New insights into trophic interactions and controls on energy flow in the Bering Sea were developed during a workshop of BEST-BSIERP PIs held in Bermuda in February 2012. The overarching question for this workshop was “How does the presence or absence of sea-ice over the eastern shelf in spring influence the flow of energy through the pelagic ecosystem in the eastern Bering Sea, particularly the distribution, standing stock, and trophic role of large crustacean zooplankton (LCZ) that are of critical importance in the diets of commercially valuable fish, marine birds and cetaceans?” This question was divided into four general hypotheses (below), each with several more specific sub hypotheses and questions.
H1: Bottom-Up Control: Differences in advection patterns between warm and cold years impact the distribution of large crustacean zooplankton (LCZ).
H2: Bottom-Up Control: Primary production is lower during warm years relative to cold years thus restricting the flow of energy to LCZ.
H3: Bottom-Up Control: Micro-zooplankton are an important but previously under-appreciated trophic level that modulates energy flow to LCZ.
H4: Top-Down Control: Predation on LCZ varies between warm and cold years.
Participants in the workshop were divided into four working groups, each assigned to address one of the hypotheses above. As the BEST-BSIERP program was conducted exclusively in cold years, addressing these hypotheses requires some level of data mining from prior research programs in the Bering Sea shelf system, which will be addressed through the course of the two-year synthesis program. Each group was responsible for presenting both initial questions and observations as well as summarizing the outcomes of breakout group discussions during the course the workshop.
New Insights included:
1. New data on current flow patterns suggest that there are substantial differences in mean on-shelf flow patterns between warm (more frequently characterized as downwelling conditions) and cold (upwelling conditions) years. The hypothesis is consistent with differences in the center of distribution of early stages of walleye pollock which shift towards the middle shelf in warm years but are centered on the outer shelf during cold years. It is still unclear if this shift is in response to temperature preferences, or due to advection.
2. New data suggest that rates of net primary production and net community production are tied to seasonal meteorological forcing (i.e. wind mixing) rather than the presence/absence of sea ice. In addition, shelf wide production appears to be lower in warm years, despite enhanced on-shelf flow and potential for nutrient recharge in the outer domain. However, because warm years have more time without ice cover, the total open water primary production is higher in warm years. These results suggest that there remain significant gaps in our understanding of the mechanisms governing primary production in this region.
3. The general importance of microzooplankton to the Bering Sea ecosystem has been well established now. Available data suggests that microzooplankton may be a more important intermediary between phytoplankton and LCZ’s in warm years than cold years. However, it is not known how this 3-step food web impacts the flow of carbon and energy between phytoplankton and LCZ’s between warm and cold years.
4. Prior to the workshop, it was generally held that top-down control on LCZ due to predation by Walleye Pollock was important due to their inverse relationship in abundance over time. From presentations at the workshop it was clear that LCZ’s are more controlled by temperature patterns than consumption by predators. Rather than serving as a top-down control, Pollock appear respond to changes in the relative abundance of LCZ’s, and the grazing impact of pollock is low to moderate (~10-30%).
These new insights will guide our work plan for the next year in preparation for the second synthesis workshop.