Abstract
Interannual variability in euphausiid (krill) abundance and population structure and associations of those measures with environmental drivers were investigated in an 11-year study conducted in late August–early September 2005–2015 in offshelf waters (bottom depth > 40 m) in Barrow Canyon and the Beaufort Sea just downstream of Distributed Biological Observatory site 5 (DBO5) near Pt. Barrow, Alaska. Statistically-significant positive correlations were observed among krill population structure (proportion of juveniles and adults), the volume of Late Season Melt Water (LMW), and late-spring Chukchi Sea sea ice extent. High proportions of juvenile and adult krill were seen in years with larger volumes of LMW and greater spring sea ice extents (2006, 2009, 2012–2014) while the converse, high proportions of furcilia, were seen in years with smaller volumes of LMW and lower spring sea ice extent (2005, 2007, 2010, 2011, 2015). These different life stage, sea ice and water mass regimes represent integrated advective responses to mean fall and/or spring Chukchi Sea winds, driven by prevailing atmospheric pressure distributions in the two sets of years. In years with high proportions of juveniles and adults, late-spring and preceding-fall winds were weak and variable while in years with high proportions of furcilia, late-spring and preceding-fall winds were strong, easterly and consistent. The interaction of krill life history with yearly differences in the northward transports of krill and water masses along with sea ice retreat determines the population structure of late-summer krill populations in the DBO5 region near Pt. Barrow. Years with higher proportions of mature krill may provide larger prey to the Pt. Barrow area bowhead whale prey hotspot. The characteristics of prey near Pt. Barrow is dependent on krill abundance and size, large-scale environmental forcing, and interannual variability in recruitment success of krill in the Bering Sea.
Highlights
Barrow during the fall are formed through a wind sequence in which local winddriven shelf-break upwelling that delivers krill to the shelf is followed by relaxation that results in krill being trapped on the shelf between westward-flowing shelf currents and the northeastward-flowing Alaskan Coastal Current in Barrow Canyon [6,7,8]
Sea ice extent, and the volume of late season melt water (LMW) were closely correlated, patterns that could be explained by preceding fall and/or late-spring sea level pressure (SLP) patterns and wind fields over the Chukchi and Beaufort Seas (Table 2)
Together with krill life history and their Bering Sea source, these associations suggested that a mechanism consisting of the interplay between wind-driven northward transport of water and sea ice, the timing of krill reproduction and development, and krill advection through the Chukchi Sea determines whether late-summer krill populations at Pt
Summary
The region near Pt. Barrow, Alaska is a known recurrent feeding region for bowhead whales as they migrate south in fall from the Canadian Arctic to overwinter south of Bering Strait [1, 2]. The physical and biological factors that generate a favorable feeding environment for bowhead whales near Pt. Barrow were explored during an 11-year oceanographic study [5]. It was found that dense aggregations of bowhead whale prey, euphausiids or krill, on the shelf northeast of Pt. Barrow during the fall are formed through a wind sequence in which local winddriven shelf-break upwelling that delivers krill to the shelf is followed by relaxation that results in krill being trapped on the shelf between westward-flowing shelf currents and the northeastward-flowing Alaskan Coastal Current in Barrow Canyon (the “krill trap”) [6,7,8]. Barrow in fall depends on the occurrence of this sequence of winds and on the characteristics of the krill (abundance, size) in the waters offshore
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