3.2.5 Intertidal Flats
Description and Distribution
This section is intended to briefly summarize the most important aspects of tidal flats which pertain directly to their function as essential fish habitat. For a more extensive and comprehensive ecological profile of tidal flats in the South Atlantic region we recommend the U.S. Department of Interior Community Profile, Peterson and Peterson (1979).
Intertidal flats are the unvegetated bottoms of estuaries and sounds that lie between the high and low tide lines. These flats occur along mainland or barrier island shorelines or can emerge in areas unconnected to dry land. Intertidal flats are most extensive where tidal range is greatest, such as near inlets and in the southern portion of the coast. Because the influence of lunar tides is minimal in the large sounds (e.g., Pamlico, Albemarle, and Currituck), true intertidal flats are not extensive, except for the area immediately adjacent to inlets (Peterson and Peterson 1979). Sediment composition on intertidal shorelines tends to shift from coarser, sandy sediment on higher portions of the shoreline, with greater wave energy, to finer, muddier sediments in the lower portion of the shoreline, with relatively less wave energy (Peterson and Peterson 1979). Conditions on intertidal flats are physically stressful for associated marine organisms. Drastic fluctuations in salinity, water and air temperature (in addition to air and wind exposure) occur during each tidal cycle. Due to physiological restraints and limited water depth, some mobile organisms are restricted to deeper waters or adjacent habitats to avoid the stressful extremes associated with low tide. However, the sediment provides a buffer from changes in temperature and salinity in the water column for benthic infauna (Peterson and Peterson 1979).
Variability in the tidal regime along the South Atlantic coast results in considerable regional variability in the distribution and character of the estimated 1 million acres (Field et al. 1991) of tidal flat habitat. Geographic patterns in sediment size on tidal flats result primarily from the interaction of tidal currents and wind energy. The coasts of North Carolina and Florida are largely microtidal (0 - 2m tidal range). In these areas wind energy has a strong affect on intertidal flats. The northern North Carolina outer banks have extensive barrier islands and relatively few inlets to extensive sound systems. South of Cape Lookout, flood-tide deltas (remnant and active) are more frequent and tidal inlets and influences on intertidal flat distribution is greater. In contrast, the coasts of South Carolina and Georgia are mesotidal (2 - 3.3m) with short barrier islands and numerous tidal inlets so that tidal currents are the primary force effecting the intertidal. In both types of systems, the substrate of the intertidal flats generally becomes finer with distance from inlets due to the progressive damping of tidal currents and wave energy in the upstream direction. Exposure of flats to wave energy, which resuspends fine particles, may cause the development of sand flats in areas where the wind fetch is sufficient for the development of significant wave energy. On the microtidal coast of North Carolina sandy flats tend to develop due to the large size of the sounds and their orientation relative to prevailing winds. In contrast in Georgia and South Carolina most flats are muddy, as the sounds and estuaries are small so that the importance of wave energy is reduced. These different depositional environments result in development of varied physico-chemical environments in and on intertidal flats which in turn cause differences in the animal populations that utilize them.
Ecological Role and Function
Intertidal flats play an important role in the ecological function of South Atlantic estuarine ecosystems, particularly in regard to primary production, secondary production and water quality. Although intertidal flats are usually classified as “unvegetated,” there is actually an extremely productive microalgal community occupying the surface sediments (MacIntrye et al., 1996). The benthic microalgal community of tidal flats consists of benthic diatoms, cyanobacteria, euglenophytes and unicellular algae. Primary production of this community can equal or exceed phytoplankton primary production in the water column, and can represent a significant portion of overall estuarine primary productivity (Pinckney and Zingmark 1993; Buzzelli et al. 1999). Benthic microalgae are resuspended into the water column and transported throughout the estuary, sometimes representing over half of the chlorophyll in the water column (de Jonge and van Beusekom 1995; Tester et al. 1995). Benthic microalgae also stabilize sediments and control fluxes of nutrients (nitrogen and phosphorus) between the sediment and the water column.
Autochthonous benthic microalgal and bacterial production and imported primary production in the form of phytoplankton and detritus support diverse and highly productive populations of infaunal and epibenthic animals. The primary factors controlling production by microalgae occupying these sediments include the amount of light they receive, community biomass (chl a), and temperature (Pinckney and Zingmark, 1993; Barranguet et al., 1998; Guarini et al., 2000).
