Cold Dome Affects Mesozooplankton Communities during the Southwest Monsoon Period in the Southeast East China Sea

In order to better understand the cold dome influence on zooplankton community structure, zooplankton samples were collected during the southwest monsoon prevailing period from the southeast waters of the East China Sea. To reduce the bias caused by different sampling months, the samples were collected in June 2018 and in June 2019. An obvious cold dome activity was proven by images of remote sensing satellites during the June 2018 cruise. In contrast, the research area was much affected by open sea high temperature and water masses during the June 2019 cruise. Significant differences in water conditions were demonstrated by surface seawater temperature, salinity, and dissolved oxygen concentrations between the two cruises. Nevertheless, no significant differences were observed concerning mesozooplankton in general, copepods, large crustaceans, other crustaceans, and pelagic molluscs between the June 2018 and June 2019 cruises. However, the mean abundance of gelatinous plankton was significantly different with 1213.08 ± 850.46 (ind./m³) and 2955.93 ± 1904.42 (ind./m³) in June 2018 and June 2019, respectively. Noteworthy, a significantly lower mean abundance of meroplankton, with 60.78 ± 47.32 (ind./m³), was identified in June 2018 compared to 464.45 ± 292.80 (ind./m³) in June 2019. Pearson’s correlation analysis also showed a highly positive correlation of gelatinous plankton and meroplankton with sea surface temperature (<i>p</i> < 0.01). The variation of salinity showed a significant negative correlation with gelatinous plankton abundance (<i>p</i> < 0.05), and a highly significant negative correlation with the abundance of meroplankton (<i>p</i> < 0.01). Only the abundance of meroplankton showed a positive correlation with dissolved oxygen concentrations (<i>p</i> < 0.05). The copepod communities were separated in two groups which were consistent with sampling cruises in 2018 and 2019. Based on the specificity and occupancy of copepods, <i>Macrosetella gracilis</i>, <i>Oithona rigida</i>, <i>Cosmocalanus darwinii</i>, <i>Paracalanus parvus</i>, and <i>Calocalanus pavo</i> were selected as indicator species for the cold dome effect in the study area during June 2018, whereas the indicator species of warm water impact in the open sea were <i>Calanopia elliptica</i>, <i>Subeucalanus pileatus</i>, <i>Paracalanus aculeatus</i>, and <i>Acrocalanus gibber</i> during the June 2019 cruise.