Amphipods and [Scolelepis] spp. in littoral medium-fine sand

Description of Amphipods and [Scolelepis] spp. in littoral medium-fine sand


flag A2.223
creatorIdentity remo
creationTime 2017-08-16T16:38:13.045
Last Maintainer Identity remo
modificationTime 2016-03-16T18:42:49
id 2775
imageSize 0 Bytes

Mobile clean sandy beaches on exposed and moderately exposed shores, with sediment grain sizes ranging from medium to fine, often with a fraction of coarser sediment. The sediment contains little or no organic matter, and usually no anoxic layer is present at all. It tends to be well-drained, retaining little water at low tide, though the sediment of the A2.2233 sub-unit may remain damp throughout the tidal cycle. These beaches usually occur under fully marine conditions, though the A2.2232 sub-unit may occur under moderately exposed lower estuarine conditions. The mobility of the sediment leads to a species-poor community, dominated by polychaetes, isopods and burrowing amphipods. Scolelepis spp. can tolerate well-drained conditions, and are often present in well-draining, coarser sand. Burrowing amphipods that often occur in this biotope include Bathyporeia spp., Pontocrates arenarius, and Haustorius arenarius. The isopod Eurydice pulchra is also often present. On semi-exposed beaches with a moderate tide range where there is a marked high-shore berm, there can be a marked seepage at the foot of the berm that probably carries the products of the organic matter derived from strand line breakdown. Here in a narrow zone, exceptionally high populations of Bathyporeia pilosa, sometimes above 10000 per square metre, may occur. The zone may be narrower than the strandline and could easily be missed on surveys were only a few levels are sampled. Three sub-biotopes are described for this biotope, based principally on differences in infaunal species composition. Situation: Situated mainly on the mid and lower shore, sometimes upper shore, of exposed to moderately exposed beaches. Under more exposed conditions, it may occur below A2.221, or A2.111. Under more sheltered conditions, it may occur above the communities of unit A2.231. Unit A2.211 may be present on the same shores as A2.223, where driftlines of decomposing seaweed and other debris occur on the upper shore. Temporal variation: Winter storms may reduce the number of or temporarily remove macroinvertebrates from exposed sandy beaches, with the sediment becoming re-colonised during the summer months.