Muddy sand shores
Image Joint Nature Conservation Committee - View across a cockle strand (biotope LMS.Pcer). Image width foreground ca 10 m.
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Ecological and functional relationships
Intertidal sand and mudflats are invaluable in supporting predator communities (Elliot et al., 1998). Large mobile epibenthic predators such as bottom-feeding fish, crabs and birds are important predators in marine soft-bottom communities and they are important regulators of species abundance (Ambrose, 1984). Exclosures set up on sandy and muddy flats in the Wadden Sea revealed that the removal of predation by shore crabs, shrimps, gobies, flatfish and birds led to a marked increase in the species diversity and the abundance of infaunal species (Reise, 1978).
The following ecological relationships are likely.
Mobile epifauna including crabs (e.g. Carcinus maenas) and shrimps (e.g. Crangon crangon) take small bivalves, polychaetes and crustacea. Carcinus maenas and Crangon crangon significantly reduce populations of Corophium volutator in estuaries, and Crangon crangon is a significant predator of small plaice during and immediately after the fish larval settlement.
The flatfish Solea solea (sole), Limanda limanda (dab), Platichythys flesus (flounder) and Pleuronectes platessa (plaice) feed on polychaetes and their tails (e.g. Arenicola and Nereis), young bivalves and their siphons (e.g. Macoma and Angulus) and tidally active crustacea such as Bathyporeia and Eurydice spp.. Gobies (e.g. Pomatoschistus spp.) prey heavily on Corophium volutator (Elliot et al., 1998). Within estuaries numerous demersal fish may be opportunistic predators (Costa & Elliot, 1991; Elliot et al., 1998).
Wildfowl feed on a variety of species, originally thought to be determined by depth of prey and bill size or shape. Recently however, waders are thought to be opportunistic feeders (McLusky, 1989). Arenicola marina reportedly provided 94% of the energy in the diet of the bar-tailed godwit (Limosa lapponica) at Lindisfarne on the north east coast of England (Baird et al., 1985). The bar-tailed godwit also feeds on large Hediste diversicolor (studied as Nereis diversicolor), which provided the main food source for this species on a reclaimed mudflat in the Tees (Evans et al., 1998). In the same area, Evans et al., (1998) reported that Hediste diversicolor was also an important component in the diet of the curlew (Numenius arquata) and grey plover (Pluvialis squatarola).
Shelducks (Tadorna tadorna) were found to feed primarily on small oligochaetes, however, copepods, Hydrobia sp. and Macoma sp. also form part of their diet (Evans et al., 1979). Eider ducks take Mytilus edulis in shallow water. Black-tailed godwit feeds mainly on Scrobicularia plana and small amounts of Nereis sp. and Hydrobia sp. (Elliot et al., 1998).
Where high densities of Hydrobia ulvae exist, the pearl bubble Retusa obtusa may also be found since Hydrobia ulvae represents an important part of its diet.
Many infauna are also important predators within marine soft-bottom communities. Polychaete worms are dominant infaunal predators that actively pursue prey and are generally opportunistic, although they have prey size preferences (Elliot et al., 1998). Nephtys sp. are usually considered to be carnivorous. However, Warwick et al. (1979) found that faecal pellets produced by Nephtys (collected fresh from the field) contained almost exclusively algal cells. They concluded that Nephtys was a ‘broad-spectrum’ omnivore and that plant material contributed about 90% of its annual production. At the sandier end of the mud-sand continuum, the speckled sea louse Eurydice pulchra is a highly predatory carnivore feeding on other infaunal invertebrates.
Nephtys sp. and Eurydice pulchra may also scavenge dead organic material. Note that the presence of various Nephtys sp. will vary along the mud-sand continuum with species such as Nephtys hombergii characteristic of muddier sediment while species including Nephtys cirrosa are more likely to be found in clean sand (Kendall, pers. comm.).
Hediste diversicolor is one of the most common intertidal estuarine polychaetes and is found in muddy habitats including sandy mud and muddy sand. It displays a variety of feeding methods and can be considered as a suspension feeder, deposit feeder, omnivore and scavenger (see MarLIN review). Tubificoid polychaetes (e.g. Tubificoides benedii) and spionid polychaetes (e.g. Pygospio elegans) are also abundant in muddier sands and all are important in the diets of wading birds (Kendall, pers. comm.).
Deposit feeding and filter feeding represent the two fundamental feeding methods among the fauna of mud and sand (Eltringham, 1971). Deposit feeders might include Corophium volutator and Arenicola marina, the former of which is also a filter / suspension feeder. Arenicola marina is a burrower and bioturbator, the activity of which can adversely effect Corophium volutator and the juveniles of various other species (see MarLIN review). Arenicola marina feeds on detritus and bacteria in the sediment.
