Salt marsh mosquitoes (Aedes taeniorhynchus and A. sollicitans) are nuisance species that affect the health of both humans and domestic animals. They do not reproduce by laying their eggs in standing water. Rather, they deposit eggs in the moist soils of high marsh above the water line in tidal wetlands. Eggs will remain dormant, often for long periods of time, until water levels rise in response to rains or tides.
Mosquito control impoundments are areas of salt marsh or mangrove forest that have been diked to allow control of water levels for mosquito mitigation. Within the dikes, perimeter ditches are flooded artificially in order to control breeding and reproduction of salt marsh mosquitoes without the use of pesticides.
Today, 192 impoundments are active on the east coast of Florida. Many of these areas are closed systems with wide variations in salinity. This, along with the flooding process, can cause dieback of natural vegetation and establishment of species that thrive in lower-salinity conditions. 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 declared extinct in 1987.
Before mosquito control methods like impoundments were in place, mosquito landings were among the highest densities ever recorded in the continental United States, reaching 500 per person per minute in some areas of Florida.
Concerted efforts aimed at controlling salt marsh mosquitoes in the Indian River Lagoon began in the mid-1920s with construction of miles of hand-dug, parallel ditches. But as the ditches required much maintenance and the tides were of low amplitude, little mosquito control was achieved.
In the 1930s, field experiments demonstrated that controlling water levels through impoundment could reduce mosquito populations by controlling reproduction. However, problematic water losses due to seepage and evaporation led to the abandonment of the impoundment strategy in favor of pesticides such as DDT. By the 1950s, concerns over pesticide resistance in insects began to emerge, and the focus of mosquito control again shifted back to source reduction.
The first impoundments in Florida were built in Brevard County in 1954, with other counties soon following. By the 1970s, more than 40,000 acres of Florida's coastal wetlands had been impounded—an area roughly equivalent to the land area of Liechtenstein. The majority of impoundments were constructed at the mean high-water level and then flooded year-round, which closed them off from adjacent estuarine waters. Others were allowed to drain during the winter months, but were flooded again as mosquito breeding season approached.
In the 1960s, in an effort to reduce impoundment impacts, natural resources managers experimented with seasonal flooding during peak mosquito breeding season. The rest of the year, dike culverts remained open to allow natural tidal fluctuation and flushing.
In 1974, seasonal impoundment was combined with active water management. Allowing tides to flush impoundments had several positive effects: continued control of salt marsh mosquitoes, retention of black mangroves and other vegetation, and the return of juvenile fishes to nursery areas unavailable to them in closed impoundments. This management strategy is currently referred to as Rotational Impoundment Management (RIM).
RIM has proven to be an effective strategy for controlling mosquitoes while minimizing serious environmental impacts to estuaries. Estuaries retain many of their natural functions, and their primary productivity can rival that of unaltered wetlands.
Culverts remain open between the impoundment and the estuary from October to May, and allow water exchange and use of impoundments by transient fish species and invertebrates. In summer, culverts are closed and impoundments flooded to the minimum levels needed to prevent egg laying by salt marsh mosquitoes. Low areas of the surrounding dike, called spillways, ensure that water levels do not exceed prescribed levels, thus preventing injury to vegetation.
RIM is currently the most commonly employed management strategy in three of the five counties adjacent to the Indian River Lagoon. Combined, St. Lucie, Brevard and Indian River counties manage nearly 6,400 acres of impoundments under this strategy.
While only a thin film of water is enough to prevent salt marsh mosquitoes from laying eggs, impoundments are typically flooded to depths of 6 to 20 inches (15 to 50 cm) above the ground surface to compensate for evaporation effects. In closed impoundments, this practice eliminated some species such as saltwort (Batis maritima), and glasswort (Salicornia bigelovii, and S. virginica). And though black mangroves’ pneumatophores enable aeration of roots during short periods of flooding, the short structures cannot withstand prolonged and deep flooding.
Closed impoundment significantly impacts soil chemistry and water quality. In soils, oxygen concentrations can decrease, while nitrogen and sulfide concentrations rise. Water effects are myriad. Some impoundments were subject to hypersaline conditions when estuarine waters were pumped in to flood them during warm summer months. Because these impoundments were closed to adjacent waters, lack of flushing and evaporation resulted in extremely high salinities, which caused local extinctions of some species. In other impoundments flooded with artesian well water, ecological turnover resulted, shifting from halophytic to freshwater communities.
Excessive freshwater flows from storms, as well as runoff from agricultural and developed areas, can cause extreme salinity fluctuations in the Indian River Lagoon estuary. Continuous exposure to lower salinity can deplete populations of shallow burrowing organisms, resulting in damaging effects on food web dynamics.
Several fish species have been greatly impacted by closed impoundments, particularly those that rely on mangrove or salt marsh for nursery grounds. Important commercial and recreational fisheries have also experienced declines, including tarpon, ladyfish, common snook and mullet. Marine invertebrates were also impacted by isolation of impounded wetlands, with biodiversity and species abundance becoming more characteristic of freshwater wetlands than marine or estuarine wetlands in some areas.
In unaltered systems, nutrients from mangrove leaf fall, which are decomposed into particulate and dissolved forms, are utilized in a variety of ways by many different organisms as mangroves are flushed by tides. In closed impoundments, natural patterns of nutrient flow between mangrove areas and adjacent waters are interrupted. Lacking the connection to estuarine waters, nutrients are never flushed from mangrove areas and remain confined within impoundments.