Plankton metabolism and bacterial growth efficiency in offshore waters along a latitudinal transect between the UK and Svalbard

Euphotic zone gross primary production, community respiration and net community production were determined from in vitro changes of dissolved oxygen, and from in vivo INT reduction capacity fractionated into two size classes, in offshore waters along a latitudinal transect crossing the North, Norwegian and Greenland Seas between the UK and Svalbard. Rates of gross primary production were higher and more variable than community respiration, so net autotrophy prevailed in the euphotic zone with an average net community production of 164±64 mmol O2 m−2 d−1. Respiration seemed to be mainly attributed to large eukaryotic cells (>0.8 µm) with smaller cells, mainly bacteria, accounting for a mean of 25% (range 5–48%) of community respiration. Estimates of bacterial growth efficiency were very variable (range 7–69%) due to uncoupling between bacterial respiration and production. Larger cells tended to contribute more towards total respiration in communities with high gross primary production and low community respiration, while bacteria contributed more towards total respiration in communities with lower gross primary production, typical of microbial-dominated systems. This suggests that community respiration is related to the size structure of the plankton community.