Suspension feeders may include Macoma balthica and Cerastoderma edule, the former of which is also a deposit feeder, feeding on detritus and deposited plankton.
Meiofauna such as harpacticoid copepods are probably important consumers of microphytobenthos in this biotope and both larger epibenthic and shallow burrowing forms are common in fine sediments.
Seasonal and longer term change
The presence of algal mats of Ulva
sp. are likely in the summer months, and microphytobenthos colouration of the sediment surface will be more noticeable to summer.
Fish species, e.g. juvenile plaice, move offshore in autumn and winter avoiding low temperature and storm induced turbulence.
Storms have significant effects on the distribution and survival of infauna as well as the success of recruitment by newly settled spat or larvae (see Hall, 1994 for review). For example:
storm events can change sediment distribution and composition significantly e.g. the removal of the top 20 cm of sand has been reported (Dolphin et al., 1995);
storms may cause dramatic changes in distribution of macro-infauna by washing out dominant species, opening the sediment to recolonization by adults and/or available spat/larvae (Eagle, 1975; Rees et al., 1977; Hall, 1994);
storms are likely to have larger effects in shallow waters and wave induced disturbance is likely to contribute to gradients in faunal composition (Hall, 1994), e.g. Emerson & Grant (1991) demonstrated that bedload sediment transport due to storms, currents and tides had a significant effect on population density and recruitment in Mya arenaria, and
storms may also cause onshore strandings and, hence, mass mortalities of infaunal organisms e.g. Rees et al. (1977) reported stranding of several intertidal and sub-tidal species due to the storms of 1975-76 in Red Wharf Bay, Anglesey.
The percentage and composition of wildfowl varies with season. Several species over-winter in UK intertidal areas, and others pass through on migration routes. Feeding times vary with season, location, tide and species. However, most shorebirds forage at low tide or on rising tides.
In cold periods shore birds require additional energy for thermoregulation and greater foraging is required since prey are scarcer at the same time (Davidson & Rothwell, 1993).
Habitat structure and complexity
Biodiversity is influenced by the stability of the habitat and the sediment type, partly because the complexity of the habitat will determine the number of available niches (Elliot et al.
, 1998). For example, muddy sand will have a higher proportion of finer particles and a greater organic content, and therefore microbial population, than cleaner sand. The productivity of muddy sands relates (in part) to the small size of clay mineral particles and the massive surface area that they provide for microbial growth (Kendall, pers. comm.).
The productivity of tidal flats is dependant on the tidal range and shore slope. Gray (1981) reported the highest abundance and biomass of in-fauna occurring at the mid-tidal level, although mid tide level was more productive because there was little true low shore. Edwards et al. (1992) found that the muddy sand / gravel lower shore of the Gann Flat contained the highest number of species. In sandier places, the shore slope continues to the sublittoral (Kendall, pers. comm.). Towards the lower shore, current speeds increase near channels whereas higher on the shore, emergence and desiccation increase.
Additional complexity may result from the presence of rocks (pebbles, cobbles, boulders), that provide substrata for rocky shore species and macrophytes, and shell fragments that alter the porosity and available niches within the sediment.
Physical habitat complexity:
- Fine and silty sands reflect low energy conditions and are characterized by small median particle size, shallow slope, high water content due to low porosity (pore space is occupied by small silt particles packed between sand grains), and low permeability.
- Muddy sands retain water at low tide as a result of their shallow gradient and the capillary action of closely packed particles (Gray, 1981). Muddy sands tends to be more freely draining than mud alone due to their increased average particle size (Jones et al., 2000).
- Muddy sands have a high organic content resulting from settlement of organic detritus and growth of heterotrophic autotrophic micro-organisms. They also have a high microbial population and high sediment stability due to cohesion. The clay mineral particles provide a massive surface area for microbial growth (Kendall, pers. comm.). Allochthonous organic material is derived from anthropogenic activity (e.g. sewerage) and natural sources (e.g. plankton, detritus). Autochthonous organic material is formed by benthic microalgae (microphytobenthos e.g. diatoms and euglenoids) and heterotrophic micro-organism production. Although the surface is well oxygenated, poor oxygenation lower down in the muds results in low degradation rates and the accumulation of organic material.
