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Big Bend Seagrasses Aquatic Preserve - Natural Communities Quick Links

Sinkhole Limestone Outcrop

Sinkholes are generally characterized as cylindrical or conical depressions with steep limestone walls. The vegetative structure of sinkholes may be that of a well-developed forest where sands cover the rock and/or the sides of the sinkholes are moderately sloped. These conditions are typically confined to the upper portions and around the rim of the sinkhole. Steeper rock walls are generally more or less covered by mosses, liverworts, and ferns with occasional herbs and shrubs. Sinkholes are extremely fragile communities. Their popularity as recreational areas subjects their flora to trampling and their steep walls to severe erosion from foot traffic and, in some cases, from dirt bikes. Sinkhole lakes attract swimmers and divers whose activities may disturb the aquatic community as well.

Shell Mound

Shell mound is unusual among the biological communities in that it is largely a result of the activities of Indians, instead of natural physical factors. Shell mound is generally characterized as an elevated mound of mollusk shells and aboriginal garbage on which a hardwood, closed-canopy forest develops. Shell mound soils are composed of shells and shell fragments with an organic component derived from forest litter. 

Alluvial Forest

Alluvial forest is a hardwood forest found in river floodplains on low levees, ridges and terraces that are slightly elevated above floodplain swamp and are regularly flooded for a portion of the growing season. The physical environment is greatly influenced by ongoing disturbances created by a fluctuating river bed which is both eroding and depositing substrates.

Floodplain Marsh

Floodplain marshes are wetlands of herbaceous vegetation and low shrubs that occur in river floodplains, mainly in Central Florida on sandy alluvial soils with considerable peat accumulation. The highest part of the marsh is often a drier, wet prairie-like zone with a large diversity of graminoids and forbs. While the progression from high to low marsh occurs generally from the upland edge to the river edge, these vegetation patches may also be scattered throughout the marsh, which provides a diversity of habitats beneficial to wildlife. Floodplain marshes are maintained by regimes of fire and water. Fires apparently burn on a one- to five-year basis under natural conditions and maintain the open herbaceous community by restricting shrub invasion, however, severe fires during drought periods will often burn the mucky peat. Floodplain marshes are flooded with flowing water for about 250 days annually.

Floodplain Swamp

Floodplain swamp is a closed-canopy forest of hydrophytic trees occurring on frequently or permanently flooded hydric soils adjacent to stream and river channels and in depressions and oxbows within floodplains. Trees are often buttressed, and the understory and groundcover are sparse. The canopy is sometimes a pure stand of bald cypress, but more commonly bald cypress shares dominance with one or more tupelo species.

Blackwater Stream

Blackwater streams are characterized as perennial or intermittent seasonal watercourses originating deep in sandy lowlands where extensive wetlands with organic soils function as reservoirs, collecting rainfall and discharging it slowly to the stream. The tea-colored waters of blackwater streams are laden with tannins, particulates, and dissolved organic matter and iron derived from drainage through swamps and marshes.

Alluvial Stream

Alluvial streams are characterized as perennial or intermittent seasonal watercourses originating in high uplands that are primarily composed of sandy clays and clayey-silty sands. Alluvial stream waters are typically turbid due to a high content of suspended particulates, including clays, silts, and sands, as well as detritus and other organic debris. Water temperatures may fluctuate substantially and are generally correlated with seasonal fluctuations in air temperature. Similarly, other water quality parameters vary substantially and generally fluctuate with seasonal rainfall patterns.

The most important characteristics of alluvial streams are the large range of flow rates and sediment loads encountered. Thus, water depth fluctuates substantially and is generally separated into two distinct stages, a normal or low flow stage and a flood or high flow stage. During the normal low flow stage the water is confined within the stream banks, while during flood stage the water overflows the banks and inundates the adjacent floodplain communities. Flood stages generally occur once or twice each year during winter or early spring and occasionally in summer.

Spring-Run Stream

Spring-run streams are perennial water courses which derive most, if not all, of their water from artesian openings in the underground aquifer. Waters issuing from the aquifer are generally clear, circumneutral to slightly alkaline (pH = 7.0 - 8.2), and perennially cool (66 – 75 °F). These conditions saturate the water with important minerals, allow light to penetrate deeply, and reduce the limiting effects of environmental fluctuations, all of which are conducive for plant growth. Thus, spring-run streams are among the most productive aquatic habitats. 

Aquatic Cave

Aquatic and terrestrial caves are characterized as cavities below the surface of the ground in karst areas of the state. The limestone aquifers that underlie the entire state of Florida could be considered vast aquatic cave communities. Troglobites (also called phreatobites) are organisms specially evolved to survive in deep cave habitats. The occasional observation of various species of troglobites in deep water wells from several regions in the state suggests that this community could be widespread. However, the dependence of troglobites on detrital inputs and other nutrients imported from the surface generally limits the distribution of well developed aquatic cave communities to karst areas with surface connections. Cave waters are generally clear, with deep water appearing bluish.

