Rockweeds dominate much of the New England and Canadian Maritime coasts and serve as food and habitat for numerous species. Cobscook Bay is unique in New England for its high tidal amplitude, wide intertidal expanse, diverse flora and fauna, and presumed high intertidal productivity, much of which is thought to be related to extensive intertidal fucoid populations. The goals of this study were: 1) to estimate intertidal rockweed biomass and productivity, 2) to quantify the variation in rockweed productivity over a range of temporal and spatial scales (high- vs low-flow sites at headlands and coves, respectively), and 3) to estimate the contribution of rockweeds to detrital pools. Net productivity was determined by separately weighing growth in the current year vs growth from preceding year classes. Sampling the same populations in both the fall and following spring allowed adjustment for winter mortality of thallus structures and productivity at two sites.

Variation in the length of apical tips of canopy shoots was high, and differences among sites were not significant. However, growth of tips of sub-canopy and lateral shoots was site-specific. Standing crop at low-flow sites ranged from 11.4 kg wet weight m⁻² at Bar Island to 28.9 kg wet weight m⁻² at Bell Farm. Standing crop at high-flow sites ranged from 8.5 kg wet weight m⁻² at Birch Point to 26.7 kg wet weight m⁻² at Mahar Point. Adjusted productivity estimates ranged from 22 to 105% greater than unadjusted values. Highest productivity estimates for Ascophyllum (14.9 kg wet weight m⁻² yr⁻¹ or 894 g C m⁻² yr⁻¹) occurred at Mahar Pt., a high-flow site. This unadjusted estimate was 50% higher than the average value (594 g C m⁻² yr⁻¹) from the two sites adjusted for winter losses, Bell Farm and Bar Island. Turnover rates of Ascophyllum ranged from 29 to 71% (mean over all sites = 54%) indicating that the biomass of this alga turns over approximately every two years. About 60% of the standing biomass (3.96 × 10⁶ g C yr⁻¹) is added to detrital pools, contributing large amounts of energy for secondary consumers. Rockweeds in Cobscook Bay are among the most productive cold-water intertidal assemblages and contribute substantial amounts of carbon to this large embayment.