- High levels of organic material support large microbial populations. The high oxygen demand of their activity, combined with the fact that much of the sediment is poorly oxygenated, means that much of the organic material undergoes anaerobic degradation releasing hydrogen sulphide, methane and ammonia, together with dissolved organic materials, which can be used by aerobic surface bacteria. Anaerobic degradation produces reducing conditions forming a 'black' layer, the depth of which depends on the depth to which oxygen can permeate (Elliot et al., 1998). Chemoautotrophs are present in the reducing layer and at depth (Libes, 1992).
- Microbial activity stabilizes organic flux in estuaries, reducing seasonal variation in productivity, cycling nutrients and making the primary production available to animal consumption ( McLusky, 1989; Elliot et al., 1998).
The food web within muddy sandy shores is reasonably well understood. Some key features that contribute to its complexity are presented below.
Factors affecting complexity
- The biomass of microbes may be of the same order of magnitude as the biomass of infauna (Elliot et al., 1998).
- The mucilaginous secretions of microphytobenthos and bacteria may stabilize the sediment. Microphytobenthos often appears as a subtle brown or greenish shading on the sediment surface. In shallow waters the biomass of microphytobenthos may exceed that of the pelagic phytoplankton (MacIntyre et al., 1996).
- The macrophyte community is invariably poorer than on rocky shores. However, fucoids can often be found on more complex LMS.MS shores with e.g. pebbles and rocks (Kendall, pers. comm.). Mats of Ulva sp. may also be found.
- In addition to increased macrophyte diversity on shores with coarser particles, barnacles, anemones and winkles may also be found (Kendall, pers. comm.).
- Muddy sands (LMS.MS) may have a lower diversity and biomass than mudflats but exhibit a higher diversity than sandflats. The infauna of mudflats often shows low species diversity but high biomass (depending on silt content). Sandflats, however, are free draining (subjected to desiccation) and less stable.
- Muddy sands support communities of amphipods, polychaetes, and molluscs (Elliot et al., 1998; Connor et al., 1997a).
- The heart urchin Echinocardium cordatum occurs in muddy and clean sand but grows more slowly in muddy sand (Buchanan, 1966).
- Several tidal migrants occur e.g. mysids, amphipods, decapods and epi-benthic fish.
- Liocarcinus depurator, Carcinus maenas, Atelecyclus rotundatus and Macropodia spp. are mobile species associated with silty sands.
- Many infauna are limited to the upper oxygenated layer, for example Abra sp., Phoronis sp. (horseshoe worms) and Venus sp. (Pearson & Eleftheriou, 1981). However, others penetrate deeper in irrigated burrows (such as the terebellid polychaete Pectinaria belagica and the capitellid worms Notomastus sp.) or extend their burrows upwards into the oxygenated layer (such as Lucinoma) (Pearson & Eleftheriou, 1981). In contrast, species such as the oligochaete Tubificoides benedii have a high capacity to tolerate anoxic conditions.
- Tidal elevation affects the distribution of fish, e.g. in summer, plaice populations are largest at the waters edge to a depth of 1-3 m and may migrate with the tide, with larger fish inhabiting greater depths (Gibson, 1973, cited in Elliot et al., 1998). The young of several fish migrate into the intertidal with the tide to feed (Elliot et al., 1998).
- Reduced salinity in estuaries will affect the communities present, e.g. with decreasing salinity (further into an estuary or in riverine inflow) Nereis diversicolor replaces Nephtys spp.
- The zonation of wildfowl on the shore is very dependent on the profile of the shore and if the shore is flat with creeks and channels, not much pattern is evident (Kendall, pers. comm.). Mohamed (1998) found that the density of waders on the shores of islands in Bahrain was largest in sites with a large dry intertidal area and a very gentle slope. The distribution of birds up the shore is also influenced by tidal level, with some birds feeding at the waters edge and other further up or down.
Physical forces are the dominant factors structuring the substratum in intertidal mud and sand flats, however, the effects of the interaction of the organisms inhabiting the substratum may modify it secondarily.
- Decreasing wave exposure is associated with finer sediments that, in turn, support more small-bodied surface dwelling species such as the small epibenthic crustacean Bathyporeia spp., which lives on the sediment surface and burrows quickly as the tide falls. Many sedentary polychaete species prefer stable sediments. Arenicola marina prefers more stable habitats because it cannot produce the large amounts of mucus that would be needed to stabilize its burrow in more fluid mobile sediment. It can be found on moderately exposed shores where it can burrow down to 40 cm to avoid the physically disturbing effects of wave action. Capitella capitata, in contrast, is more tolerant of mobile sediments. The sand digger shrimp Bathyporeia sarsi is found in both stable and unstable sites.
- Deposit feeders dominate over suspension feeders in areas with high percentages of silt.