Consolidated Substrate

Marine and estuarine consolidated substrates are mineral based natural communities generally characterized as expansive, relatively open areas of subtidal, intertidal and supratidal zones which lack dense populations of sessile plant and animal species. Consolidated substrates are solidified rock or shell conglomerates and include coquina, limerock or relic reef materials. These communities may be sparsely inhabited by sessile, planktonic, epifaunal, and pelagic plants and animals but house few infaunal organisms (i.e., animals living within the substrate). The three kinds of consolidated substrate communities occurring in Florida are of limited distribution. The type present along the Big Bend is limerock substrates which occur as outcrops of bedded sedimentary deposits consisting primarily of calcium carbonate.

Unconsolidated Substrate

Marine and estuarine unconsolidated substrates are mineral based natural communities generally characterized as expansive, relatively open areas of subtidal, intertidal, and supratidal zones which lack dense populations of sessile plant and animal species. Unconsolidated substrates are unsolidified material and include coralgal, marl, mud, mud/sand, sand or shell. While these areas may seem relatively barren, the densities of infaunal organisms in subtidal zones can reach the tens of thousands per meter square, making these areas important feeding grounds for many bottom feeding fish, such as red drum (redfish), southern flounder, spot and sheepshead . The intertidal and supratidal zones are extremely important feeding grounds for many shorebirds and invertebrates.

Unconsolidated substrates are important in that they form the foundation for the development of other marine and estuarine natural communities when conditions become appropriate. Unconsolidated substrate communities are associated with and often grade into beach dunes, salt marshes, mangrove swamps, seagrass beds, coral reefs, mollusk reefs, worm reefs, octocoral beds, sponge beds, and algal beds.

Unconsolidated substrate communities which are composed chiefly of sand (e.g., sand beaches) are the most important recreational areas in Florida, attracting millions of residents and tourists annually. This community is resilient and may recover from recreational disturbances. However, this community is vulnerable to compaction associated with vehicular traffic on beaches and disturbances from dredging activities and low dissolved oxygen levels, all of which can cause infaunal organisms to be destroyed or to migrate out of the area. Generally these areas are easily recolonized either by the same organisms or a series of organisms which eventually results in the community returning to its original state once the disturbance has ceased.

Mollusk Reef

Marine and estuarine mollusk reefs are faunal based natural communities typically characterized as expansive concentrations of sessile mollusks occurring in intertidal and subtidal zones to a depth of 40 feet. In Florida, the most developed mollusk reefs are generally restricted to estuarine areas and are dominated by the Eastern oyster. Mollusk reefs that are exposed during low tides are frequented by a multitude of shorebirds, wading birds, raccoons and other vertebrates. One of the United States' largest wintering populations of American oystercatchers is situated in the heart of the Cedar Keys. The success of this rookery can be attributed to the oyster reefs located here, which are an excellent and tremendously important food source.

In the Big Bend region, research has shown a 66 percent net loss of oyster bar area between 1982 and 2011. This rapid loss is very likely due to a departure from historical norms, and stems from multiple factors. Extended periods of high salinity are likely stressors of oyster populations, particularly on offshore bars, to the extent that the physical structure of bars are affected by both mortality of older oysters, and the loss of significant recruitment. Once the structure of bars is weakened, bars became less resilient to wave action, particularly during storm events. Evidence suggest that the primary mechanism is reduced survival and recruitment as a result of decreased freshwater inputs, thus causing existing bars to be vulnerable to wave action and sea level rise; once bar substrate becomes unconsolidated, the breakdown of the bar may not be reversible.

Oystercatchers on oyster bar

Oystercatchers loafing on an oyster bar.

Octocoral Bed

Marine and estuarine octocoral beds are soft faunal based natural communities characterized as large populations of sessile invertebrates of the Class Anthozoa, Subclass Octocorallia, Orders Gorgonacea and Pennatulacea. The dominant animal species are soft corals such as gorgonians, sea fans, sea feathers and sea plumes, sea fingers, sea pansies, sea rods, and sea whips.

Sponge Bed

Marine and estuarine sponge beds are soft faunal based natural communities characterized as dense populations of sessile invertebrates of the phylum Porifera, Class Demospongiae. Sponge beds require hard bottom (consolidated) substrate (i.e., coquina, limerock, relic reefs) on which to anchor. Hard bottom substrate occurs sparsely throughout Florida in marine and estuarine areas; however, sponges prefer the warmer waters of the southern portion of the state, significantly limiting the distribution severely.

