Subtidal Biotope Complex: Subtidal Mobile Sandbanks

Organic Production and Phytobenthos

Benthic fauna in relation to hydrography

Organic Production and Phytobenthos

The physical environment of subtidal mobile sandbanks with stronger currents is often too harsh for vegetation to become established; they are less productive with lower levels of organic matter. However, sheltered subtidal sandbanks may support the sugar kelp Laminaria saccharina attached to stones and shallow conditions with adequate light will maintain a microphytobenthic community. Similarly, adequate light conditions will allow maerl to develop (see Volume V).

Benthic fauna in relation to hydrography

Macrofauna

Mobile sandbanks are colonised by infaunal/epifaunal small crustaceans, polychaetes and molluscs which are adapted to the changing hydrography and substratum; they are able to reburrow rapidly following being washed-out of the sediment during storms (Vanosmael et al, 1982). For example, the body form and mobility of magelonid polychaetes and species such as Nephtys cirrosa and Micropthalamus similis are well suited to burrowing in mobile sands. These features indicate that the communities are clearly shaped by physical rather than biological forces.

The sediment in a mobile sandbank system may range from fine to coarse clean sands, and the density of individuals and species richness is often highest in the coarsest grade, mainly due to large numbers of interstitial polychaetes (Vanosmael et al, 1982). The mean macrobenthic diversity and species richness of clean mobile sandbanks is generally lower than the surrounding sea bed (reflecting the greater stresses inherent in these environments) although the fauna is essentially comparable with that of the open sea. The mouth of the Teign estuary has been noted to only contain Scolelepis squamata, Eteone longa, Anonides oxycephala, Nemertea indet. and an occasional juvenile Mytilus edulis in the mobile sandbanks.

Due to the continual sediment disturbance, the community may have a large opportunistic component with species such as Chaetozone setosa and may be prevented from reaching a climax community. For example, subtidal areas along the eroding Holderness Coast have large populations of C. setosa indicating a community held in a disturbed condition (Allen, in prep.).

The fauna of nearshore sandbanks vary geographically and are often impoverished extensions of the communities found at exposed intertidal areas, especially where currents are high and the substratum is clean sand. The MNCR classification of clean mobile subtidal sands describes the fauna as barren or characterised by Pagurus bernhardus and Ammodytes species (Connor et al, 1997). However if pockets of silt develop e.g. between ripples on the sand banks, a richer fauna may develop (Vanosmael et al, 1982) and substratum with an increased silt content may be characterised by epifaunal and infaunal echinoderms. As in intertidal areas, species which help to consolidate the substratum such as the reef forming polychaete Sabellaria spinulosa will help promote the settlement of other fauna.

The characteristic fauna of the subtidal mobile sandbanks is similar to Peterson’s Venus community (in broad terms) which is comparable to the ‘boreal offshore sand association’ (Table 3.0) with the number of species reflecting the stability of the area. Depending on the hydrodynamic regime of the area, the assemblage may also have elements of the ‘boreal off-shore gravel association’ of Jones (1950) (Vanosmael et al, 1982). More stable sandbanks comprised of finer sediments may also resemble the ‘boreal offshore muddy sand association’. Such an association is often better characterised by less frequent species than by dominant ones, and the interstitial polychaetes and archiannelids, especially, seem to be characteristic of the communities described by Petersen (1913) and Jones (1950).

As an example, Vanosmael et al (1982) found that sandbanks off the Dutch coast were characterised by mobile and rapid-burrowing crustaceans and polychaetes such as Nephtys cirrosa, Hesionura elongata and Microphthalmus listensis. Sessile tube building polychaetes were represented by only a small number of individuals.

Hesionura augeneri (an interstitial polychaete) made up 55% of the macrofaunal population which occurred in the highest densities in the coarser sediments. The polychaetes Micropthalmus listensis, Nephtys cirrosa and N. hombergii occurred separately because of differing sediment preferences. The mollusc Spisula elliptica was common in muddy sand, fine sand and shell gravel banks and the crustaceans, Tanaissus lilljeborgi and Bathyporeia elegans were the most common species on the sand banks, both showing a preference for fine sand. The macro-crustacea may be generally more abundant in finer sediments (Willems et al, 1982a).

Various epifaunal brittle stars are associated with this biotope complex e.g. Amphiura filiformis which extends its arms up into the water column to feed on suspended material. The heart urchin Echinocardium cordatum may also be common but replaced by another heart urchin, Brissopsis lyrifera in siltier areas. Sandeels e.g. Ammodytes tobianus and A. marinus, are widespread (and Corbins sandeel and the greater sandeel to a lesser extent) on subtidal mobile sandbanks.

Meiofauna

The meiofauna also form an important component of the sandbank fauna. Interstitial organisms occur in sediments with a median grain size above 200 Ám and polychaetes are found to be abundant (although they also live in finer sediments) in sediments with a particle size above 300 Ám (Willems et al, 1982a). The meiofauna may be characterised by low densities of nematodes and high densities of copepods, annelids and halacarid mites. The meiofauna (particularly nematodes and copepods) are not correlated with sediment type although ostracods and halacarids may be more numerous in coarser sediments. In finer sediments, studies showed most copepods were species of Cylindropsillidae and Parameschridae, (the smallest harpacticoids). In coarser sands, above 300 Ám median particle diameter, interstitial fauna included the nematode families, Ameiridae, Ectinosomatidae and Diosaccidae.

The occurrence of rare species and very high diversity is unusual in mobile sandbanks. However, three important species of interstitial polychaetes were recorded by Vanosmael et al (1982) in mobile subtidal sandbanks in the North Sea: Polygordius appendiculatus, (with a preference for coarse and medium sands), Protodriloides chaetifer (fine medium and coarse sands) and a species of the genus Protodrilus. The nematode densities in these sandbanks were generally higher than the surrounding seabed. Whilst the generic composition of the nematode communities in mobile sandbanks is similar to those of other clean sandy biotopes the large number of Epsilonematidae and Draconematoidea (Nematoda) found in this study is exceptional for offshore communities. These are adapted to the extreme instability of the substratum of the sandbanks and are confined to these biotopes. The fauna of these unstable habitats consists largely of erratic colonists brought in by water movement (Willems et al, 1982).

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