Ocean Acidification

Highlights

• Remote sensing indicates declining bloom in cold and developing bloom in warm water
• Primary production peaks in polar and arctic surface water masses
• Bacterial production is coupled to primary production despite thermal sensitivity
• Oxygen-based community respiration is highest in warmer water masses
• Oxygen-based and INT-based respiration estimates differ by an order of magnitude

Abstract

The Fram Strait is the primary oceanic gateway to the Arctic Ocean and has highly dynamic oceanographic conditions. Oceanographic conditions can shape community compositions, which is increasingly shown using molecular studies, but rate measurements remain scarce, especially for respiration in the Arctic Ocean. Here, we assessed primary production (PP), bacterial production (BP), bacterial abundances (BA), and community respiration (CR) using Winkler titrations and in vivo Iodo-Nitro-Tetrazolium (INT) reduction within the upper 50 m across water masses of the Fram Strait that had varying bloom conditions in spring 2021. We complemented in situ observations with remote sensing of sea surface temperature (SST) and chlorophyll-a (SSC) to infer bloom phenology in warm (θ>2°C), intermediate (2>θ>0°C), and cold (θ<0°C) waters using remote sensing machine-learning. In the cold, nutrient-rich surface waters, a subsiding spring bloom was associated with elevated PP, BP, and BA alongside a high temperature sensitivity (Q₁₀) that indicates active microbial turnover. In contrast, the warm Atlantic-influenced waters exhibit a relatively lower PP but moderate cell-specific BP, suggesting bacterial maintenance metabolism under pre-bloom conditions. Notably, bacterial respiration (BR) estimates differed by 5- to 80-fold between the Winkler and in vivo INT methods. Although the true BR likely lies between these estimates, our results highlight the substantial microbial activity and underscore the need for more accurate BR measurements in Arctic studies.

von Jackowski A., Lampe V., Hirschmann S., Devresse Q. & Engel A., 2026. Water mass-driven variations in primary production and bacterial respiration during the spring bloom in the Fram Strait. Deep Sea Research Part II: Topical Studies in Oceanography: 105683. doi: 10.1016/j.dsr2.2026.105683. Article.