Microbial respiration in the mesopelagic zone at Station ALOHA

Measurements of the temporal and depth variability of microbial community respiration (MCR) in the mesopelagic zone (200-1,000 m) at Station ALOHA were made approximately monthly from November 2011 to October 2012 using the in vivo INT method. MCR in the summed and in each of the 0.2-0.8 mu m and >0.8 mu m size-fractions in the upper mesopelagic zone (200-350 m) (mean [+/- SE], 107.1 [+/- 16.3], 57.5 [+/- 8.4], and 49.6 [+/- 9.0] mu mol O-2 m(-3)day(-1), respectively) were higher than in the lower mesopelagic zone (350-1000 m) (56.1 [+/- 4.5], 30.8 [+/- 2.9], and 25.3 [+/- 3.1] mu mol O-2 m(-3)day(-1), respectively). The average > 0.8 mu m/0.2-0.8 m MCR in the mesopelagic zone was 0.97 [+/- 0.16]) and was indistinguishable in the upper and lower portions of the mesopelagic zone. A recurrent local peak in MCR was found at 600-650 m for both non-winter and winter measurements. Total depth-integrated MCR (200-1,000 m) was higher during the non-winter than during winter months (55.7 [+/- 3.4] and 44.8 [+/- 2.4] mmol O-2 m(-2) day(-1)). Variability of total MCR rates was controlled by changes in the >0.8 mu m size fraction, and was possibly related to higher primary production and export from the euphotic zone during the non-winter period. These findings suggest seasonal variability in respiration rates in the mesopelagic zone at Station ALOHA and support the current steady-state model where export flux controls respiration in the mesopelagic zone