The large-bodied copepods off Masirah Island, Oman: An investigation of Southwest Monsoon onset and die-off

Abstract

Abstract Masirah Island, situated off the southern coast of Oman in the Arabian Sea, is surrounded by upwelled water during the Southwest Monsoon (SWM) and was used as a base for time-series studies of the copepod community for a decade beginning in 2006, covering the Spring Intermonsoon (SIM), April–May, and including June, and the SWM period, August–October. Subeucalanus pileatus and Calanoides natalis were the two most abundant copepod species (33.9% and 32.7% respectively of the total abundances). These two species shifted their dominance during the two main seasonal periods. During the SWM, C. natalis was the dominant species occurring mainly at sea surface temperatures (SST) between 18 and 27 °C while S. pileatus dominated during the SIM occurring at warmer SSTs, frequently between 25 and 28 °C. Results from generalized linear models indicated that the season and the SST were the best predicted variables affecting the abundances of C. natalis and S. pileatus. Calanoides carinatus was also affected by wind speed and wind direction, meaning that strong SWM winds and low temperatures (seasonal upwelling) favor the occurrence of this species. Upwelling favorable winds lasted longer in 2015/2016 than in 2009/2010; peak winds shifted from June in 2009–2011 to July in 2012–2016. The SST was higher in 2016 than in the previous five years. Time-series of C. natalis life stages showed three large peaks of early copepodites from June through September 2016 during the SWM season, in contrast to only one peak observed at the beginning of the SWM season from previous years. This result suggests that C. natalis was able to capitalize on the longer SWM in 2016 by continuing to reproduce throughout September. Diapausing C. natalis reached the surface in mid-to-late June as stage CV, ruling out any reproduction by this species at its overwintering depth. Unfavorable SST at the end of the SWM corresponded to a downward migration by C. natalis in 2015. Overall, the physical forcing of this upwelling region appears to be changing and C. natalis seems to be adapting to these changing conditions by continuing to reproduce at the surface as long as SST is favorable.