Transition from a Mixotroph/Heterotroph Protist Community During the Dark Winter to a Photoautotrophic Spring Community in Arctic Surface Waters

Abstract

Unicellular plankton communities (protists) are the basis of the marine food web. The spring bloom is especially important, because of its high biomass. However, it is poorly described how the protist community structure in Arctic surface waters develops from winter to spring. We show that mixotrophy and parasitism are the prominent trophic modes in the dark winter period. The transition period was characterized by a high relative abundance of mixotrophic dinoflagellates, while centric diatoms and the haptophyte Phaeocystis pouchetii dominated the successive phototrophic spring bloom event. Our observations indicate the presence of a characteristic winter community and a community shift from winter to spring, and not just a dormant spring community waiting for better circumstances. The spring bloom initiation commenced while sea ice was still obstructing the light penetration into the water column. The initiation coincided with a change in day length and spectral composition of the light, rather than with an increased light intensity. The initial increase in fluorescence, and therefore photosynthetic activity, was detected relatively deep in the water column, at ~55 m depth. This suggests that water column stratification and a complex interplay of abiotic factors eventually promote the spring bloom initiation.

Beyond blooms: A novel time series analysis framework predicts seasonal keystone species and sheds light on Arctic pelagic ecosystem stability

“…It is remarkable that Dinophyceae are present in almost all seasons, with the exception of the low light period, which is predominantly occupied by heterotrophic Syndiniales. This indicates that these mixotrophic organisms play a crucial role particularly in the intermediate phases of spring and autumn (Jassey et al, 2015;Bruhn et al, 2021;Mitra et al, 2014).…”

“…change from darkness to constant daylight within 20 days). The predominance of dinoflagellates in the intermediate phases of spring and autumn indicates that these mixotrophic organisms play a crucial role during transition phases (Jassey et al, 2015; Bruhn et al, 2021; Mitra et al, 2014). According to traditional ecological theory, keystone species are often defined as those with the most biomass (Kang and Fryxell, 1992; Sergeeva et al, 2018).…”