This study describes a hyporheic zone that existsbeneath a river-dominted estuary in Norht Wales,and which spans the freshwatr/saltwater boundary.A series of 72 cores was taken from the Aber Estuary,in September,at depths from 10-60 cm below the bed surface.Site 1 was above the extreme high watermrk and therefore was never inundated by salt water.Site 5 was the furthest downstream and was inundated twice daily by incoming tides.Substrates contined more gravel at the upstream sites and closer the bed surface,with more sand and silt at downstream sites and at depth.Salinity of interstitial watr increased both with distance away from Site 1 and with increasing depth into the zone,reaching a maximum of 15per thousand at 60 cm at Site 5,asmeasured at low tide.Longitudinal and vertical zontions of the invertebrate taxa were evident,and densities were greater in the top 40 cm than below.Nematodes were especially abundant from 10-60 cm,and genera differed along the estuary.Oligochaetes were well represented at most sites,with the less saline sites yielding Naididae,Enchytraeidae,Aeolosomtidae and Tubificidae;Site 5 was dominated by Tubifex costatus.Chironomid larvae were most abundant to 40 cm at Sites 1 and 2(primarily Brillia modesta,Corynoneura sp.,Tanytarsus sp.,and unidentified Tanypodinae),but they were also found at Site 3 (to 60 cm),and at 10 cm at Site 5 (Orthocladius sp.).Nymphs of mayflies and caddisfly larvae were found primarily at Site 1,but small stonefly nymphs and elmid beetle larvae were taken at Site 2 depths where the interstitial water was brackish.The most saline sediments were populated by the triclad Uteriporus vulgaris,the polychaete Nereis sp.,and snails (hydrobia sp.).Site,salinity,pH,and sand and silt contents appeared to influence species distriution most;together,theseexplained 37.8% of the variance(CANOCO).There was an overall negative relationship between silt content of the sediments and total invertebrate density.The hyporheic zone of the Aber Estuary is thus not only influenced by surface water and,presumably,estuarine groundwater exchanges,but is further complicated,biologically,by having a salinity gradiet running through it.The term 'brackishwater hyporheic zone'(BHZ) is proposed to describe this and similar systems.
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