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Patterns and controls of reef‐scale production of dissolved organic carbon by giant kelp Macrocystis pyrifera

We investigated the patterns and controls of dissolved organic carbon (DOC) production by the giant kelp (Macrocystis pyrifera) using data from short‐term in situ incubations of entire blades and portions of stipes. These data were incorporated into an empirical model of reef‐scale net primary production (NPP) at Mohawk Reef in southern California, U.S.A. for an 8‐yr period. Rates of DOC release of incubated blades varied unpredictably with time of year, but were significantly related to the irradiance at the sea surface during the incubations. The growth stage, C/N ratio, and epiphyte load of the blades and the temperature of the ocean during the incubations had no discernable effect on rates of DOC release. Blades produced on average 2–3 times more DOC than stipes, and stipes and blades produced on average 30% and 80% more DOC respectively during the day compared to the night. Modeled DOC NPP at the reef scale was on average highest in summer and spring (∼0.5 g C m−2 d−1) and lowest in winter and autumn (∼0.31 g C m−2 d−1), but it varied greatly among years for any given season as large oscillations in standing biomass led to corresponding fluctuations in reef‐scale DOC NPP. The fraction of NPP released as DOC was highly variable when examined at the monthly time scale, but became much more stable at seasonal and annual time scales averaging 14% of total NPP.