The overall objective of this synthesis was to better understand climatological, physical, chemical and biological processes that influence the flow of energy from primary producers to crustacean zooplankton on the Bering Sea shelf, and how a warming climate may influence these processes. Hypotheses were addressed both by PIs on this proposal, and through synthesis workshops (described below). Because the eastern shelf has considerable spatial heterogeneity, we examined these questions in defined regions of the shelf to elucidate how differences in hydrographic structure and mixing processes affect the flow of carbon and energy. In the hypotheses and questions, cold and warm were used to define years in which sea-ice was present or absent over the southern shelf in spring respectively.
The entirety of the hypotheses and questions set forth in this project could not be answered without the participation of many other PIs from the joint BEST-BSIERP program, as well as PIs from other research programs in the Bering Sea. This is especially true for contrasting the impact of warm and cold years on the trophic structure and carbon and energy flow within the ecosystem. To accomplish this we held two synthesis workshops designed to bring together national and internationally-recognized researchers in the Bering Sea who were considered essential for fully addressing our hypotheses and questions.
The two workshops built upon progress that each research team made during their multi-disciplinary analysis of various data sets in the intervening months. These workshops focused on identifying and filling gaps, presenting new approaches to challenges, molding synthesis products, and developing new paradigms.
Results have been presented through ~35 journal articles, with additional manuscripts in preparation, and from ~50 presentations at numerous conferences and science/agency advisory meetings including the Alaska Marine Science Symposium (2013 and 2014) and AGU/ASLO Ocean Sciences Meetings (2012, 2014).
Through the BEST program and synthesis, we have achieved a new understanding of trophic dynamics and how the ecosystem responds to variations in climate. A short list of significant results from the multitude of publications that resulted from this synthesis include:
· Seasonal variability in the transport of ice, heat, salt, nutrients and zooplankton over the shelf
· Spatial and temporal distribution of nutrients, chlorophyll and primary production over the shelf
· Physical and biological controls of energy flow from primary producers and microzooplankton to large crustacean zooplankton (LCZ) and fish.
The major factor causing declines of LCZ in the Middle Domain of the southeast Bering Sea shelf was not differences in production between warm and cold years, but the absence of the pool of cold bottom water which provides an over-wintering habitat for LCZ during cold years.
In addition, we examined the potential for top-down and bottom-up control of euphausiids in the period 2001 -2012 and found that, on an annual basis, primary production was more than sufficient to support euphausiid populations. Evidence of top-down control by pollock included a strong inverse relationship between pollock biomass and euphausiid biomass, the substantial percentage of euphausiid production consumed by pollock, and the modeled steep decline in euphausiid biomass between spring and fall, which may be ascribed to predation, though other possibilities could not be ruled out. Contrary evidence came from a paper by Ressler et al., 2014, Marine Ecology Progress Series, that showed water temperatures were a strong predictor of euphausiid distribution and abundance, whereas pollock biomass had little or no predictive power. Finally, comparing the relative rates of euphausiid production, pollock consumption, and other sources of euphausiid mortality, it is clear that factors other than pollock consumption are likely important in determining euphausiid standing stock. We developed a simple conceptual model of how climate variability, in particular stanzas of years with early or late ice retreat such as that observed in 2001 - 2012, might affect the likelihood of top-down or bottom-up control of euphausiids. The strong density-dependent coupling of pollock with euphausiids (and large, lipid-rich copepods), and the sensitivity of these prey to variability in climate, suggests that the eastern Bering Sea pollock stock will be vulnerable to future climate shifts.