Phenology of Group 2i Isochrysidales
Within the same location where annual temperature varies from around freezing to over 10 ˚C, Group 2i only occurs in winter and spring when the water temperature is below ~12˚C (Fig. 3). Group 2i was detected in water samples with 1) temperatures below 0 ˚C, 2) temperature above 0˚C collected under-ice or samples collected shortly after ice melt in spring; but is absent in samples collected during the summer. Group 2i was not detected in any samples collected during an extensive survey of haptophytes conducted in the central North Pacific from 0˚ to 60˚ N along the transect of 170˚W during the summer of 2014 where seawater temperatures ranged between 9.8-29.3˚C32.
We examined Isochrysidales DNA data from ice and seawater samples collected across seasonal cycles from two regions in the Baltic Sea, Gulf of Finland and Oslofjorden (Fig. 2). Isochrysidales sequences were found in the ice and water column during the cold seasons (Oct.-May) in the two monitored sites at the northwest of the Gulf of Finland—Krogarviken and Storfjärden by Enberg et al.21. Both of the sites were covered by sea ice from January to April, and the total algal biomass was higher in the ice than in the water column during the ice-covered season and slowly increased as ice thickness increased (Fig. 4; Enberg et al., 2018). Isochrysidales communities in the ice, under ice water (collected at the ice-water interface), and water bottom were composed of Group 2i and Group 1 Isochrysidales. Group 2i was the predominant haptophyte in the ice samples based on relative read abundance, where the brine salinity ranged widely from approximately 4-45 psu. It made up 40-85% of the total haptophyte read in Krogarviken and 58-86% in Storfjärden, and accounted for up to 0.2-2.0%, and 0.3-2.2% of the total Eukaryotic reads in the Krogarviken and Storfjärden sea ice from January to April (Fig. 4). In contrast, zero or only a few (<5 reads) Group 1 were detected in each ice sample, the majority of Group 1 was detected in low salinity (~4 psu) water samples. Group 2i’s relative read abundance to total eukaryotes and absolute read count were lower in the water column than in the ice from February through early April, and the abundance in the water decreased sharply as the ice started to melt in mid-April. The Group 2i population in the water column decreased as water temperatures rose and was absent in most of the water samples collected during July-November at both sites (Fig. 4).
Egge et al.20 collected water samples in outer Oslofjorden (59.19˚N, 10.69˚E) at 1 m depth monthly and sequenced haptophytes through NGS from September 2009 to June 2011. Group 1, 2i, 2w, and 3 were all detected at this site. Emiliania huxleyi was detected in all months except April and is one of the predominant haptophyte species during the high-temperature months from summer to early autumn (Fig. 5). Group 2w was detected in September 2009 and May 2011 when the surface temperature reached 16˚C. Group 2i, in contrast, was absent during the warm season and only detected from December 2010-April 2011 when the surface water temperature was around freezing for a prolonged period and upper Oslofjorden was frozen extensively. The relatively low sequence reads of Group 2i at this site suggest that Group 2i might have been transported from the ice in Upper Oslofjorden. Group 1 was detected in February and May 2010, and March-May 2011, likely controlled by freshwater input and reduced surface salinity during those periods. The sequences in Oslofjorden further demonstrate the substantial difference between the key environmental controls on the bloom phenology of the three groups.