|Shallow Water Habitats|
|...key features of Virginia's bays and estuaries|
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|Nereis succinea, a common nereid worm in the Chesapeake Bay. Photo: Jenny Dreyer, VIMS.|
|Hydroids, whelk, and anemone on bottom. Note worm holes, indicating the presence of benthic infauna. Photo: Linda Schaffner, VIMS.|
Macrofauna, also called macrobenthos, are invertebrates that live on or in sediment, or attached to hard substrates. A majority of recognized animal phyla (18 out of 34) have benthic macrofaunal representatives living in marine, estuarine or freshwater environments. Annelid worms, bivalves, gastropods, crustaceans, tunicates, and insect larvae are the most commonly encountered macrofauna in estuarine and coastal areas. They are usually collected using 0.5 or 1 mm mesh screens.
When macrofauna live within the substrate they are called infauna. Even though muddy sediment can be full of noxious or even poisonous compounds such as ammonium and sulfide, which form as a result of microbial processes, many aquatic invertebrates have evolved an infaunal lifestyle. Living within the sediment offers considerable protection from predators, so it becomes a tradeoff between adapting to the sedimentary environment or being eaten.
The infauna is comprised of annelids and bivalves, larval insects, phoronids, amphipod crustaceans, anthozoans, brittle stars and more. While many species of infauna are relatively sedentary, others burrow freely through soft sediment. Some species, such as the thin-shelled clam Macoma balthica, burrow tens of centimeters below the sediment surface as adults in order to avoid predation. However, even deep burrowers must maintain a connection to the sediment-water interface in order to obtain the oxygen they need for respiration from the overlying water.
Epibenthic macrobenthos (also called epifauna) live on or just above the substrate. They may be firmly attached (sessile), relatively sedentary, or highly motile. Barnacles, oysters, sponges, tunicates, entoprocts, gastropods, anthozoans, mud crabs, and certain species of amphipods are common representatives of the epibenthos.
Many sessile epibenthic species are vulnerable to predators as juveniles, but then grow hard shells for protection (oysters, barnacles), concentrate compounds that make them unpalatable (tunicates), or have stinging cells (anthozoans) that provide defense. Other epibenthic species are motile and live within the interstices of the substrate (Neanthes succinea), emerging to graze on trapped detritus, algae, or smaller invertebrates and retreating to avoid predation.
Most of the macrobenthos living in muddy sediments are deposit feeders. These animals ingest sediment and digest associated bacteria, microalgae and organic matter.
Filter-feeding macrofauna are also common in the benthos. They feed on suspended algae and detrital particles. Filter feeders are mostly sedentary and depend on water currents to deliver their food. They may be limited from areas with high sediment loading or sediment disturbance because high turbidity interferes with feeding. Other filter feeding taxa, such as annelids, tunicates and hydrozoans, can also be highly abundant in estuarine and coastal marine habitats.
Some macrofauna depend on benthic microalgae as a food source. A good example is the mud snail, which can sometimes be found in dense aggregations while grazing on mats of benthic microalgae in shallow waters.
Carnivores also abound in the benthos. Examples include the predatory blood worm, ribbon worms, some species of snails, and the well-known blue crab, Callinectes sapidus.
Macrofauna are important components of estuarine and coastal ecosystems, because they serve as critical links between a variety of primary producers and organic matter sources (e.g., phytoplankton, benthic microalgae and macroalgae, detritus) and economically, ecological, and recreationally important fish and crustaceans. It has been estimated that approximately 50% of the fish production in Chesapeake Bay is directly linked to a benthic food web. Some benthic macrofauna such as the hard shelled clam (Mercenaria mercenaria) and Eastern Oyster (Crassostrea virginica) are economically important species.
Communities of macrobenthos provide many ecosystem services that help to maintain good water and sediment quality. Filter feeders remove particles from the water column, which may result in enhanced water clarity. Given the importance of light in shallow water estuarine ecosystems, filter feeding may improve shallow water habitat for submerged aquatic plants and benthic microalgae.
Bioturbation (sediment mixing) of the bottom by infaunal macrobenthos has been show to enhance the degradation of some pollutants due to stimulation of microbial processes. The enhanced coupling of key nitrogen transformations in the presence of benthic macrofauna can lead to the production of nitrogen gas, which escapes to the atmosphere, thereby reducing nitrogen loading in the ecosystem.
Benthic macrofauna have been used for decades as indicators of environmental status and trends in estuaries and coastal areas because: (1) most infauna are sedentary and respond to local environmental impacts; (2) they encompass a wide range of physiological tolerances, living positions, feeding modes and trophic interactions;(3) assemblages respond relatively quickly to habitat disturbances and (4) they are important components of aquatic food webs and they affect transport and cycling of nutrients and toxicants. Benthic ecologists have developed a number of indicators of ecosystem health, such as the Benthic Index of Biotic Integrity (B-IBI), using macrofauna-based measures.
For further information about macrofauna, refer to the following: