Rosr, A. & SEIFRIED, S. (2006): Small-scale diversity of Harpacticoida (Crustacea, Copepoda) from an intertidal sandflat in the Jade Bay (German Bight, North Sea). -Senckenbergiana marŸ 36 (2): 109-122, 6 Figs., 3 Tabs., Frankfurt a. M.]Harpacticoid copepod assemblages from an intertidal sandflat in the Jade Bay (German Bight, North Sea) were investigated during a snapshot study, uncovering a relatively high small-scale organismic diversity: 1952 adult Harpacticoida of 32 species were recorded, belonging to 19 genera and 13 families. With 1232 adult specimens (63.1%), Ectinosomatidae were the most abundant family, containing Pseudobradya minor (T. & A. ScoTm) as the dominating species (57.4 %). For five species, Dac(ylopusia vulgaris (SaRs), Remanea arenicola K~I~, Mesochra pygmaea (CI~aus), Arenopontia subterranea KtrNZ, and Enhydrosoma gariene Gumqtv, the distributional range along the German coast Iine was extended to the area west of the river Elbe. Two species, ArenoseteIla tenuissirna (K~IE) and Pseudobradya beduina MONae, D, were recorded for the first time in the Jade region. Most species recorded in our study ate typical inhabitants ofintertidal sandy and/or muddy environments.Taking into account different aspects ofdiversity (species richness: species numbers, estimated species numbers by Jack-knife 1 and 2, species richness plots; heterogeneity: Shannon index, rarefaction curves; phylogenetic/taxonomic relationship: taxonomic distincmess) we found only minor diversity differences between four investigated small spatial scales (circles from 18 cm to 20 m diameter). Hypothesis [a] stating increasing diversity with increasing scale had to be rejected. ~lhis unexpected result could be caused by the presence of a mosaic of more or less randomly arranged micropatches of species over the sampling domain. Obviously, all circles were more or less within the range oŸ single ecological scale. A slight diversity change at the 20 m scale, mainly based on lower species density, may be due to changing tide, or to a beginning overlap of small-scale random micropatches by a larger-scale environmental pattern. Since we found only minor diversity changes from the smallest (centimetres) to the largest investigated scale (tens of metres), biotic and/or abiotic factors and processes acting at spatial scales of centimetres are assumed to be ofspeciaI relevance for intertidal harpacticoid diversity.Hypothesis [b] of no significant harpacticoid assemblage differences between the investigated scales was tested byANOS[M for two similarity measures, leading to different results. Cosine similarity revealed no significant difference in assemblage composition between all possible pairwise combinations of circles, whereas [b] had to rejected for Bray-Curtis dissimilarity which produced significant results for two pairs (A/D, C/D). ~he difference between the test results is suggested to be caused by a lower species density in circle D, since Bray-Curtis dissimilarity is separating similar monitoring units if the difference is only du...