Krill diel vertical migration: A diagnostic for variability of wind forcing over the Beaufort and Chukchi Seas

Abstract

Measurements of echo intensities were acquired in shelf waters of the western Beaufort Sea near Utqiaġvik (formerly Barrow), Alaska by upward-looking 307 kHz acoustic Doppler current profilers during a 2008–2015 series of late-summer mooring deployments. These echo signals were analyzed for characteristic patterns of krill diel vertical migration (DVM) from which daily and seasonally-averaged DVM indices (DVMI) were derived. Time varying relationships among DVMIs (inferred krill biomasses) and local and regional wind regimes were diagnosed statistically. The threshold wind speed at which easterly winds promote upwelling of krill onto the western Beaufort shelf occurs at about 6 m s−1. Inferred krill biomass increases on the shelf as upwelling winds relax. Years (2009, 2012) in which inferred krill biomasses were higher on the western Beaufort shelf occurred when average mid-summer winds over the Chukchi Sea were from the south and average late-summer winds over the Beaufort shelf were weak and variable. In contrast, years (2008, 2010–11, 2013–15) in which inferred krill biomasses were lower occurred when average mid-summer winds over the Chukchi Sea were weak and variable and average late-summer winds over the Beaufort shelf were generally easterly and strong.