Production, abundance and mortality of the copepod Acartia tonsa were studied for a period of 9 d in a hyper-eutrophic estuary, Mariager Fjord, Denmark. The estuary is characterised by oxygen-depleted and often sulphidic bottom water, and a relatively sparse mesozooplankton commu- nity with low species diversity. During the study, an intense phytoplankton bloom consisting mainly of the diatom Skeletonema costatum developed with chlorophyll a concentrations reaching 46 μg l -1 . Egg production rate (EPR) in A. tonsa ranged from 30 to 65 eggs female -1 d -1 , and egg hatching suc- cess was >90%, yet the abundance of copepods remained low (1 to 3 nauplii l -1 , 0.3 to 1.5 copepodites l -1 ). Calculated daily copepod mortality ranged from 18% for nauplii, 16% for Copepodite Stage C1, up to 70% for C2 and C3, then declining for older stages. The vertical distribution of copepodites in relation to the depth range of mussel beds suggests strong predation by suspension-feeding Mytilus edulis at depths of 5 to 10 m. Moreover, anoxia and the presence of sulphide in deep water, which prevented hatching of A. tonsa eggs and decreased the survivorship of older life stages sinking below 20 m depth, further contributed to the low copepod abundance.