Important benthic animals in and on the sediments include ciliates, rotifers, nematodes, copepods, annelids, amphipods, bivalves and gastropods. This resident benthos is preyed upon by mobile predators that move onto the flats with the flood tide. These predators do not always kill their benthic prey and many ―nip‖ appendages of buried animals such as clam siphons and polychaete tentacles that can be regenerated. An important aspect of the function of these systems is the regular ebb and flood of the tide over the flats and the corresponding rhythm that exists among animals and microalgae adapted to life in the intertidal zone. The flooding tide brings food and predators onto the flat while the ebb provides residents a temporal refuge from the mobile predators.
This constantly changing system provides the following ecological functions: 1) nursery grounds for early stages of development of many benthically oriented estuarine dependent species; 2) refuges and feeding grounds for a variety of forage species and juvenile fishes; 3) significant trophic support to fish and shellfish, including oysters and clams (Riera and Richard 1996; Kreeger et al. 1997; Sullivan and Currin 2000; Page and Lastra 2003; Currin et al. 2003); 4) stabilization of sediments via the production of exopolymers (Yallop et al. 1994, 2000) and 5) modulation of sedimentary nutrient fluxes (Miller et al. 1996; Cerco and Seitzinger 1997; Sundback et al. 1991). Although it is recognized that tidal flats provide these important ecological functions, the relative contribution of intertidal flats of different types and in different locations within coastal systems is not well known.
Intertidal flats also provide habitat for a large and diverse community of infauna and epifauna, which in turn may become prey for transient fish species utilizing the intertidal flat. The faunal communities associated with intertidal flats will be described below.
Coastal development and human activities can have direct and dramatic impacts on tidal flats, although subtler impacts may occur from activities that alter current patterns, wave energy or the supply of sediment. Examples of direct impacts include on-site dredging and contaminant spills. Indirect impacts include dredging that significantly alters current patterns, dam construction that traps sediment, beach re-nourishment projects and jetty construction.
Although intertidal flats are protected by the same permitting process that regulates activities impacting other estuarine habitats, the perception that flats are of minor importance relative to vegetated habitats increases pressure on intertidal flats. Flats have the same legal protection afforded vegetated intertidal areas, however; the importance of intertidal flats is not generally recognized and the relative value of intertidal flats is not understood. As a consequence, permits may be more easily granted for filling/dredging tidal flats than for salt marshes and salt marsh may be planted on a natural intertidal flat when mitigation for marsh destruction is required. Increased recognition of the ecological value of tidal flats by resource managers and permitting agencies is necessary to preserve these valuable habitats, and research on the different types of intertidal flats and their relative value in coastal systems should be encouraged.
Species composition and community structure
Both plankton and benthic feeding herbivorous fish are found in abundance on intertidal flats. Schools of baitfish, small pelagic fish that tend to group together, are common over subtidal soft bottom and very abundant on shallow intertidal flats. These baitfish, such as anchovies, killifish, and menhaden, feed on the abundant supply of phyto- and zooplankton in the water column, but also consume resuspended benthic algae, microfauna, and meiofauna (Peterson and Peterson 1979). Although the majority of detritivores of the soft bottom habitat are invertebrates, striped mullet, white mullet, and pinfish also feed on detritus on subtidal bottom and intertidal flats. Other fish species use detritus as an alternate food source when preferred items are not available.
Most fish that forage on soft bottom are predaceous. Predators of benthic invertebrates include juveniles and adults of the following species (Peterson and Peterson 1979; Bain 1997):
∙ rays and skates,
∙ flatfish (southern flounder, summer flounder, hogchoker, tonguefish),
∙ several species of drum (spot, Atlantic croaker, red drum, kingfishes, silver perch),
∙ Florida pompano,
∙ sea robins,
∙ gobies, and
∙ shortnose and Atlantic sturgeons.
The compressed body forms of flatfish, rays, and skates assist in prey acquisition and predator avoidance on shallow intertidal flats (Peterson and Peterson 1979). For example, flounder forage on shallow flats by laying still, by concealing themselves under a thin layer of sediment, or by changing skin color. Small flatfish, including the bay whiff, fringed flounder, hogchoker, and tonguefish, feed mostly on copepods, amphipods, mysids, polychaetes, mollusks, and small fish. By way of comparison, summer and southern flounder primarily consume fish, such as silversides and anchovies, as well as shrimp and crabs, small mollusks, annelids, and amphipods (Peterson and Peterson 1979). Various rays excavate large pits while feeding, creating slightly deeper pockets of water that other fish and invertebrates use as refuge. Mollusks, annelids, crustaceans, and fish comprise the typical diet of rays.