Algal Bed

Marine and estuarine algal beds are floral based natural communities characterized as large populations of nondrift macro or micro algae. Vascular plants (e.g., seagrasses) may occur in algal beds associated with soft bottoms. Sessile animals associated with algal beds will vary based on bottom type. For algal beds associated with hard bottom substrate (lithophytic), faunal populations will be similar to populations associated with octocoral beds and sponge beds. Those associated with soft bottom substrate (psammophytic) may have similar benthic and pelagic species in addition to infauna species. Recent research has shown that algal beds provide critical habitat for juvenile spiny lobsters, a species of great commercial importance.

Distribution information for algal beds is lacking. The location of major beds must be determined before this natural community can be managed adequately. The primary threat to algal beds are dredging and filling activities which physically remove or bury the beds. Other damage occurs from increased turbidity in the water column which reduces available light; pollution, particularly from oil spills; and damage from boats.

Seagrass Bed

Seagrass beds are plant-based natural communities typically characterized as expansive stands of vascular plants. This community occurs in subtidal (rarely intertidal) zones, in clear, coastal waters where wave energy is moderate. Seagrasses are not true grasses. The three most common species of seagrasses in Florida are turtle grass, manatee grass, and shoal grass. Nearly pure stands of any one of these species can occur, but mixed stands are also common. Attached to the seagrass leaf blades are numerous species of epiphytic algae and invertebrates. Together, seagrasses and their epiphytes serve as important food sources for manatees, marine turtles, and many fish. The dense seagrasses also serve as shelter or nursery grounds for many marine invertebrates, and fish. Marine and estuarine seagrass beds occur most frequently on unconsolidated substrates of marl, muck or sand, although they may also occur on other unconsolidated substrates. The dense blanket of leaf blades reduces the wave-energy on the bottom and promotes settling of suspended particulates. The dense roots and rhizomes of the seagrasses stabilize settled particles. Thus, marine and estuarine seagrass beds are generally areas of soil accumulation. 

Seagrass beds are extremely vulnerable to human impacts. Many have been destroyed through dredging and filling activities or have been damaged by sewage outfalls and industrial wastes. In these instances, the seagrasses are either physically destroyed or succumb as a result of decreased solar radiation resulting from increased water turbidity. Seagrass beds are also highly vulnerable to oil spills. Low concentrations of oil are known to greatly reduce the ability of seagrasses to photosynthesize. Extreme high temperatures also have adverse impacts on seagrass beds. The area surrounding power plant outfalls, where water temperatures may exceed 95 degrees F has been found to be lethal to seagrasses. Seagrass beds are susceptible to long term scarring cuts from boat propellers, anchors and trawls. Such gouges may require many years to become revegetated. When protected from disturbances, seagrasses have the ability to regenerate and recolonize areas. Additionally, some successful replantings of seagrass beds have been conducted. However, the best management is to preserve and protect seagrass beds in their natural state.

Salt Marsh

Salt marsh is a largely herbaceous community that occurs in the portion of the coastal zone affected by tides and seawater and protected from large waves, either by the broad, gently sloping topography of the shore, by a barrier island, or by location along a bay or estuary. The width of the intertidal zone depends on the slope of the shore and the tidal range. Salt marsh may have distinct zones of vegetation; each dominated by a single species of grass or rush.

Tidal fluctuation is the most important ecological factor in salt marsh communities, cycling nutrients and allowing marine and estuarine fauna access to the marsh. This exchange helps to make salt marsh one of the most biologically productive natural communities in the world. Salt marshes are also extremely important because of their storm buffering capacity and their pollutant filtering actions. The dense roots and stems hold the unstabilized soils together, reducing the impact of storm wave surge. The plants, animals, and soils filter, absorb, and neutralize many pollutants before they can reach adjacent marine and estuarine communities. These factors make salt marshes extremely valuable as a natural community.

Mangrove Swamp

Mangrove swamp is a dense forest occurring along relatively flat, low wave energy, marine and estuarine shorelines. The dominant plants of mangrove swamp are red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans), white mangrove (Laguncularia racemosa) and buttonwood (Conocarpus erectus). Red mangrove often dominates the lowest (or deep-water) zone, followed by black mangrove in the intermediate zone, and white mangrove and buttonwood in the highest, least tidally-influenced zone. The density and height of mangroves and the diversity of associated herbaceous species can vary considerably within a mangrove swamp. Mangroves are vulnerable to cold, with red mangroves the least resistant to freezes.

Composite Substrate

Composite substrates consist of a combination of natural communities such as "beds" of algae and seagrasses or areas with small patches of consolidated and unconsolidated bottom with or without sessile floral and faunal populations. Composite substrates may be dominated by any combination of marine and estuarine sessile flora or fauna, or mineral substrate type. Typical combinations of plants, animals and substrates representing composite substrates include soft and stony corals with sponges on a hard bottom such as a limerock outcrop; algae and seagrasses scattered over a sand bottom; and patch reefs throughout a coralgal bottom. Any of the remaining marine and estuarine natural communities can grade into composite substrate communities.

Big Bend Seagrasses Aquatic Preserve

Last updated: November 24, 2015

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