Intertidal Mud and Sandflats

Subtidal Mobile Sandbanks

The previous section details the components and structure of the communities found in these biotope complexes. However, it is emphasised that these communities are relatively poor in diversity but that, especially in the case of intertidal mud and sandflats, they have high abundances. Because of this, it is necessary to detail the functioning of the ecosystems and the support by prey for predator populations. Important predator populations in the intertidal and subtidal sedimentary areas include bird and fish species. In addition to the effects of other major biotic and environmental factors, the size of the any productive will area affect its carrying capacity in supporting wading birds (Meire, 1993) and fish.

Intertidal mud flats are important in the functioning of estuarine systems and may have a disproportionately high productivity compared to subtidal areas (Elliott & Taylor, 1989b). Conversely, coastal sandflats have a very poor productivity (McLachlan, 1996). Epifaunal organisms associated with these biotope complexes are predominantly mobile predatory species such as crabs e.g. Carcinus maenus and shrimps e.g. Crangon crangon, which take infaunal populations of small bivalves, polychaetes and crustacea. Organisms associated with silty sands are predominantly mobile species, including the crabs Liocarcinus depurator, Atelecyclus rotundatus and Macropodia spp.

Carcinus maenus has been shown to significantly reduce the numbers of Manayunkia aesturina on mudflats (McLusky, 1989) and Carcinus and Crangon may reduce the population of Corophium volutator in estuaries by over 50% (Pihl, 1985). The shrimp Crangon crangon is a significant predator of the smallest sizes of plaice during and immediately after the fish settle on sandy beaches when predation rate is strongly dependent on the size of both the predator and the prey (Gibson et al, 1995). This size dependency is caused principally by the superior escape capabilities of larger fish once captured rather than differences in the ability of different sizes of shrimps to capture prey.

Polychaete worms are dominant predators within the substratum and tend to be opportunistic and actively pursue prey (although they may have size preferences); their numbers may be closely related to those of their prey which includes other worms and crustaceans (Meire et al, 1994). Many infaunal species also scavenge e.g. Nephtys and the isopod Eurydice pulchra and quantity of food input determines the density of scavengers (Hayward, 1994, Ansell et al, 1972). Scavengers may dominate on coarse steep shores and are found in high numbers near kelp beds where there is a large amount of macrodebris (McLachlan, 1983).

Intertidal areas are well-defined as juvenile fish feeding areas (Costa & Elliott, 1991). Mud and sandflats are important nursery areas for plaice (Lockwood, 1972; Marshall, 1995; Marshall & Elliott, 1997), as well as feeding areas for sea bass and flounder (Elliott & Taylor, 1989). Fish such as dover sole, Solea solea and gadoids frequent sandy areas, but many also occur on coarser and mixed grades of sediment. Smaller fish (e.g. plaice) may settle on mudflats while larger fish are found on sandflats (Gibson, 1973; Gibson & Robb, 1992). Migratory species such as salmon and shad can also be found in these areas on passage to other wetlands, e.g. saltmarshes and freshwater areas, although they appear to have no requirement for the mud and sandflats.

The most important marine predators on intertidal sand and mudflats are particularly the flatfish Solea solea (sole), Limanda limanda (dab), Platichthys flesus (flounder) and Pleuronectes platessa (plaice) which feed on polychaetes and their tails (e.g. of Arenicola and Nereis), bivalve young and siphons (e.g. of Macoma and Angulus) and tidally active crustaceans such as Bathyporeia and Eurydice species (Croker & Hatfield, 1980; McDermott, 1983; McLachlan, 1983; Zwarts et al, 1985). In summer, large numbers of plaice and dab juveniles move over flats at high tide to feed on mobile epifauna, sedentary infauna and protruding siphons and tentacles (Elliott & Taylor 1989a). Within estuaries and on mud and sandflats, however, many demersal fish are opportunistic predators and the prey choice will reflect the infaunal species distribution of the area (Marshall & Elliott, submitted; Costa & Elliott, 1991).

Flatfish use several feeding strategies in estuarine areas with plaice and flounder using tidal migration feeding only at high tide on the intertidal flats. Dab and sole do not migrate tidally and feed continuously in the subtidal areas (McLusky, 1989). Gobies e.g. Pomatoschistus spp. are another important predator on mudflats and prey heavily on Corophium volutator and they have a significant impact as both predator and prey in estuarine ecosystems. Small juveniles (e.g. plaice) settling on fine sediments are less likely to be predated than those that are settled but do not bury in coarser sediments (Gibson & Robb, 1992).