To avoid predation, small fish commonly feed on open, unvegetated bottom at night and hide near structure during the day (Peterson and Peterson 1979). Larger predators that feed on smaller, benthic-feeding fish and invertebrates typically move onto the flats during high water to feed on schools of fish. These predators include sharks (sandbar, dusky, smooth dogfish, spiny dogfish, Atlantic sharpnose, scalloped hammerhead), drum (weakfish, spotted seatrout), striped bass, and estuarine dependent reef fish (black sea bass, gag grouper, sand perch, sheepshead) (Peterson and Peterson 1979; Thorpe et al. 2003).
Due to their size and shape, small baitfish and flat bodied rays, skates, and flounders have a feeding advantage over other fish in that they can forage on intertidal flats for greater amounts of time than larger fish. These fish groups are considered to be most characteristic of intertidal flats and would be most affected by habitat degradation and loss of intertidal flats from dredging, filling, bulkheading, or other anthropogenic causes (Peterson and Peterson 1979).
Fish species and age composition over soft bottom vary seasonally. Baitfish are present on shallow flats throughout the year. In the spring, large schools of baitfish are joined by juvenile fish that were spawned offshore in the winter (spot, Atlantic croaker, menhaden). In the summer, these species remain abundant on shallow unvegetated bottom; flatfish and rays also appear at this time. By fall, fish species diversity is at a maximum since summer residents and fall migrants are both present. Migratory fish feeding on the soft bottom include bluefish, striped mullet, kingfish, spotted seatrout, red drum, and many others (Peterson and Peterson 1979).
Intertidal Flats as Essential Fish Habitat
Benthic Nursery Function
Many species whose larval stages are planktonic but are benthically oriented as juveniles utilize intertidal flats as primary nursery ground. Intertidal flats are particularly suited for animals to make the shift from a pelagic to benthic existence. During this habitat shift these small animals are expected to be particularly vulnerable to adverse physical forces, predation and starvation, and flats may provide a relatively low energy environment where predation pressure is low and small benthic prey abundant. These animals may develop a tidal rhythm of behavior and move off and on the flat with the ebb and flood of the tide. This provides them an area of retention as currents over the flats are reduced, a refuge from a variety of predators due to the shallow water and excellent feeding conditions as the abundant meiofauna emerge to feed with the flooding tide. A wide variety of important fishes and invertebrates utilize intertidal flats as nurseries (Table 3-2.7) including the commercially important paralichthid flounders, many members of the drum family including red drum, and spotted seatrout, the mullets, gray snapper, the blue crab, and penaeid shrimps.
Table 3.2-7. Species‘ utilization of intertidal flats.
Letter codes for function use are N=benthic nursery function, R=refuge function, and F=feeding ground function. Life stage codes are PL=post-larval, J=juvenile, and A=adult.
A variety of pelagic and benthic species utilize the intertidal flats as a refuge from predation and adverse physical conditions (Table 3.2-7). Predation pressure in the subtidal, particularly in the vicinity of inlets may increase during the rising tide due to the influx of coastal predators. Intertidal flats provide energetic advantages for animals seeking to maintain their position within the system as current velocities are generally low relative to deeper areas. Schools of planktivores including anchovies, silversides and menhaden and schools of benthic feeding juveniles such as the spot and croaker, pinfish and mojarras, move onto flats with the rising tide to take advantage of the favorable conditions flats provide. More solitary species such as black seabass and gag grouper also appear to utilize flats as a refuge during their emigration from structured estuarine nursery habitats to the sea in the fall. Flats also can provide a refuge from low oxygen levels that may develop in deeper areas of estuaries during summer months.
Feeding Ground Function
Several groups of specialized feeders utilize intertidal flats as feeding grounds (Table 3.2-7). The depositional nature of intertidal flats provide a rich feeding ground for detritivores such as mullet and predators of small benthic invertebrates such as spot and mojarra. A variety of invertebrate predators such as whelks and blue crabs feed on tidal flats as do their bivalve prey such as oysters and hard clams, important filter feeding residents of tidal flats. Another group that relies on flats as feeding grounds is predatory fishes such as rays, a wide variety of flatfishes and lizard fish whose form makes them well adapted to feed in shallow water. Other more conventionally shaped fishes whose prey concentrate on flats use these areas as feeding grounds and red drum can be found hunting blue crabs on flats. Because flats are ―dry‖ much of the time activity is concentrated during high water making tidal flats rich feeding grounds for species adapted to shallow waters.