Spatial Considerations

Spatial Considerations | Bottom-up/Top-down Controls of Zooplankton | Impacts of Sea-ice on Bering Sea

During winter, the entire eastern Bering Sea shelf is well mixed from about 100 m to the coast. During summer, fronts divide the Bering Sea shelf into three distinct hydrographic regions: the Inner, Middle and Outer Domains (Coachman, 1986). The shallow Inner Domain is unstratified due to mixing of the upper and lower water columns by wind and tides, respectively. The Inner Front, usually located near the 50 m isobath (Stabeno et al., 2001), separates the Inner and Middle Domains. While during winter the middle domain is well mixed, during summer it is a two-layered system with an upper mixed layer separated from the deeper layer by a seasonal pycnocline at 15-35 m depth. The Outer Domain, from 100 to 200 m depth, consists of a wind mixed surface layer and a tidally mixed bottom layer, separated by a transition zone. The Middle Transition and shelf-break fronts separate the Middle Domain and Outer Domain from outer shelf and slope waters respectively (Coachman, 1986, Stabeno et al., 2001).

During most winters, sea-ice is present over the southern portion (south of 59°N) of the shelf, but the percent of coverage is highly variable, as is the timing of ice arrival and retreat (Stabeno et al., 2007, 2010). If ice is present after mid-March, an ice-associated phytoplankton bloom occurs; otherwise the bloom is delayed until May or even June (Stabeno et al., 2007, 2010). However, it is the intensity of stratification and storm events, not the presence of ice that determines if the primary production will be concentrated in spring or will continue throughout the summer (Hunt et al., 2002). This is especially true in the southern Middle Domain. If stratification is intense and is established early in the season, most production will occur as a sharp peak in spring and zooplankton will be food-stressed at some time during the summer production season. If stratification is weak (or winds particularly strong), production events will occur during summer (Sambrotto et al., 1986; Bond and Overland, 2005, Stabeno et al., 2010) utilizing the nutrient reservoir beneath the pycnocline (Whitledge et al., 1986; Rho et al., 2005); thus, zooplankton will have a more continuous food supply between the spring and fall blooms. The crustacean zooplankton community in the middle shelf is dominated numerically by small neritic species, though the biomass is dominated by the LCZ Calanus marshallae and the euphausiid Thysanoessa raschii (Cooney & Coyle, 1982; Smith & Vidal, 1986). These LCZ may require colder water (<3°C) on the shallow middle shelf in winter. With climate warming, elevated metabolic rates may cause them to exhaust their lipid reserves before phytoplankton production returns in spring (Coyle et al., In Press). They will leave diapause early, reproduce early and their nauplii-copepodids will experience a poor food environment.

Spring blooms in the Outer Domain usually occur after those in the Middle Domain. The Outer Domain and Slope region are dominated by oceanic zooplankton, which are not food limited there because production is elevated for most of the summer (Green Belt, Springer et al., 1996). This is apparent from examining the distributions of the LCZ Calanus marshallae, Neocalanus spp. and T. inermis during the warm period (Coyle et al., 2008; BASIS unpubl. data). These zooplankton can survive near the shelf edge because the water is deep and they can find favorable habitat to diapause by descending to warmer, saltier waters. Ice-cooled water is not the determinant of their survival there.

The northern portion of the shelf is ice-covered every winter. Ice-melt in spring brings a strong salinitymediated stratification and a spring bloom in April or May.  Although there is a summer draw-down of nutrients, benthic regeneration and cross-shelf advection may replenish them, and there is a subsurface chlorophyll maximum just below the pycnocline. The northern Middle Domain between St. Matthews Island and St Lawrence Island will have a cold pool more frequently than the southern Middle Domain, and therefore may have more LCZ present in summer.