Pleurophycus gardneri (Laminariales) is common in the low intertidal of the Northeast Pacific, but dominates many deep (30 to 40 m) rocky reefs in central California. Seasonal dynamics of productivity and resource allocation of a deep-water population of this deciduous, stipitate kelp were studied to understand how blade abscission affects the annual carbon budget. Patterns of growth, metabolism, and carbon storage and mobilization were measured monthly for 1 yr relative to in situ light and temperature, and used to model the annual carbon budget. The resulting carbon budget was used to determine if blade abscission effectively reduced respiratory demand during the winter period of low light availability. Metabolic properties (photosynthesis, photoacclimation, and respiration) were seasonally constant and showed evidence of photoacclimation to this deep, low-light environment. Blades grew between February and July, followed by senescence and sloughing from August to December. Concentrations of laminaran and mannitol increased in the blades from the onset of sloughing in August until just prior to blade abscission in mid-December, suggesting translocation of these carbohydrates may have occurred from the blade to the stipe and holdfast. Carbon budget estimates revealed that scalar irradiance measures overestimated the light available for photosynthesis of these paddle-shaped kelp blades by 50 to 75%. The calculations also revealed that blade retention allowed for the maintenance of positive carbon balance throughout the year. Thus, conservation of the internal carbon reserve for metabolic survival during the low-light period does not appear to be a viable explanation for the deciduous life history of P. gardneri. Abscission may reduce hydrodynamic drag, thus minimizing the probability of dislodgment of entire plants during winter storm events, or promote spore dispersal as abscised blades and sori drift away from the parent holdfast. KEY WORDS: Carbon budget · Photosynthesis · Abscission · Light acclimation Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 309: [143][144][145][146][147][148][149][150][151][152][153][154][155][156][157] 2006 excluded by light limitation (Edwards 1998). However, there is no clear evidence that the newly settled algae undergo a seasonally adjusted metabolic response to light availability. In northern latitudes where light limitation is seasonally predictable, perennial kelps have evolved a variety of physiological mechanisms that allow them to survive through summer periods of nutrient limitation and winter periods of light limitation (Gagne et al. 1982). Laminaria solidungula, for example, uses stored carbohydrates accumulated during summer to complete 90% of its annual linear blade growth during the virtual darkness of the Arctic winter (Mann 1973, Chapman & Craigie 1977, 1978, Dunton & Schell 1986. Another mechanism may involve cellular photoacclimation to increased photosynthetic efficiency in low-light environments, thr...