Annual and seasonal changes in the assemblage of planktonic copepods and appendicularians in Funka Bay before and after intrusion of Coastal Oyashio Water

Abstract

Annual and seasonal changes in the abundance and the assemblage of planktonic copepods and appendicularians were clarified in Funka Bay. Zooplankton samples were obtained by vertical hauls with a NORPAC net (0.10-mm mesh aperture) from December to May over a 4-year period (2014–2017). Temporal fluctuations of mean abundance of copepods were small from December 2013 to May 2014 under the low water temperature condition (maximum 1.55 times difference between adjacent survey periods), while those were large in other 3 years (1.60–1.93 times). Maximum mean abundance of appendicularians in 2014 (6.0\u2009×\u2009104 ind./m2) was lower than those in 2015–2017 (8.6\u2009×\u2009104–1.4\u2009×\u2009105 ind./m2). Less saline (≤\u200933.3) and cold Coastal Oyashio Water (COW) intruded in early February 2014, late February 2015, and late March 2016 and 2017. Phytoplankton blooms occurred in March of all 4 years. Copepods and appendicularians increased nearer periods of phytoplankton bloom than after COW intrusion. In the results of canonical correspondence analysis, the assemblage of copepods and appendicularians gradually changed from small-sized and warm-water species to large-sized and cold-water ones. Northwesterly wind speed had an effect equal to COW intrusion (82% of COW intrusion) to the change in taxonomic compositions of copepods and appendicularians.