Responses of hyperiid (Amphipoda) communities to monsoon reversal in the central South China Sea

Abstract

Significant seasonal variations in surface circulation are driven by the East Asian Monsoon in the South China Sea (SCS). Seasonal variations in copepod and hyperiid communities following the monsoon have been reported. The mechanisms controlling these monsoon-driven seasonal variations in hyperiid communities were analyzed using data from the central SCS (CSCS) during 2014–2015. Hyperiids mainly aggregated at the transitional water zones driven by the interactions of the cyclonic gyre and anti-cyclonic gyre or the western boundary current (WBC) in the CSCS. The total hyperiid abundance was related to the Northwest Luzon Cyclonic Gyre during the northeast monsoon and the East Vietnam Cyclonic Gyre during the southwest monsoon. The hyperiid community structure was significantly affected by the temperature of the upper 100-m water layer and gelatinous plankton in the CSCS from summer to fall. Different hyperiid ecological groups occurred in different water masses in the CSCS. The areas of interaction between cyclonic and anti-cyclonic gyres or the WBC and the distributions of water masses changed following seasonal variations in surface currents. The seasonal change in transitional water zones and water masses caused the seasonal variations of hyperiid community to track the monsoon reversal. Comparison of our data with previous data for the tropical SCS and other tropical oceanic areas confirms that the species composition of hyperiids is similar to that of the tropical Pacific, and that the most abundant species of hyperiid communities are very much more abundant than other species in subtropical–tropical oceans. Confirmation of symbiosis between three abundant hyperiid species (Lestrigonus bengalensis, Primno latreillei and Phronima sedentaria) and some abundant species of gelatinous zooplankton in CSCS suggests that the symbiosis helps the most abundant species to dominate significantly. The seasonal species diversity and total abundance of hyperiids usually remain relatively constant, the occurrence of strong El Niño events leads to the abnormal low total hyperiid abundance in fall, and the weak Kuroshio intrusion causes the abnormally low hyperiid species diversity in fall in the CSCS.