- Competitive interactions can play a significant role in determining the temporal and spatial abundance of macrobenthos in muddy sand communities (Peterson, 1977). Organisms may compete for, for example, space and / or food and competitive exclusion may occur. Experimental manipulation revealed that the total abundance of three tube-building polychaetes negatively affected the abundance of a burrowing polychaete (Woodin, 1974). Within particular trophic guilds (feeding types), competition may result in resource partitioning, e.g. Hydrobia sp. and Corophium sp. ingest different size particles (Fenchel, 1972).
- The substratum characteristics may be modified by organisms. Spionid tubes and microphytobenthic mats, for example, may stabilize the sediment surface whereas excessive reworking of the sediment (bioturbation) by mobile infauna (e.g. Macoma balthica) may destabilize the sediment. Biosedimentation may increase supply of sediment from the water column, e.g. through the activity of suspension feeders such as Cerastoderma edule. Bioturbation by burrowing infauna such as Arenicola marina rework sediment bringing material and nutrients to the surface while allowing oxygenated water to reach deeper sediment (Elliot et al., 1998; see Hall, 1994 for review).
Allochthonous organic material is derived from anthropogenic activity (e.g. sewerage) and natural sources (e.g. plankton, detritus). Autochthonous organic material is formed by benthic microalgae (microphytobenthos e.g. diatoms and euglenoids) and heterotrophic micro-organism production. Organic material is degraded by micro-organisms and the nutrients recycled. The high surface area of fine particles provides a surface for microflora.
Microphytobenthos, water-column phytoplankton and deep sediment chemoautotrophs provide primary productivity to sediments although opportunistic algal mats of Ulva sp. may develop. However, photosynthesis is light limited in turbid conditions. Most macrofauna productivity is secondary, derived from detritus and organic material.
Intertidal mudflats in estuarine systems may have a higher productivity than subtidal sediments although coastal sandflats have very poor productivity (McLachlan, 1996, cited in Elliot et al., 1998).
Some macrofauna in LMS.MS breed several times in their life history (iteroparous) such as Arenicola marina
and Nephtys hombergii
while others, such as Hediste diversicolor
, are semelparous. Some species are planktonic spawners producing large numbers of gametes (depending on food availability) with fertilization in the water column. In these species, dispersal potential is high, although in sheltered bays the larvae may be entrapped, and recruitment is linked to the hydrographic regime with regards to dispersal. Small scale eddy's (e.g. over obstacles and inconsistencies in the surface of the substratum) may result in concentration of larvae or propagules. High densities of adults, suspension feeders and surface deposit feeders together with epibenthic predators and physical disturbance result in high post settlement mortality rate of larvae and juveniles (Olafsson et al.
, 1994). For many species, larval development is either direct (e.g. Corophium volutator
) or lecithotrophic (e.g. Hydrobia ulvae
and Hediste diversicolor
) (see MarLIN
reviews). Certainly for direct development, larval dispersal is limited as is the case for Corophium volutator
review) although compensated by significant adult mobility. Arenicola
larvae settle outside usual habitat preferences away from areas dominated by adults, although juvenile Nephtys
can migrate to more favourable areas. Overall recruitment is likely to be patchy and sporadic, with high spat fall occurring in areas devoid of adults, perhaps lost due to predation or storms. Similarly, larvae may be concentrated by the hydrographic regime or swept to neighbouring or removed sites.
Time for community to reach maturity
Little information was found concerning the development of muddy sand intertidal communities. However, one study in Massachusetts focused on the colonization of different sized defaunated plugs of sediment implanted into a mudflat (silty-sand, median grain size 63-125 µm) (Smith & Brumsickle, 1989). Postlarval immigration was found to be one of the most important factors in recolonization. In the small plugs (50 cm2), the abundance of the most dominant polychaete species had almost reached background levels 40 days after planting. Rates of colonization of the small plugs, faunal abundance and species numbers were also higher than in the large plugs (1750 cm2), partly because postlarval immigration was inversely proportional to patch size. However, the experiment was conducted in summer months when most of the abundant polychaetes were available for recruitment. This is a key point concerning community development and colonization and time taken for the community to develop may take significantly longer during other times of the year. Community development is likely to depend on the species present, the hydrographic regime (for example, stronger currents may resuspend and transport larvae and juveniles into the developing area) and recruitment. Colonization may occur through various routes including adult and postlarval migration, and larval settlement. Time taken to reach maturity will also vary spatially and temporally.
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This review can be cited as follows:
Tyler-Walters, H. & Marshall, C.
Muddy sand shores.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=21&code=2004>