Seasonal and spatial variability in rates of primary production and detritus release by intertidal stands of and on wave-exposed shores in the northeast Atlantic.

Coastal 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- (oarweed) and (sugar kelp)-on four NE Atlantic rocky shores over 22 months. The density of was greater at exposed compared to moderately exposed shores but remained consistently low for throughout the survey period. Individual productivity and erosion rates of did not differ between exposed and moderately exposed shores but differed across exposure levels throughout the year at moderately exposed sites only. Productivity and erosion of remained low on moderately exposed shores and showed no clear seasonal pattern. Patterns of productivity and total detrital production (erosion and dislodgement) per m of both and followed closely that of densities per m, peaking in May during both survey years. Temperature and light were key factors affecting the productivity rates of and . Erosion rates of were affected by wave exposure, temperature, light, grazing, and epiphyte cover, but only temperature-affected erosion of . Production of biomass and detritus was greater in than in and exceeded previous estimates for subtidal and warmer-water affinity kelp populations (e.g., ). 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.