The population dynamics and production of Nereis oligohalina Rioja, one of the numerically dominant macrofaunal species in salt marshes of SE Brazil, were studied from May 1987 to June 1988 in Paranaguá Bay. Despite large fluctuations in population density (490 ind 0.45
m−2 in June 1987 to 15 ind 0.45 m−2 in February 1988), N. oligohalina was present throughout the year. Biomass peaks up to 8.23 g m−2 were evident in June and September 1987, and January 1988. N. oligohalina undergoes at least
two annual reproductive peaks, superimposed upon a background of continuous low-level recruitment. Separation of monthly modal components and the later subjective linkage of modal progressions of average sizes allowed for the identification of four annual cohorts which grossly corresponded
to autumn, winter, spring, and summer periods. Instantaneous rates of total mortality were lower for the autumn and winter cohorts. Life spans varied from 5.74 (autumn cohort) to 6.42 mo (spring cohort). Increased rates of total mortality and increased life span in the summer cohort were significantly
correlated to higher precipitation rates during this period. The annual production was estimated as 17.94 g m−2 yr−1 (Crisp's method) and the P:B ratio as 4.5, for formalin fixed wet weight. Temporal variations in population density and biomass of N. oligohalina
are strongly correlated to seasonal variations in below ground biomass of Spartina alterniflora. This suggests that the reproductive strategies of the polychaete and the seasonal allocation of energy to below ground tissues of the marsh plant are synchronized.
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