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Abstract

Patches of benthic diatoms covered 23 to 73% of the sediment area at water depths down to 30 m of a high Arctic fjord system (Young Sound, NE Greenland). Mapping of the in situ chlorophyll (chl a) fluorescence by a pulse amplitude modulated (PAM) fluorometer demonstrated the presence of diatom-covered sediment patches with a characteristic size scale of 5 to 8 m. The benthic diatoms were well adapted to the ambient irradiance as demonstrated by in situ measurements of the relative electron transport rate (ETR) versus irradiance. However, the characteristics of the PE (photosynthesis versus downwelling irradiance) relations changed within minutes after exposure to changed light conditions neutralizing any depth related light acclimation. This demonstrated that the diatoms efficiently optimized their photosynthetic apparatus to the current light conditions. Steady state O-2 microprofiles measured in situ and in the laboratory showed a gradual increase in the O-2 penetration depth and O-2 concentration at the sediment surface with increasing irradiance. Net photosynthesis showed a compensation irradiance of 4.5 mumol photons m(-2) s(-1) and a net photosynthetic capacity (P-max) of 85 O-2 mmol m(-2) d(-1). No photoinhibition was observed at the maximum irradiance of 140 mumol photons m(-2) s(-1). Total exchange rates Of 02 and DIC mirrored each other and confirmed the depth distribution of benthic diatoms and the microsensors measurements. The compensation irradiance for the integrated benthic community (including macrofauna) decreased with water depth to a minimum of 12 mumol photons m(-2) s(-1) at 30 m water depth. The mean O-2/DIC exchange ratio across the benthic interface was approximately 1.2 and was independent of water depth. Extrapolation of our productivity data to the outer fjord system by accounting for the in situ light regime showed that the primary production of the benthic diatoms was equivalent to that of benthic macroalgae. Benthic net photosynthesis was almost 7 times higher than the gross photosynthetic rates of the pelagic community for water depths <30 m. However, integrated for the entire outer Young Sound, benthic net photosynthesis only accounted for similar to70 % of the pelagic production as measured by the C-14-incubation technique.