AbstractCoastal habitats are increasingly recognized as fundamentally important components of global carbon cycles, but the rates of carbon flow associated with marine macrophytes are not well resolved for many species in many regions. We quantified density, rates of primary productivity, and detritus production of intertidal stands of two common intertidal kelp species—Laminaria digitata(oarweed) andSaccharina latissima(sugar kelp)—on four NE Atlantic rocky shores over 22 months. The density ofL. digitatawas greater at exposed compared to moderately exposed shores but remained consistently low forS. latissimathroughout the survey period. Individual productivity and erosion rates ofL. digitatadid not differ between exposed and moderately exposed shores but differed across exposure levels throughout the year at moderately exposed sites only. Productivity and erosion ofS. latissimaremained low on moderately exposed shores and showed no clear seasonal pattern. Patterns of productivity and total detrital production (erosion and dislodgement) per m2of bothL. digitataandS. latissimafollowed closely that of densities per m2, peaking in May during both survey years. Temperature and light were key factors affecting the productivity rates ofL. digitataandS. latissima. Erosion rates ofL. digitatawere affected by wave exposure, temperature, light, grazing, and epiphyte cover, but only temperature‐affected erosion ofS. latissima. Production of biomass and detritus was greater inL. digitatathan inS. latissimaand exceeded previous estimates for subtidal and warmer‐water affinity kelp populations (e.g.,Laminaria ochroleuca). These biogenic habitats are clearly important contributors to the coastal carbon cycle that have been overlooked previously and should be included in future ecosystem models. Further work is required to determine the areal extent of kelp stands in intertidal and shallow subtidal habitats, which is needed to scale up local production estimates to entire coastlines.