The seasonal variation of biomass and the annual net primary production for above- and belowground components of Spartina alterniflora Loisel intermediate form were evaluated by harvest methods over a 14-month period in Paranaguá Bay (SE Brazil). Belowground components were characterized by a larger biomass, ranging from 172±48 g m−2 at the end of summer to 569±211 g m−2 in late spring. The biomass of live aboveground tissue was usually higher than dead tissue, ranging from 51±21 g m−2 in winter to 116±36 g m−2 in summer. The overall mean root to shoot ratio was 4·8, ranging from 1·7 in the summer to 8·5 in late spring, reflecting storage and translocation events. Net belowground primary production was 358 g m−2 year−1, largely exceeding aboveground production, which ranged from 101 to 179 g m−2 year−1, depending on how data were handled. Turnover of above- and belowground material, expressed by a production to maximum biomass ratio, was fast, ranging from 0·66 to 1·52 years, respectively. In comparison with temperate Spartina marshes, the low productivity and low biomass of marshes of the high-energy sector of Paranaguá Bay are probably a result of different growth strategies, high salinities and fast sediment accretion, which usually lead to their ultimate replacement by mangroves.