Sampling of 57 littoral stations in Manukau Harbour, Auckland, (37° 02’ S, 174° 41’ E) for species composition, distribution, and abundance as part of a base‐line ecological survey of the harbour shows that the community trophic structure is related to the sediment grain size. Deposit feeders such as the bivalvesMacomona lilianaandNucula hartvigianaand the gastropodZeacuman‐lus lutulentusare most abundant in fine sand, whereas suspension feeders such as the bivalvesChione stutchburyiandPaphies australeare most abundant in medium sand. The proportion of deposit feeders in the sample increases with decreasing grain size of the sediment. Most stations conform to this general trend, although when the results are plotted as the percentage of deposit feeders against the median and mean grain sizes of the sediment in which they occur, three groups of stations appear anomalous. These stations, at Waiau Pa, Pollok Beach, and Fosters Bay, may be unstable and undergoing a change in community structure or sediment characteristics. Identification of unstable areas using this method may be useful to monitor effects caused by environmental changes.