they may have lasting effects on the ecosystems. Hurricanes produce storm surges, wind and waves that can impact mangroves and marshes in several ways. Upper marshes and mangrove swamps can experience an influx of seawater at a salinity to which vegetation is not accustomed, causing dieback of several plant species. Wind can strip trees and bushes of foliage and damage the trunk. The white mangrove, Laguncularia racemosa, is the mangrove species most susceptible to wind damage (Doyle et al. 1995).
In addition, lower elevations can experience extreme rates of sedimentation or erosion. Sediment erosion can wash away much of the vegetation, reducing habitat acreage. However, sediment accretion could be more harmful, essentially covering marsh and mangrove areas (Rejmanek et al. 1998) and smothering sessile benthic invertebrates. One example of rapid sedimentation occurred in the upper Chesapeake Bay, when over a 70-year period 50% of the sediment accumulation was attributed to one flood event and a single hurricane (Schubel & Hirschberg 1978). Regeneration of mangrove forests following substantial storm damage may take decades, and restored swamps may have altered biodiversity and plant zonation (Ellison & Farnsworth 1990).

Sea Level Rise

Much attention has been given to the effects of rising sea level on coastal ecosystems throughout the world. As intertidal communities, salt marshes and mangroves are at risk from both the amplitude and rate of this rise. For the ecosystems to thrive, they must occur at the appropriate elevation and slope. In fact, one of the most common reasons for restoration failure in salt marshes is choosing an improper site based on these parameters (Crewz & Lewis 1991). As sea level rises, it is possible for marshes and mangroves to shift in a landward direction if the rate of rise is slow enough for sediment accretion to occur (Montague & Wiegert 1990). However, coastal development and steep terrain may inhibit plant migration, changing zonation in these habitats or flooding them completely. In addition, compression of the intertidal zone can lead to increased interspecific competition and loss of biodiversity. See Climate Change and the IRL.

References & Further Reading

Brockmeyer JR., RE, Rey, JR, Virnstein, RW, Gilmore, RG & L Earnest. 1997. Rehabilitation of impounded estuarine wetlands by hydrologic reconnection to the Indian River Lagoon, Florida (USA). Wetlands Ecol. Manag. 4: 93-109.

Crewz, DW & RR Lewis III. 1991. An evaluation of historical attempts to establish vegetation in marine wetlands in Florida. Florida Sea Grant technical paper TP-60. Sea Grant College, University of Florida. Gainesville, FL. USA.

David, JR. 1992. The Saint Lucie County Mosquito Control District summary workplan for mosquito impoundment restoration for the salt marshes of Saint Lucie County. Saint Lucie County Mosquito Control District. Saint Lucie, FL. USA.

Doyle, TW, Smith III, TJ & MB Robblee. Wind damage effects of Hurricane Andrew on mangrove communities along the southwest coast of Florida, USA. J. Coast. Res. 21: 159-168.

Ellison, AM & EL Farnsworth. 2001. Mangrove communities. In: Bertness, MD, Gaines, SD & ME Hay. Marine community ecology. Sinauer Associates, Inc. Sunderland, MA. USA. 550 pp.

Faunce, CH & R Paperno. 1999. Tilapia-dominated fish assemblages within an impounded mangrove ecosystem in east-central Florida. Wetlands. 19: 126-138.

Ford, MA & JB Grace. 1998. Effects of vertebrate herbivores on soil processes, plant biomass, litter accumulation and soil elevation changes in a coastal marsh. J. Ecol. 86: 974-982.

FWS. 1999. Coastal Salt Marsh. In: Multi-species recovery plan for South Florida. US Fish & Wildlife Service. 553-595.

Kale II, HW. 1996. Recently extinct: dusky seaside sparrow, Ammodramas maritimus nigrescens. In: Rodgers, JA, Kale II, HW & HT Smith, eds. Rare and endangered biota of Florida. Volume V. Birds. 7-12. University Presses of Florida. Gainesville, FL. USA.

Kemp, SJ. 2008. Autecological effects of habitat alteration: trophic changes in mangrove marsh fish as a consequence of marsh impoundment. Mar. Ecol. Prog. Ser. 371: 233-242.

Klassen, CA. 1998. The utilization of a Florida salt marsh mosquito impoundment by transient fish species. Master's Thesis. Florida Inst. of Technology. 87 pp.

Leenhouts, WP. 1983. Marsh and water management plan, Merritt Island National Wildlife Refuge. US Fish and Wildlife Service. Merritt Island National Wildlife Refuge. Titusville, FL. USA.

Levine, JM, Brewer, JS & MD Bertness. 1998. Nutrients, competition and plant zonation in a New England salt marsh. J. Ecol. 86: 285-292.

Middleton, B, Devlin, D, Proffitt, E, McKee, K & KF Cretini. 2008. Characteristics of mangrove swamps managed for mosquito control in eastern Florida, USA. Mar. Ecol. Prog. Ser. 371: 117-129.

Montague, CL & RG Wiegert. 1990. Salt marshes. In: Myers, RL & JJ Ewel, eds. Ecosystems of Florida. UCF Press. Orlando, FL. USA. 765 pp.

Poulakis, GR, Shenker, JM & DS Taylor. 2002. Habitat use by fishes after tidal reconnection of an impounded estuarine wetland in the Indian River Lagoon (USA). Wetlands Ecol. Manag. 10: 51-69.

Provost, MW. 1949. Mosquito control and mosquito problems in Florida. Proc. Annu. Meet. Calif. Mosq. Control Assoc. 17th. 32-35.

Rejmanek, M, Sasser, C & GW Peterson. 1988. Hurricane-induced sediment deposition in a Gulf Coast marsh. Est. Coast. Shelf Sci. 27: 217-222.

Rey, JR & T Kain. 1989. A guide to the salt marsh impoundments of Florida. University of Florida, Florida Medical Entomology Laboratory. Vero Beach, FL. USA.

Rey, JR & T Kain. 1993. Coastal marsh enhancement project. Indian River National Estuary Program. Final report contract CE004963-91. University of Florida IFAS. Vero Beach, Florida. USA. 29 pp.

Schubel, JR & DJ Hirschberg. 1978. Estuarine graveyards, climatic change, and the importance of the estuarine environment. In: Wiley, ML, ed. Estuarine Interactions. 285-303. Academic Press. New York. USA.

Trefry, JH, Metz, S, Trocine, RP, Iricanin, N, Burnside, D, Chen, NC & B Webb. 1990. Design and operation of a muck sediment survey. Final report to the St. Johns River Water Management District. Available from the St. Johns River Water Management District. Palatka, FL. USA.

Trefry, JH & RP Trocine. 2002. Pre-dredging and post-dredging surveys of trace metals and organic substances in Turkey Creek, Florida. Final report to the St. Johns River Water Management District. Available from the St. Johns River Water Management District. Palatka, FL. USA.

Trefry, JH, Trocine, RP & DW Woodall. 2007. Composition and sources of suspended matter in the Indian River Lagoon, Florida. Florida Sci. 70: 363-382.


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