Tidal elevation influences population size in fish, for example, plaice populations are largest at the water’s edge at a depth of 1-2m suggesting that they migrate with the tide up and down shore (Gibson, 1973). The young of many species, such as plaice, enter the intertidal zone to feed as the tide floods. There is also a relationship between the size of the fish and depth (for plaice specifically), as the body length increases, the depth of the water that the fish inhabits increases (Gibson et al, 1995). Depth and salinity may also influence flounder distribution (Armstrong, 1997).

These biotope complexes are used by important wintering and passage birds for feeding and roosting.

Figure - a simplified food web of a depositing shore

Shorebirds form important predators on NW European intertidal mud and sandflats during long migrations over long distances from breeding to wintering grounds. Particularly dependent species are brent geese, shelduck, pintail, oystercatcher, ringed plover, grey plover, bar-tailed and black-tailed godwits, curlew, redshank, knot, dunlin and sanderling, whilst grey geese and whooper swan may use this habitat for roosting (Jones & Key, 1989; Davidson et al, 1991). Bird communities are highly mobile and usually exhibit patterns of activity related to tidal water movements. However, the carrying capacity, in terms of space or food, of these biotope complexes has not been determined. The carrying capacity is reached when every new individual entering the habitat causes emigration or death of another bird (Goss Custard, 1984).

Where mudflats occur within estuaries, any change in salinity will affect their prey structure but not necessarily their functioning. For example, on mud flats Nereis may be replaced by Nephtys following an increase in salinity with reduced river flows (McLusky & McCrory, 1989). Although the species composition is seen to have changed along the environmental gradient, the community still functions as prey for the birds.

In analysing the feeding preferences of different species, it was initially suggested (Green, 1968) that both physical and behavioural adaptations have been shown for shore birds including bill lengths which correspond to the depth of specific prey items. Species such as the bar-tailed godwit takes cues from new Arenicola casts and the shelduck which feeds extensively on Hydrobia ulvae has five distinct feeding methods relating to the tidal state and the behavioural patterns displayed by Hydrobia (Bryant & Leng, 1976). However, more recently, waders are regarded as opportunistic feeders with only a general relationship between depth of prey and size of bill (McLusky, 1989).

Different bird species exploit different areas of an intertidal area, for example the redshank and the shelduck feed on the intertidal area at low water or the water’s edge, usually on Macoma, Hydrobia and other small invertebrates such as Corophium. Waders such as sanderling are efficient croppers of macrofauna at the low tide waters’ edge both in summer and winter (McLachlan, 1983). Eider ducks, however, feed in the shallow water at low tide on species such as Mytilus edulis. The diet of the black-tailed godwit consisted mainly of the bivalve mollusc Scrobicularia plana with a small proportion of ingested biomass being Nereis and Hydrobia (Moreira, 1994). The sizes taken reflect those available although optimal foraging occurs.

Feeding behaviour differs within intertidal sand and mudflats with variations in the percentage of shorebirds engaged in feeding, roosting and other behaviours dependent on location, date, time, tide and species. Burger et. al (1997), studying spring migrations in Delaware Bay, found that a higher percentage of shorebirds fed during the middle of migration, in early to mid-morning, and during low and rising tides than at other times. Some shorebirds fed on the marshes and mudflats during all tidal states, but none fed on beaches at high tide (beaches were too narrow). Within each habitat, the highest percentage of shorebirds foraged during low tide (marshes) or rising tides (mudflats and beaches). The percentage of shorebirds engaged in foraging as an indication of foraging value for each habitat type, showed that a mosaic of habitat types ranging from mudflats to high marshes is essential to sustain the high populations of shorebirds.

The subtidal mobile sandbanks provide prey for demersal fishes, especially the mobile small crustaceans which migrate from the sediment and thus become available for predation (Costa & Elliott 1991; Marshall & Elliott, submitted). The areas are often important as fish nursery areas, e.g. plaice (Gibson, 1973), and may be characterised by low organic enrichment though there may be localised pockets of organic matter or areas which receive anthropogenic waste, e.g. the Dogger bank.

The sandbanks are also important areas for crab populations, for example the Race Bank and Docking Shoal off the Norfolk coast support a large population as well as numerous other epifauna, particularly echinoderms. The epifaunal component may represent a large proportion of the biomass of the sand bank fauna with large numbers of echinoderms such as Asterias rubens and brittle stars such as Ophiura albida. Predatory fauna such as hermit crabs e.g. Eupagurus bernhardus, Liocarcinus depurator and the edible crab Cancer paguras may also be present.

Birds such as the guillemot, razorbill, puffin and the terns will feed on the fish such as sandeels (Ammodytes spp.) which are found in mobile subtidal sands (Batten et al, 1990). Both the arctic tern and the puffin rely on populations of sandeel as their predominant food source. The sandeel is also an important food source for wintering birds such as scoters, little terns and the red-throated diver (Gibbons et al, 1993). Guillemots and razorbills although not as selective as puffins and terns will also eat sandeels.

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