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In sheltered brackish muddy sand and mud, beds of Ruppia maritima and more rarely Ruppia spiralis may occur. These beds may be populated by fish such as Gasterosteus aculeatus which is less common on filamentous algal-dominated sediments. Seaweeds such as Chaetomorpha spp., Ulva spp., Cladophora spp., and Chorda filum are also often present in addition to occasional fucoids. In some cases, the stoneworts Lamprothamnium papulosum and Chara aspera can occur. Infaunal and epifaunal species may include mysid crustacea, the polychaete Arenicola marina, the gastropod Peringia ulvae, the amphipod Corophium volutator and oligochaetes such as Baltidrilus costatus. In some areas Zostera subg. Zostera marina may also be interspersed with the Ruppia beds.

Low and variable salinity infralittoral mud with beds of Stuckenia pectinata. Other associated species are broadly similar to that of SS.SMp.SSgr.Rup, with blankets of filamentous green algae such as Ulva intestinalis, Cladophora liniformis, and Rhizoclonium riparium. The grazing gastropod Potamopyrgus antipodarum is found in this biotope and juvenile Mytilus edulis have been observed settled on Potamogeton leaves and amongst the algae. The nationally scarce charophyte Lamprothamnium papulosum may be found to some extent in this biotope but more often in neighbouring habitats (see Plaza & Sanderson 1997). Mysids, trout (Salmo trutta), and sticklebacks Gasterosteus aculeatus can be found swimming amongst the vegetation. Mya arenaria may be found in some examples of this biotope. The infaunal component of this biotope requires further investigation but is likely to contain oligochaetes, Arenicola marina, Corophium volutator and Gammarus spp. This biotope is furhter described as NVC type A12 (Rodwell 1995).