Return of warm conditions in the southeastern Bering Sea: Phytoplankton - Fish

Abstract

In 2014, the Bering Sea shifted back to warmer ocean temperatures (+2 oC above average), bringing concern for the potential for a new warm stanza and broad biological and ecological cascading effects. In 2015 and 2016 dedicated surveys were executed to study the progression of ocean heating and ecosystem response. We describe ecosystem response to multiple, consecutive years of ocean warming and offer perspective on the broader impacts. Ecosystem changes observed include reduced spring phytoplankton biomass over the southeast Bering Sea shelf relative to the north, lower abundances of large-bodied crustacean zooplankton taxa, and degraded feeding and body condition of age-0 walleye pollock. This suggests poor ecosystem conditions for young pollock production and the risk of significant decline in the number of pollock available to the pollock fishery in 2–3 years. However, we also noted that high quality prey, large copepods and euphausiids, and lower temperatures in the north may have provided a refuge from poor conditions over the southern shelf, potentially buffering the impact of a sequential-year warm stanza on the Bering Sea pollock population. We offer the hypothesis that juvenile (age-0, age-1) pollock may buffer deleterious warm stanza effects by either utilizing high productivity waters associated with the strong, northerly Cold Pool, as a refuge from the warm, low production areas of the southern shelf, or by exploiting alternative prey over the southern shelf. We show that in 2015, the ocean waters influenced by spring sea ice (the Cold Pool) supported robust phytoplankton biomass (spring) comprised of centric diatom chains, a crustacean copepod community comprised of large-bodied taxa (spring, summer), and a large aggregation of midwater fishes, potentially young pollock. In this manner, the Cold Pool may have acted as a trophic refuge in that year. The few age-0 pollock occurring over the southeast shelf consumed high numbers of euphausiids which may have provided a high quality alternate prey. In 2016 a retracted Cold Pool precluded significant refuging in the north, though pollock foraging on available euphausiids over the southern shelf may have mitigated the effect of warm waters and reduced large availability of large copepods. This work presents the hypothesis that, in the short term, juvenile pollock can mitigate the drastic impacts of sustained warming. This short-term buffering, combined with recent observations (2017) of renewed sea ice presence over southeast Bering Sea shelf and a potential return to average or at least cooler ecosystem conditions, suggests that recent warm year stanza (2014–2016) effects to the pollock population and fishery may be mitigated.

Copepod dynamics across warm and cold periods in the eastern Bering Sea: Implications for walleye pollock (Gadus chalcogrammus) and the Oscillating Control Hypothesis

“…The open water spring bloom likely consists of larger‐sized diatoms and possibly dinoflagellates (Moran et al., ), although observational data are limited. The phytoplankton community prior to the bloom is light‐limited and is assumed to consist mostly of small cells (<5 μm or 10 μm in diameter) (Duffy‐Anderson et al., ; Lomas et al., ). Under these conditions, smaller‐bodied copepods would have a more favorable PPMR with smaller phytoplankton cells and all copepods would benefit from preying upon microzooplankton that graze on these smaller phytoplankton (Figure ).…”

“…Recent, large‐scale climatic fluctuations have been observed in the Bering Sea (Danielson, Curchitser, Hedstrom, Weingartner, & Stabeno, ; Hunt et al., ; Stabeno et al., ; Yeo et al., ). The result was a period of warm and cold “stanzas” (anomalous temperatures that persist over multiple years) that affected the temporal and spatial coverage of sea ice (Duffy‐Anderson et al., ; Stabeno et al., ).…”

Acoustic documentation of temperate odontocetes in the Bering and Chukchi Seas

“…) following the 2014 to 2016 warm climate regime that began attenuating after a warm peak in 2015 (Duffy‐Anderson et al . ). In fact, nearly all of the clicks and buzzes from typically temperate species occurred in the PAL data sets from 2008 to 2012, then were not detected again until 2017 coincident with the PDO easing out of its strong positive phase.…”

Observational Needs Supporting Marine Ecosystems Modeling and Forecasting: From the Global Ocean to Regional and Coastal Systems

“…In subarctic regions like the Bering Sea, sea ice and bottom temperature are major drivers and determinants of ecosystem state, with the timing and extent of sea ice cover and open waters influencing the evolution of the seasonal cycle in the region. Date of ice retreat, combined with solar heating and wind mixing determine the timing of the spring bloom on the southern Bering Sea shelf, with early ice retreat favoring late blooms in May or June, and late ice retreat (after March) favoring early phytoplankton blooms (Stabeno et al, 2007). Warm and cold years, as defined by water temperatures between the surface and 70 m depth, correlate to low and high abundance of large zooplankton in the southeastern Bering Sea shelf (Coyle et al, 2011), and the "cold pool, " the area where the bottom temperature is less than 2 • C, has been found to influence the distribution of major commercial groundfish (Lauth and Kotwicki, 2013).…”