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Coastal systems serve many human uses and as a result are susceptible to anthropogenic activities such as nutrient loading and overfishing. In soft sediments, infauna frequently serve as key indicators of such activities. To use infauna effectively as bioindicators, it is important to understand how infaunal abundances and community patterns vary naturally within ecosystems. We examined the spatial and temporal dynamics of infaunal annelids in four tidal creeks of the Plum Island Estuary, Massachusetts, USA, from June to October 2003, sampling along a tidal inundation gradient that crossed five distinct habitats from creek bottoms to the vegetated high marsh platform. Annelids comprised 97% of the total number of macroinfauna. Highest densities were found in creek wall habitats (33,418–65,535 individuals m-2), and lowest densities (2,421–10,668 individuals m-2) were found in Spartina patens habitats. Five numerically abundant species comprised 87% of the annelid assemblage and three species, Manayunkia aestuarina (Polychaeta), Paranais litoralis (Oligochaeta), and Cernosvitoviella immota (Oligochaeta), were broadly distributed across the marsh landscape. Streblospio benedicti (Polychaeta) and Fabricia sabella (Polychaeta) were abundant only in mudflat and creek wall habitats, respectively. P. litoralis experienced a summer decline in all habitats, whereas M. aestuarina abundance increased 4–5 fold in October relative to June in creek wall and tall-form Spartina alterniflora habitats. Hierarchical spatial analysis revealed that > 90% of the variability in annelid abundances was found at the mesospatial scale (< 50 m). Variation among the four creeks (> 1 kin) was relatively small. [ABSTRACT FROM AUTHOR]