Effects of El Niño on Northeastern Pacific Blue Whale Population Dynamics

Effects of El Niño on northeastern Pacific blue whale population dynamics

Jessica Klack (0833002)

ZOO*4570

March 28, 2017

Introduction

        The California Current System (CCS) is an eastern boundary current of the North Pacific driven by large scale patterns of atmospheric pressure and associated winds off the west coast of North America (Checkley and Barth 2009). This current experiences coastal upwelling events, which are driven by atmospheric pressure gradients between the northeast Pacific high-pressure region off western North America and the Aleutian low-pressure region southwest of Alaska (Jacox et al. 2015). During periods of El Niño, there are anomalously warm surface waters pushed to the eastern boundaries of the Pacific basin due to weakened equatorial trade winds (Marinovic et al. 2002; Cowles et al. 1977). This diminishes coastal upwelling from below the deepened thermocline, causing reduced nutrient fluxes to the sunlit surface waters (Escribano et al. 2001; Marinovic et al. 2002). This warm, nutrient depleted water limits phytoplankton growth, causing reduced populations of prey species, such as euphausiids, for blue whales across the eastern Pacific (Escribano et al. 2001; Marinovic et al. 2002) This paper will identify different patterns in primary and euphausiid productivity associated with El Niño and their effects on the population dynamics and migration of northeast Pacific blue whales.

        This paper will proceed to discuss the alterations in primary productivity and phytoplankton community composition in response to climatic mechanisms associated with El Niño events (Du et al. 2015; Bograd and Lynn 2001). Changes in physical oceanographic conditions due to El Niño events are driving increases in sea surface temperature (SST) in the northeastern Pacific, causing ecological impacts on euphausiid abundance and community composition (Fisher et al. 2015; Marinovic et al. 2002). Finally, responses of blue whale populations to these alterations in preferred prey availability will be addressed, including alterations to their reproductive and migration ecology (Burtenshaw et al. 2004; Greene 2016).

Changes in Primary Productivity

        El Niño acts to weaken the equatorial trade winds resulting in reduced upwelling from below the deepened thermocline (Jacox et al. 2014). This contributes to a lack of nutrients distributed to the euphotic zone, as nutrients normally circulated from below the thermocline are suspended in the deeper waters of the aphotic zone (Escribano et al. 2001; Du et al., 2015). Bograd and Lynn (2001) found the nutricline deepened to 80 m along the southern California coastline during the 1997-1998 El Niño event, which was associated with significant reductions in surface water nitrate concentrations from 10 umol/L NO3 to a mere 1.7 umol/L. These changes in the physical environment have been shown to invoke unambiguous reductions in surface chlorophyll-a concentrations, as a measure of phytoplankton biomass, reflecting nutrient limitations (Bograd and Lynn 2001). This reduction in phytoplankton biomass is associated with reductions in primary productivity due to lower photosynthesis carbon fixation rates (Chavez et al. 1999) Limited nutrient input because of weakened upwelling imposes a large restriction on the primary production available for growth and reproduction of higher trophic level organisms.