To determine the organization, dynamics, and controlling mechanisms of an estuarine benthic copepod assemblage, 2 stations were established 1 km apart in a South Carolina estuary in January 1973, and sampled at monthly intervals for 3 yr. These sand and mud faunas displayed unique dynamic properties, yet species diversity, used as a measure of community structure, did not discriminate community differences seasonally, within or between the 2 locations. The mud fauna, distributed to a depth of 1 cm, has a randomly varying community abundance and diversity over time, and diversity was controlled by a marked seasonal cycling of species suites which recurred regularly. The sand fauna diversity decreased with time and was correlated to an increasing abundance trend. Sand faunal seasonality was not marked, however, diversity was maintained by a high spatial niche overlap of fauna through a 10—cm depth distribution. The communities were different in species composition, seasonal responses, environmental regimes, dominance relationships, and controlling factors, yet maintained equal diversity. Diversity, however, was maintained by different and independent mechanisms.