Temperature
Temperature is the main factor governing the distribution of pteropods. They prefer a warmer water, as there is a well-defined latitudinal temperature gradient for their distribution. There is only one pteropod species, Limacina helicina Phipps lived in the two polar seas, whereas the warm, tropical zones are populated by many genera and species. Here in Daya Bay, as shown in Fig. 5, there was a positive relationship between the spatial distribution of C. acicula and water temperature, indicating C. acicula also favor of warmer water. The highest abundance (5595 ind. m-3) matched the highest temperature (~32.3°C). Also, most of the stations with higher abundance of C. acicula were located in areas with sea surface temperature ≥30°C. Furthermore, according to buoy measurements from May to July 2020 (Fig. 6), there were two peaks of sea surface temperature (SST), the first one was in June when the surface water temperature increased from 29.8°C on June 9 to 32.8°C on June 13, 2020; the second one was on July 2 when the temperature increased to 32.4 °C. It is worth noting that the observed high abundance of C. acicula population were coincident with the two temperature peaks. In fact, the bloom was observed on June 12, 2020, right falling in the first intensive warming period of 4 days. The highest value of C. acicula population density was observed during the second temperature peak. These findings thus suggested that higher water temperature could be one of the essential environmental conditions for the outbreak ofC. acicula in Daya Bay, or at least facilitated their accumulating in sea surface.