Transition from a mixotrophic/heterotrophic protist community during the dark winter to a photoautotrophic spring community in surface waters of Disko Bay, Greenland

Unicellular eukaryotic plankton communities (protists) are the major 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
composition in Arctic surface waters develops from winter to spring. We show
that mixotrophic and parasitic organisms are prominent in the dark winter period. The transition period toward the spring bloom event 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 during the study. The data shows a continuous community
shift from winter to spring, and not just a dormant spring community waiting
for the right environmental conditions. The spring bloom initiation commenced
while sea ice was still scattering and absorbing the sunlight, inhibiting its
penetration into the water column. The initial increase in fluorescence was
detected relatively deep in the water column at ~55 m depth at the halocline, at
which the photosynthetic cells accumulated, while a thick layer of snow and sea
ice was still obstructing sunlight penetration of the surface water. This suggests
that water column stratification and a complex interplay of abiotic factors
eventually promote the spring bloom initiation.