Because the distribution of species is determined so closely by physical and chemical parameters, these offer an alternative basis for classifying benthic communities, primarily with respect to substrate, depth, temperature and salinity. Jones (1950) divided the shelf communities of the north-east Atlantic in this way as follows. Obviously the communities defined below can be further subdivided into narrower zones of distribution with more precisely defined environmental parameters.
(1) Shallow water and brackish communities with upper limits of distribution extending on to the shore. Eurythermal and euryhaline within wide limits. Temperature range 3-16°C, salinity 7-34%o, but always exposed to periodical diminution below 23%o.
(A) Shallow soft bottom community
(i) Boreal shallow sand association - occurring on relatively exposed coasts in north-west Europe. Important species include Arenicola marina, Tellina tenuis, Donax vittatus, Nephtys caeca, Bathyporeia pelagica.
(ii) Boreal shallow mud association - equivalent to Petersen's Macoma community, occurring on more sheltered coasts of north-west Europe, and in estuaries and in the Baltic.
Arenicola marina, Macoma baltica, Mya arenaria, Cerastoderma (Cardium) edule, Corophium volutator.
(B) Shallow hard bottom community
(i) Boreal shallow rock association - shores and shallow water with rocky substrates off north-west Europe and in the Baltic.
Balanus balanoides, Mytilus edulis, Littorina spp., Patella vulgata, Nucella lapillus.
(ii) Boreal shallow vegetation association - algae of the shore and shallow water with associated fauna.
Hyale prevosti, Idotea spp., Hippolyte varians, Littorina littoralis ( = L. obtusata and L. mariae), Rissoa spp., Lacuna vincta.
(2) Offshore communities having upper limits of distribution below extreme low-water spring tidal level. Eurythermal and euryhaline, but between narrower limits than (A). Temperature range 5-15°C, salinity 23- 35.5%o.
(A) Offshore soft bottom community
(i) Boreal offshore sand association - equivalent to Petersen's Venus community, occurring offshore on sandy bottom. Sthenelais limicola, Nephtys spp., Ampelisca brevicornis, Bathyporeia guilliamsoniana, Dosinia lupinus, Venus striatula, Tellina fabula, Gari fervensis, Abra prismatica, Ensis ensis, Echinocardium cordatum.
(ii) Boreal offshore muddy sand association - equivalent to Petersen's Echinocardium-filiformis community, occurring on muddy sand offshore and in modified form in sheltered and estuarine situations. Nephtys incisa, Goniada maculata, Lumbriconereis impatiens, Pec-tinaria auricoma, Diplocirrus glaucus, Eumenia crassa, Scalibregma inflatum, Notomastus latericeus, Owenia fusiformis, Ampelisca spinipes, A. tenuicornis, Nucula turgida, Dosinia lupinus, Abra alba, A. prismatica, A. nitida, Arctica (Cyprina) islandica, Acanthocardia (Cardium) echinatum, Parvicardium (Cardium) ovale, Cultellus pellucidus, Spisula subtruncata, Corbula gibba, Dentalium entalis, Turritella communis, Aporrhais pespelicani, Philine aperta, Ophiura texturata, Amphiura filiformis, Echinocardium cordatum, E. flaves-cens, Leptosynapta inhaerens.
(iii) Boreal offshore mud association - equivalent to Petersen's Brissopsis chiajei community, occurring on soft mud, usually outside the muddy sand association but gradually grading into it.
Leanira tetragona, Nephtys incisa, Glycera rouxi, Lumbriconereis impatiens, Maldane sarsi, Notomastus latericeus, Eudorella emar-ginata, Calocaris macandreae, Nucula sulcata, N. tenuis, Abra nitida, A. prismatica, Amphiura chiajei, Brissopsis lyrifera. (B) Offshore hard bottom community
(i) Boreal offshore gravel association - occurring at moderate depth wherever the deposit is very coarse, whether sand, gravel, stones or shells. Masses of Modiolus modiolus sometimes form a numerous epifauna.
Polygordius lacteus, Glycera lapidum, Potamilla spp., Serpula vermicularis, Crania anomala, Balanus balanus (=porcatus), Galathea spp., Eupagurus spp., Hyas coarctatus, Nucula hanleyi, Glycymeris glycymeris, Lima loscombi, Venus casina, V. fasciata, V. ovata, Venerupis rhomboides, Gari tellinella, Spisula elliptica, Modiolus modiolus, Buccinum undatum, Asterias rubens, Ophiothrix fragilis, Ophiopholis aculeata, Echinus spp., Echinocyamus pusillus, Echinocardium flavescens, Spatangus purpureus. (3) Deep communities with upper limits of distribution not above 70 m depth and usually much lower. Stenothermal and stenohaline. Temperature range 3-7°C, salinity 34-35.5%o. (A) Deep soft bottom community
(i) Boreal deep mud association - equivalent to Petersen's Brissopsis sarsii and Amphilepis-Pecten communities, occurring below the offshore mud association.
Glycera alba, Spiophanes kroyeri, Chaetozone setosa, Maldane sarsi, Clymene praetermissa, Sternaspis scutata, Notomastus latericeus, Melinna cristata, Proclea graffi, Eriopisa elongata, Nucula tenius, Nuculana minuta, Chlamys vitrea, Thyasira flexuosa, Abra nitida, Portlandia lucida, Parvicardium (Cardium) minimum, Ophiura sarsi, Amphilepis norvegica, Brissopsis lyrifera. (B) Deep hard bottom community
(i) Boreal deep mud association - a little-known, deep-water epifauna of corals and associated animals.
Lophelia pertusa, Paragorgia arborea, and Gorgonocephalus caput-medusae.
Although some parts of the sea floor present sharp discontinuities, for instance the change from rock to sand, alterations of substrate from one place to another are mostly gradual. Furthermore, each species has its particular distribution which is never identical with that of any other. Consequently boundaries between communities are usually indefinite with intergrading along transitional zones. On almost every part of the sea-bed the inhabitants comprise a climax community for that particular area, stable in composition within natural, short-term fluctuations. Wherever environment changes with locality there are corresponding adjustments in the make-up of the assemblage of species. The concept of a community is essentially an abstraction from studies of overlapping distributions of many species along various ecological gradients. For further discussion of the ideas underlying these classifications of benthic communities, the student may consult the paper by Glemarec (1973).
The facility with which communities may be characterized by particular conspicuous species, sometimes rather misleadingly described as 'dominants', although evident in marine benthos around the British Isles, is less apparent at low latitudes where the composition of communities generally shows a greater diversity. This diversification may be a feature of more mature communities which have evolved in stable conditions over a long period, permitting the survival of species specialized for narrow ecological niches. The communities of the north-east Atlantic may be regarded as relatively immature, having evolved in fluctuating conditions since the extremes of the Pleistocene period. This favours the evolution of polymorphic populations which survive by virtue of their wide variability, with 'dominant' species occupying broad ecological niches.
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