Decoupling of Environmental Conditions, Primary, and Secondary Producers within Cold Regimes in the Southeastern Bering Sea: Insights from a Mooring Time-series and Ship-based Studies

Environmental conditions in the sub-Arctic Bering Sea are characterized by alternating warm and cold regimes driven largely by atmospheric processes and their effects on the seasonal formation and retreat of sea ice. Variability of environmental conditions and primary and secondary productivity between these regimes has been the focus of much study in recent years, while interannual variability within single cold or warm regimes and among multiple trophic levels has been less well-documented. A mooring (M2) instrumented with oceanographic and multi-frequency acoustic sensors was deployed on the southeastern shelf of the Bering Sea during the cold regime years of 2009–2012. Data from the spring-to-summer transitions (March-September) in each of these years showed extensive interannual variability of hydrographic conditions, primary producer biomass, and secondary producer and consumer relative abundance and community structure. Overall, cross-covariance analyses indicated that primary producers and secondary producers were more tightly coupled to each other and to environmental conditions in the coldest year (2012) than in the warmer years (2009, 2011) of this cold regime dataset. Specifically, primary producer biomass (as chlorophyll fluorescence), was more tightly coupled to environmental conditions in 2012 than in the other years, results that were reflected in ship-based observations of more classical spring bloom phytoplankton communities in 2012 (e.g. diatom-dominated) than in 2011 (e.g. prymnesiophyte-dominated). These between-year differences in phytoplankton community composition and the apparent independence of shallow (< 20 m) and mid-column (20 - 40 m) phytoplankton blooms likely explain the mooring-based observations of variable stimulation of consumer classes by algal blooms in each year. The results of these studies highlight the highly variable nature of interactions between the atmospherically-driven oceanographic environment and food webs in this ecosystem and underscore the need for the use of diverse methodologies and tools in the study of ecosystem function across trophic levels and through time.